The Human Body: October 2009

JOINT DISEASES

Posted by Prakash

Monday, October 5, 2009

Joint diseases are any of the diseases or injuries that affect human joints. Arthritis is no doubt the best-known joint disease, but there are also many others. Diseases of the joints may be variously short-lived or exceedingly chronic, agonizingly painful or merely nagging and uncomfortable; they may be confined to one joint or may affect many parts of the skeleton. For the purposes of this article, two principal categories are distinguished: joint diseases in which inflammation is the principal set of signs or symptoms and joint diseases, called noninflammatory in this article, in which inflammation may be present to some degree (as after an injury) but is not the essential feature.

INFLAMMATORY JOINT DISEASES: TYPES OF ARTHRITIS

Arthritis is a generic term for inflammatory joint disease. Regardless of the cause, inflammation of the joints may cause pain, stiffness, swelling, and some redness of the skin about the joint. Effusion of fluid into the joint cavity is common, and examination of this fluid is often a valuable procedure for determining the nature of the disease. The inflammation may be of such a nature and of such severity as to destroy the joint cartilage and underlying bone and cause irreparable deformities. Adhesions between the articulating members are frequent in such cases, and the resulting fusion with loss of mobility is called ankylosis. Inflammation restricted to the lining of a joint (the synovial membrane) is referred to as synovitis. Arthralgias simply are pains in the joints; as ordinarily used, the word implies that there is no other accompanying evidence of arthritis. Rheumatism, which is not synonymous with these, does not necessarily imply an inflammatory state but refers to all manners of discomfort of the articular apparatus including the joints and also the bursas, ligaments, tendons, and tendon sheaths. Inflammation of the spine and joints is called spondylitis.

Bursitis

Inflammation of a synovial bursa, the lubricating sac located over a joint or between tendons and muscles or bones, is called bursitis (or bursal synovitis). Bursas sometimes are affected along with the joints and tendon sheaths in rheumatoid arthritis and gout. Infectious agents introduced by penetrating wounds or borne by the bloodstream also may result in bursitis, but this is unusual. The prepatellar bursa, located on the lower part of the kneecap, is especially subject to involvement in brucellosis (undulant fever).

The cause of most cases of bursitis appears to be local mechanical irritation. Often the irritation is of occupational origin and occurs in the shoulder region, at the knee, or near the hip. The inflammatory reaction may or may not include deposition of calcium salts. The border between bursitis and other painful rheumatic conditions of the soft tissues is indistinct in many instances.
The most common form of bursitis affects the subdeltoid bursa, which lies above the shoulder joint. Bursitis in this circumstance is not the primary abnormality but results from degeneration and calcification of the adjacent rotator tendon. Direct injury is not usually the cause of calcium deposits and inflammation in the tendon; indeed, heavy labourers are less frequently affected than persons employed in less-strenuous occupations. The bursa proper is affected only when debris from the tendon extends into it, this intrusion being the principal cause of an acutely painful shoulder. The condition occurs most often in middle age and is infrequent among young children. Women are twice as likely to have the condition as men. The onset may be sudden and unprovoked. Pain and tenderness are great, and there is difficulty in raising the arm. Resting the arm and use of analgesics tend to lessen the discomfort; corticosteroids may reduce inflammation; and carefully graduated exercises may be used to lessen the possibility of lasting stiffness of the shoulder. Many months may pass before complete recovery is attained. Chronic inflammation of the bursa at the side of the hip joint—trochanteric bursitis—has a similar course.
The more clearly traumatic forms of bursitis are exemplified by "beat knee," a bursitis that develops below the kneecap because of severe or prolonged pressure on the knee. Bloody fluid distends the bursa and, unless removed early, may cause the walls of the bursa to become thickened permanently. Treatment, which involves protection from further irritation to the extent that this is possible, is otherwise similar to that for subdeltoid bursitis. A fair proportion of these lesions become infected as a consequence of injury to the overlying skin.

Infectious arthritis

Joints may be infected by many types of microorganisms (bacteria, fungi, viruses) and occasionally by animal parasites. There are three routes of infection: by direct contamination, by way of the bloodstream, and by extension from adjacent bony infections (osteomyelitis). Direct contamination usually arises from penetrating wounds but may also occur during surgery on joints. Blood-borne infections may enter the joints through the synovial blood vessels. Commonly, however, foci of osteomyelitis occur first in the long bones near the end of the shaft or next to the joint. The infection then extends into the joint through natural openings or pathological breaches in the outside layer, or cortex, of the bone. Characteristically, hematogenous (blood-borne) infectious arthritis affects one joint (monarthritis) or a very few joints (oligoarthritis) rather than many of them (polyarthritis) and usually affects large joints (knee and hip) rather than small ones. Infections of the joints, like infections elsewhere in the body, often cause fever and other systemic indications of inflammation.

Joint cartilage may be damaged rapidly by formation of pus in infections by such bacteria as staphylococci, hemolytic streptococci, and pneumococci. Tuberculosis of the joint can result in extensive destruction of the adjacent bone and open pathways to the skin. Tuberculous spondylitis, also known as Pott disease, is the most common form of this infection. It occurs mostly in young children. Treatment is with the antibiotic streptomycin and with antituberculous medications such as isoniazid and rifampin. A frequent fungal infection in the United States is caused by Coccidioides immitis, an organism indigenous to the arid regions of the southwestern United States. As in tuberculosis, seeding from the lung to the bone usually precedes involvement of a joint. Brucellosis, like tuberculosis, has a particular affinity for the spine. Brucella suis is the most likely brucellar organism to cause skeletal disease. Deformities and destructive changes in the joints in leprosy (Hansen disease) arise from infection of the nerves by the leprosy bacillus or from infection by other bacteria.

Among the better-recognized viral infections that can cause joint discomforts are rubella (German measles) and serum hepatitis, both of which usually are of short duration and have no permanent effect. Several tropical forms of synovitis are also viral. Dranunculiasis (Guinea worm disease) is an infection caused by the Guinea worm, a parasitic nematode that affects persons in tropical countries, and may involve the joints.

Infectious arthritis complicates several sexually transmitted diseases, including gonorrhea. Early treatment with penicillin may provide a prompt cure and may prevent the marked destruction of the joint that could otherwise ensue. Reactive arthritis (Reiter disease), which may occur after food poisoning or infection with some sexually transmitted diseases, usually improves spontaneously over the course of several months. Characteristically, reactive arthritis involves inflammation of the joints, the urethra, and the conjunctiva of the eyes. Syphilis appears not to infect the joints directly except in the most advanced stage of the disease and in congenital syphilis. The latter frequently causes destructive inflammation in the growing cartilaginous ends of the bones of newborn infants. Untreated, it leads to deformity and restriction of growth of the involved part, but early treatment with penicillin may result in complete recovery. Clutton joint is another type of congenital syphilitic lesion. It is a true inflammation of the synovial membrane that occurs in children between ages 6 and 16; although it causes swelling of the knees, it is a relatively benign condition. Lesions characteristic of tertiary syphilis sometimes occur in the joints of children who have congenital syphilis. Yaws, a nonvenereal infection by an organism closely related to that causing syphilis, leads to similar skeletal lesions. The condition has largely been eradicated but still affects persons in tropical areas.

Rheumatoid arthritis and allied disorders

In several types of arthritis that resemble infectious joint disease, no causative agent has been isolated. Principal among these is rheumatoid arthritis. This disorder may appear at any age but is most usual in the fourth and fifth decades. A type that affects children is called juvenile rheumatoid arthritis. Rheumatoid arthritis typically affects the same joints on both sides of the body. Almost any movable joint can be involved, but the fingers, wrists, and knees are particularly susceptible. The joints are especially stiff when the affected person awakes. Rheumatoid arthritis is not only a disease of the joints; fatigue and anemia indicate that there is a more generalized systemic involvement. A slight fever may sometimes be present. Lesions also occur in sites outside the joints. Involvement of bursas, tendons, and tendon sheaths is an integral part of the disease. Approximately one of five affected persons has nodules in the subcutaneous tissue at the point of the elbow or elsewhere. Inflammatory changes also are found sometimes in small arteries and the pericardium—the membrane enclosing the heart.

The course of the disease varies greatly from person to person and is characterized by a striking tendency toward spontaneous remission and exacerbation. With continuing inflammation of the joints, there is destruction of the joint cartilage. The degree of articular (joint) disability present in rheumatoid arthritis depends in large measure upon the amount of damage done to this cartilage. If the injury is severe, large areas of bone may be denuded of cartilage, so that adhesions form between the articular surfaces. Subsequent transformation of these adhesions into mature fibrous or bony connective tissue leads to firm union between the bony surfaces (ankylosis), which interferes with motion of the joint and may render it totally immobile. In other instances, the loss of cartilage and bone, coupled with the weakening of tendons, ligaments, and other supporting structures, results in instability and partial dislocation of the joint. In a small minority of cases, the disease pursues a rapidly progressive course marked by relentless joint destruction and evidence of diffuse vasculitis (inflammation of blood vessels). Many affected persons are benefited over the course of several months by rest, analgesic medications, and therapeutic exercises. In approximately one-third of the instances of the disease, it progresses and causes serious incapacity. In the absence of proper physical therapy, the joints may become greatly deformed and ankylosed.

There is now convincing evidence that immunologic reactions play an important role in the causation of rheumatoid arthritis. The blood of approximately 80 to 90 percent of persons with rheumatoid arthritis contains an immunoglobulin called rheumatoid factor that behaves as an antibody and reacts with another class of immunoglobulin. This immunoglobulin is produced by plasma cells that are present in sites of tissue injury. There is evidence that suggests that this agent may be one or more viruses or viral antigens that persist in the joint tissues.

Although there is no cure, corticosteroid medications and nonsteroidal anti-inflammatory drugs (NSAIDs) may be helpful in reducing pain and inflammation. The effectiveness of corticosteroids generally diminishes with time, and there are definite disadvantages in their use, such as a greater susceptibility to infection and peptic ulcers. Disease-modifying antirheumatic drugs (DMARDs) may slow the progression of the disease by inhibiting further joint damage. Surgery is often of value in correcting established deformities. A mild dry climate seems to be beneficial in some cases, but the improvement is generally not sufficient to justify a move that would disrupt the affected person's life.

There is at times a close association between rheumatoid arthritis and seemingly unrelated disorders. In about one-third of the cases of Sjögren syndrome, there is also rheumatoid arthritis, and high levels of rheumatoid factors are usually present in the bloodstream. In Felty syndrome, rheumatoid arthritis coexists with enlargement of the spleen and diminution in the number of circulating blood cells, particularly the white blood cells. Removal of the spleen restores the number of blood cells to normal but has no effect on the arthritis.

Several other types of polyarthritis resemble rheumatoid arthritis but characteristically lack the rheumatoid factors in the bloodstream. Psoriatic arthritis, associated with the skin disease psoriasis, differs from rheumatoid arthritis insofar as it has a predilection for the outer rather than the inner joints of the fingers and toes; furthermore, it results in more destruction of bone. Another type of arthritis is associated with chronic intestinal diseases—ulcerative colitis, regional enteritis, inflammatory bowel disease, cirrhosis, and Whipple disease. Ankylosing spondylitis, also known as Marie-Strümpell disease or Bechterew disease, affects some of the peripheral joints, such as the hip; but its principal location is in the spine and sacroiliac joints. In the spine the small synovial joints and the margins of the intervertebral disks are both involved. These structures become bridged by bone, and the spine becomes rigid. Ankylosing spondylitis affects approximately eight times as many men as women. The age of onset is lower than that of rheumatoid arthritis. The general management of the two disorders is much the same, but phenylbutazone is more effective in ankylosing spondylitis than in rheumatoid arthritis.

Collagen diseases

The collagen diseases are so called because in all of them abnormalities develop in the collagen-containing connective tissue. These diseases are primarily systemic and are frequently accompanied by joint problems. One of these diseases, systemic lupus erythematosus (SLE), may affect any structure or organ of the body. An association with rheumatoid arthritis is suggested by the fact that one-quarter of those with SLE have positive serological tests for rheumatoid factor, and perhaps as many patients with rheumatoid arthritis have positive lupus erythematosus tests. In another collagen disease, generalized scleroderma, the skin becomes thickened and tight. Similar changes occur in other organs, particularly the gastrointestinal tract.
Rheumatic fever often is classified with the collagen diseases. It has certain similarities to rheumatoid arthritis, as the name suggests, but the differences are more notable. In both conditions, arthritis and subcutaneous nodules occur, and inflammation of the pericardium is frequent. Nevertheless, the joint manifestations of rheumatic fever typically are transient, while those of rheumatoid arthritis are more persistent. The reverse is true of cardiac involvement in the two disorders. There is no compelling evidence that streptococcal infection is an important causative factor in rheumatoid arthritis, but it appears well established in rheumatic fever.

Arthritis more or less resembling rheumatoid arthritis occurs in roughly one-fourth of children who lack gamma globulins in the blood. In this circumstance there is a deficit in the body's mechanisms for forming antibodies.

Miscellaneous arthritides

Several types of arthritis appear to be related to a hypersensitivity reaction. Erythema nodosum is a skin disease characterized by the formation of reddened nodules usually on the front of the legs. In the majority of cases, pain may arise in various joints, and sometimes swelling appears. Lymph nodes at the hilus of the lung (the site of entrance of bronchus, blood vessels, and nerves) are enlarged. The synovitis disappears in the course of several weeks or months. Many cases of erythema nodosum are associated with drug hypersensitivity, with infections such as tuberculosis, coccidioidomycosis, and leprosy, and with sarcoidosis, a systemic disease in which nodules form in the lymph nodes and other organs and structures of the body. Synovitis of this sort occurs in 10 to 15 percent of patients with sarcoidosis.

Palindromic rheumatism is a disease of unknown cause that is characterized by attacks that last one or two days but leave no permanent effects. Nevertheless, palindromic rheumatism rarely remits completely, and approximately one-third of cases result in rheumatoid arthritis. Polymyalgia rheumatica, a relatively frequent condition occurring in older people, is characterized by aching and stiffness in the muscles in the region of the hips and shoulders, but the joints proper do not seem to be involved. There does seem to be some relationship to a type of arterial inflammation called giant cell arteritis. Polymyalgia rheumatica is not usually accompanied by serious systemic abnormalities and is treated with corticosteroids or NSAIDs.

NONINFLAMMATORY JOINT DISEASES: INJURY AND DEGENERATIVE DISORDERS

Traumatic joint diseases

Blunt injuries to joints vary in severity from mild sprains to overt fractures and dislocations. A sprain is ligament, tendon, or muscle damage that follows a sudden wrench and momentary incomplete dislocation (subluxation) of a joint. There is some slight hemorrhage into these tissues, and healing usually takes place in several days. More-violent stresses may cause tears in ligaments and tendons. Because the ligaments and tendons are so strong, they frequently are torn from their bony attachments rather than ripped into segments. Ligamentous, tendinous, and capsular tears are able to heal by fibrous union, provided that the edges are not totally separated from each other. Internal derangements of the knee most often arise from tears in the semilunar cartilages (menisci). Usually it is the medial meniscus that is disrupted. These tears are particularly frequent in athletes and develop as the knee is twisted while the foot remains fixed on the ground. Locking of the knee is a characteristic symptom. Because the semilunar cartilages have little capacity for repair, they must be removed surgically. Bleeding into the joint, called hemarthrosis, may also result from injuries.

Most traumatic dislocations are treated by prolonged immobilization to permit the capsular and other tears to heal. In some instances, surgical repairs are required. Fractures of bone in the vicinity of joints may or may not extend into the joint space. Whether they do or not, the normal contour of the joint must be restored or arthritic complications are likely to develop.

Degenerative joint disease

Osteoarthritis is a ubiquitous disorder affecting all adults to a greater or lesser degree by the time they have reached middle age. The name osteoarthritis is a misnomer insofar as its suffix implies that the condition has an inherently inflammatory nature. For this reason it frequently is called degenerative joint disease, osteoarthrosis, or arthrosis deformans. When the spine is involved, the corresponding term is spondylosis. Unlike rheumatoid arthritis, osteoarthritis is not a systemic disease and rarely causes crippling deformities. In the majority of instances, the milder anatomical changes are not accompanied by appreciable symptoms. The changes are characterized by abrasive wearing away of the articular cartilage concurrent with a reshaping of adjacent ends of the bones. As a result, masses of newly proliferated bone (osteophytes) protrude from the margins of the joints.

The clinical manifestations of osteoarthritis vary with the location and severity of the lesions. The most disabling form of the disorder occurs in the hip joint, where it is known as malum coxae senilis. Osteoarthritis of the hip, like that of other joints, is classified as primary or secondary. In secondary osteoarthritis the changes occur as a consequence of some antecedent structural or postural abnormality of the joint. In about half the cases, however, even rigorous examination fails to disclose such an abnormality; in these instances the osteoarthritis is called primary.

Probably the most frequent cause of osteoarthritis of the hip is congenital dysplasia (dislocation or subluxation of the hip). This term refers to a poor fit of the head of the femur, the long bone of the thigh, with its socket in the pelvis, the acetabulum. There is evidence that many cases arise in infancy as a consequence of swaddling infants or carrying them in headboards, procedures that keep the thighs in an extended position. Before the child is able to walk, the hip joint has frequently not yet fully developed, and the head of the femur is forced out of its normal position by this extension.

Osteoarthritis of the hip occurring in relatively young persons—in their 30s or 40s—frequently follows a progressive course and requires surgical treatment. Two rather different strategies of surgery are employed: one, an osteotomy, involves reshaping the upper end of the femur so that the load borne by the joint is distributed more efficiently; the other requires removal of the diseased tissue and replacement by an artificial joint.

Aside from the rapidly developing forms, osteoarthritis of the hips also appears frequently in elderly persons. Aging is an important factor in the development of other forms of degenerative joint disease as well, since the lesions increase in frequency and severity as time passes.

Considerations like these have led to the view that the principal causative factors in degenerative arthritis are faulty mechanical loading and senescent deterioration of joint tissue. Single injuries, unless they leave a joint permanently deformed, rarely result in osteoarthritis. Recurrent small athletic and occupational injuries, such as those arising from heavy pneumatic drill vibrations, apparently are more likely to do so. Lifting heavy weights has been implicated in some studies of spinal involvement.

Aside from surgery of the sort noted in the hip and sometimes the knee, treatment includes rest and proper exercise, avoidance of injury, the use of analgesics, NSAIDs, and corticosteroids to relieve pain, and several types of physical therapy.

Chondromalacia patellae is a common and distinctive softening of the articular cartilage of the kneecap in young persons, particularly young athletes. It results in "catching" and discomfort in the region of the patella, or kneecap, as the knee is bent and straightened out. Pathologically, the changes are indistinguishable from changes that occur early in osteoarthritis. Treatment includes rest, NSAIDs, and physical therapy. More-serious cases of chrondromalacia patellae may require surgery.

Degeneration of the intervertebral disks between the vertebrae of the spine is a frequent and in some ways analogous disorder. Often this occurs acutely in young and middle-aged adults. The pulpy centre of the disk pushes out through tears in the fibrous outer ring, resulting in a slipped disk. When this takes place in the lumbosacral region, the displaced centre (the nucleus pulposus) impinges on the adjacent nerve roots and causes shooting pains in the distribution of the sciatic nerve—hence the name sciatica. Pain in the small of the back may be associated not only with degeneration of the intervertebral disk and spondylosis but also with structural abnormalities of the region. Principal among these is spondylolisthesis, in which there is an anterior displacement of one lumbosacral vertebral body on another. The episodes respond to bed rest and mechanical support from wearing an abdominal brace. Muscle relaxants and muscle-strengthening exercises also may be of value. Recurrences are prevented by avoidance of back strains. The protruding tissue is removed by surgery only in cases in which pain and neurological defects are severe and fail to improve after less drastic measures.

CONGENITAL AND HEREDITARY ABNORMALITIES

Congenital abnormalities are not necessarily transmitted from generation to generation but can be acquired during fetal life or soon after delivery. The latter abnormalities usually are structural; the inherited defects may be structural or appear later in life as the consequence of a systemic metabolic defect present from conception. Mention has already been made of congenital dysplasia of the hip. The joint proper may be initially normal in this condition and in several other congenital disorders; only after other supporting tissues have altered the proper relationships does the contour of the bone and joint become distorted. In arthrogryposis multiplex congenita (multiple congenital crooked joints), many joints are deformed at birth, particularly the hip. The deformities are the consequence of muscle weakness that in turn sometimes results from spinal cord disease. Clubfoot (talipes equinovarus) is a congenital deformity in which the foot is twisted downward and inward because the ligaments and tendons are too short. Only infrequently are the muscles at fault. Idiopathic scoliosis (lateral curvature of the spine) usually makes its appearance during early adolescence. There is considerable plasticity of the tissues with latitude for correction of these deformities and for preventing their progression. For this reason the application of splints and other mechanical supports as soon as the condition is recognized is the major part of treatment. Surgery is resorted to when other measures have failed.

Structural variations in the lumbosacral spine are common and often harmless. Incompletely ossified interarticular portions of the neural arches of a vertebra constitute a congenital anomaly referred to as spondylolysis; it predisposes to forward slipping of the vertebra later in life and so to the congenital type of spondylolisthesis described above. By contrast, when the failure of bony fusion exists between the right and left halves of the neural arch, the condition is called spina bifida occulta.

Several genetically influenced metabolic diseases have articular manifestations. Gout is the most frequent of these. Chalklike masses of sodium acid urate crystals are deposited in and about the joints. Acute episodes of gouty arthritis are extremely painful. There is a tendency toward involvement of the big toe, a condition known as podagra.

Ochronotic arthropathy results from another, rarer inborn error of metabolism. It is characterized by pigmentation and degeneration of hyaline cartilage and by defective breakdown of the amino acids tyrosine and phenylalanine, causing large amounts of homogentisic acid to accumulate in body fluids and the urine. The urine turns black when exposed to air, a phenomenon called alkaptonuria. After many years, severe degenerative changes occur in the peripheral joints and in the spine.

In yet another metabolic disease, chondrocalcinosis, or pseudogout, crystals of calcium pyrophosphate are deposited in joint cartilages. There are several forms of the disease. Sometimes there are no symptoms; in other cases, symptoms are sufficiently severe to cause confusion with rheumatoid arthritis. Some cases run in families.

Joints also are affected by several relatively rare hereditary diseases collectively called the mucopolysaccharidoses, which result from defects in the metabolism of connective tissue matrices. In Hurler syndrome, for example, manifestations include mental retardation and heart failure, although skeletal growth also is abnormal. Most affected persons do not survive adolescence. Morquio disease, by contrast, is a recessively inherited form of severe dwarfism that is not associated with mental deficiency or cardiac insufficiency. X-rays of the spine reveal a characteristic misshapen flattened appearance of the vertebral bodies. Premature and severe degenerative changes in the peripheral and spinal joints are common. Polyepiphyseal dysplasias (abnormal development in childhood of a number of epiphyses—the ends or outlying portions of bones separated from the main body of the bone by cartilage) are a vaguely similar, though much milder, group of conditions in which precocious osteoarthritis and spondylosis are the first abnormalities to appear. Preexisting changes in the skeleton, resembling a milder form of Morquio disease, may then be discovered upon X-ray examination. The hip joint is most severely affected. In some cases the inheritance is dominant, in others recessive. Abnormalities in the fibrous components of connective tissue matrices are characteristic of Marfan syndrome. Many organs are affected by this condition, and the articular manifestations are less important. The joints are excessively loose, however, and painful complications develop in about half of affected individuals.

SECONDARY JOINT DISEASES

Hemorrhagic joint diseases

Hemarthrosis (bleeding into the joints) is a major complication of hemorrhagic disorders. Aside from the life-threatening episodes of bleeding, it constitutes the principal disability arising from the hemophilias. Most persons with these clotting defects are affected and usually within the first years of life. Bleeding into the joints is usually caused by relatively minor injury but may leave several residual deformities and loss of mobility of the part. Recurrent hemorrhage into an isolated joint, in the absence of a systemic tendency to bleed, is characteristic of pigmented villonodular synovitis, a tumour characterized by abnormal thickening and coloration of the synovial membrane. This is not a primary inflammatory disease of joints, despite the name. Large joints, usually of the lower extremity, are affected.

Aseptic necrosis

Because joint cartilages are without blood vessels, they are not destroyed by failures in the blood supply. Nevertheless, several joint diseases arise in association with aseptic necrosis—tissue death not caused by infection—of bone next to the joints. The precise nature of the failure of the blood supply is not always known. Fractures are one obvious cause. In decompression sickness (caisson disease) the obstructive elements are minute gas bubbles formed in the circulating blood from excessively rapid decompression. Decompression syndromes occur principally in divers and tunnel workers. Acute cases take the form of the "bends" and frequently are fatal. However, in a large proportion of workers in these occupations, even those who have not experienced the bends, extensive infarcts (areas of dead tissue) of bones and secondary osteoarthritis develop after many years. Analogous changes in sickle cell anemia presumably result from blood clotting related to the abnormality of the red blood cells. There is no entirely persuasive explanation for other types of aseptic necrosis that occur in adults. In each instance the hip is the joint most affected. Osteochondritis dissecans is a similar disorder in which a piece of joint cartilage and of underlying bone breaks off and lodges in the joint cavity. Usually the person affected can remember having injured the joint. The knee is the most frequent site. The condition usually occurs during the second and third decades of life. The displaced fragment causes a creaking sound when the joint is moved and must be removed by surgery.

Two different patterns of aseptic necrosis with joint involvement occur in growing children. One type (slipped epiphysis) is characterized by partial or complete tearing away of an epiphysis, usually as the result of injury. The epiphysis at the upper end of the thighbone is particularly susceptible. Osgood-Schlatter disease is an analogous lesion, but it affects a growth centre (anterior tibial tubercle) at a slight distance from the joint rather than in its immediate vicinity. In the second type of aseptic necrosis in children, the necrosis is not the consequence of mechanical tearing away of the part. The most frequent site is in the head of the thighbone; necrosis at this site is known as Legg-Calvé-Perthes disease. It occurs in children between ages 3 and 13 and is much more frequent in boys than in girls. Persistent pain is the most prominent symptom. Uncorrected severe lesions lead to arrest of growth, deformity, and arthritic changes in the hip joint.

Endocrine factors

The only joint lesion clearly related to a malfunctioning of the ductless (endocrine) glands is acromegaly. This disease results from excessive secretion of growth hormone by a tumour of the anterior pituitary gland. The hormone stimulates the proliferation of several skeletal soft tissues and bone including the joint cartilage. This causes the enlargement of the fingers that is characteristic of the disease. The enlarged joints are particularly prone to undergo osteoarthritic degeneration. Cretinism, which is related to hypothyroidism, causes dwarfism and abnormally developed bony epiphyses but apparently does not lead to joint disease. Severe diabetes mellitus, however, may result in Charcot joint (see below) arising from the effect of diabetes on the nervous system.

Neurogenic arthropathy

Neurogenic arthropathy, also known as Charcot joint, is a severe degenerative disease related to nerve lesions that develops when the sensory mechanisms of joints are impaired. The current view is that these joints become excessively strained because the ability to receive stimuli from bodily structures and organs necessary for normal limitation of motion is lacking. As a result, the supporting tissues are torn, and there is extreme disintegration of the structure. Neurogenic arthropathy is most often associated with diabetes mellitus, tabes dorsalis (a late form of syphilis affecting the posterior columns of the spinal cord), syringomyelia (a disease in which cavities develop in the gray substance of the spinal cord), pernicious anemia, and leprosy. The disease usually is localized to one joint or one group of joints, depending on the location of the nerve defect. Pain is frequently mild considering the massive distortion of the joint. Treatment is difficult and is based primarily on immobilization and restriction of weight bearing.

Hypertrophic osteoarthropathy

In approximately 5 to 10 percent of persons who have primary tumours within the chest, the ends of the bones near the joints become enlarged and painful. New bone is formed in the periosteum, and only occasionally do abnormalities develop within the joints themselves. Just how the chest abnormality leads to hypertrophic osteoarthropathy (disease of bones and joints with abnormal growth of bone) is somewhat of a mystery, but there is reason to believe that the vagus nerve is involved, since the condition is usually relieved promptly by cutting the vagus. It is also relieved by removal of the tumour. In this disorder the tips of the fingers become club-shaped, a painless lesion that occurs in many other circumstances as well.

Reflex sympathetic dystrophy

Reflex sympathetic dystrophy—also called shoulder-hand syndrome because pain in the shoulder is associated with pain, swelling, and stiffness of the hand—only rarely develops in the wake of external injury. Most often it follows a heart attack (myocardial infarction) or is associated with disease in the neck vertebrae; frequently there is no apparent cause. Most often the syndrome begins with pain and stiffness of a shoulder, followed shortly by pain and swelling of the hand, with vascular (blood vessel) changes in the skin of the hand. Over the course of several months, the swelling and vascular changes subside, but the skin and soft tissues become tightened. These changes sometimes disappear completely, but in other cases they leave permanent contractures—i.e., flexion and loss of mobility due to the tightening of the fingers. Loss of mineral occurs in the bones of the shoulder, wrist, and fingers. Blocking (interruption of functioning) of sympathetic nerves serving the area, administration of corticosteroids, and therapeutic exercises are used in the management of the condition.

Tumours of joints

Tumours of joints are uncommon. In synovial chondromatosis, a benign condition, numerous cartilaginous nodules form in the soft tissues of the joint. The lesion is usually confined to one joint, particularly the knee, and occurs in young or middle-aged adults. It may or may not cause pain or swelling and usually is cured by excision of a portion of the synovial membrane. The tumour rarely becomes malignant. The cartilaginous nodules sometimes also contain islands of bone; in this circumstance the lesion is called synovial osteochondromatosis. Like synovial chondromatosis, synovial osteochondromatosis is often a spontaneous or primary disorder of unknown cause. In many cases, however, it is a development secondary to other diseases of the synovium, such as rheumatoid arthritis and even osteoarthritis.

Synoviomas, or synovial sarcomas, are malignant tumours that arise in the tissues around the joints—the capsule, the tendon sheaths, the bursas, the fasciae, and the intermuscular septa, or divisions—and only rarely within the joint proper. Although they may occur at any age, they are most frequent in adolescents and young adults. The legs are more often involved than the arms. The tumours spread locally and also to regional lymph nodes and lungs. Synoviomas are not particularly sensitive to X-rays, and treatment with drugs has been ineffective. If distant spread has not occurred at the time the condition is identified, radical excision, which may include amputation of the part, is the recommended treatment.

JOINTS

Posted by Prakash

Sunday, October 4, 2009

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Joint in anatomy is a structure that separates two or more adjacent elements of the skeletal system. Depending on the type of joint, such separated elements may or may not move on one another. This article discusses the joints of the human body—particularly their structure but also their ligaments, nerve and blood supply, and nutrition. Although the discussion focuses on human joints, its content is applicable to joints of vertebrates in general and mammals in particular. For information about the disorders and injuries that commonly affect human joints, see joint disease.

JOINT MOVEMENTS

In order to describe the main types of joint structures, it is helpful first to summarize the motions made possible by joints. These motions include spinning, swinging, gliding, rolling, and approximation.

Spin is a movement of a bone around its own long axis; it is denoted by the anatomical term rotation. An important example of spin is provided by the radius (outer bone of the forearm); this bone can spin upon the lower end of the humerus (upper arm) in all positions of the elbow. When an individual presses the back of the hand against the mouth, the forearm is pronated, or twisted; when the palm of the hand is pressed against the mouth, the forearm is supinated, or untwisted. Pronation is caused by medial (inward) rotation of the radius and supination by lateral (outward) rotation.

Swing, or angular movement, brings about a change in the angle between the long axis of the moving bone and some reference line in the fixed bone. Flexion (bending) and extension (straightening) of the elbow are examples of swing. A swing (to the right or left) of one bone away from another is called abduction; the reverse, adduction.

Approximation denotes the movement caused by pressing or pulling one bone directly toward another—i.e., by a "translation" in the physical sense. The reverse of approximation is separation. Gliding and rolling movements occur only within synovial joints and cause a moving bone to swing.

TYPES OF JOINT

Joints can be classified in two ways: temporally and structurally. Each classification is associated with joint function.

Considered temporally, joints are either transient or permanent. The bones of a transient joint fuse together sooner or later, but always after birth. All the joints of the skull, for example, are transient except those of the middle ear and those between the lower jaw and the braincase. The bones of a permanent joint do not fuse except as the result of disease or surgery. Such fusion is called arthrodesis. All permanent and some transient joints permit movement. Movement of the latter may be temporary, as with the roof bones of an infant's skull during birth, or long-term, as with the joints of the base of the skull during postnatal development.

There are two basic structural types of joint: diarthrosis, in which fluid is present, and synarthrosis, in which there is no fluid. All the diarthroses (commonly called synovial joints) are permanent. Some of the synarthroses are transient; others are permanent.

A . SYNARTHROSES

Synarthroses are divided into three classes: fibrous, symphysis, and cartilaginous.

Fibrous joints

In fibrous joints the articulating parts are separated by white connective tissue (collagen) fibres, which pass from one part to the other. There are two types of fibrous joints: suture and gomphosis.

A suture is formed by the fibrous covering, or periosteum, of two bones passing between them. In the adult, sutures are found only in the roof and sides of the braincase and in the upper part of the face. In the infant, however, the two halves of the frontal bone are separated by a suture (the metopic suture), as are the two halves of the mandible at the chin. Excepting those of the fetus and newborn infant, all sutures are narrow. In the late fetus and the newborn child, the sagittal suture, which separates the right and left halves of the roof of the skull, is quite wide and markedly so at its anterior and posterior ends. This enables one of the halves to glide over the other during the passage of the child through the mother's pelvis during birth, thus reducing the width of its skull, a process called molding. (The effects of molding usually disappear quickly.) After birth, all sutures become immobile joints. The expanded anterior and posterior ends of the sagittal suture are called fontanels; they lie immediately above a large blood channel (superior sagittal sinus).

Sutures are transient; they are unossified parts of the skeleton that become fused at various times from childhood to old age. The fusion is effected by direct conversion of the sutures into bone. Until maturity the sutures are active sites of growth of the bones they separate.

A gomphosis is a fibrous mobile peg-and-socket joint. The roots of the teeth (the pegs) fit into their sockets in the mandible and maxilla and are the only examples of this type of joint. Bundles of collagen fibres pass from the wall of the socket to the root; they are part of the circumdental, or periodontal, membrane. There is just enough space between the root and its socket to permit the root to be pressed a little farther into the socket during biting or chewing. Gomphoses are permanent joints in the sense that they last as long as do the roots of the teeth—unless, of course, they are damaged by disease.

The movement of the root within a gomphosis has a threefold effect. It lessens some of the impact between the upper and lower teeth in biting; it pumps blood and lymph from the periodontal membrane into the dental veins and lymph channels; and it stimulates sensory nerve terminals in the membrane to send signals to the brain centres that control the muscles of mastication.

Symphyses

A symphysis (fibrocartilaginous joint) is a joint in which the body (physis) of one bone meets the body of another. All but two of the symphyses lie in the vertebral (spinal) column, and all but one contain fibrocartilage as a constituent tissue. The short-lived suture between the two halves of the mandible is called the symphysis menti (from the Latin mentum, meaning "chin") and is the only symphysis devoid of fibrocartilage. All of the other symphyses are permanent.

The symphysis pubis joins the bodies of the two pubic bones of the pelvis. The adjacent sides of these bodies are covered by cartilage through which collagen fibres run from one pubis to the other. On their way they traverse a plate of cartilage, which in some instances (especially in the female) may contain a small cavity filled with fluid. Surrounding the joint and attached to the bones is a coat of fibrous tissue, particularly thick below (the subpubic ligament). The joint is flexible enough to act as a hinge that allows each of the two hip bones to swing a little upward and outward, as the ribs do during inspiration of air. This slight movement is increased in a woman during childbirth because of the infiltration of the joint and its fibrous coat by fluid toward the end of pregnancy; the fluid makes the joint even more flexible. In both sexes the joint acts as a buffer against shock transmitted to the pelvic bones from the legs in running and jumping.

The symphysis between the bodies of two adjacent vertebrae is called an intervertebral disk. It is composed of two parts: a soft centre (nucleus pulposus) and a tough flexible ring (anulus fibrosus) around it. The centre is a jellylike (mucoid) material containing a few cells derived from the precursor of the spine (notochord) of the embryo. The ring consists of collagen fibres arranged in concentric layers like those of an onion bulb. These fibres reach the adjacent parts of the vertebral bodies and are attached firmly to them.

There are 23 intervertebral disks, one between each pair of vertebrae below the first cervical vertebra, or atlas, and above the second sacral vertrebra (just above the tailbone). The lumbar (lower back) disks are thickest, the thoracic (chest or upper back) are thinnest, and the cervical are of intermediate size. These differences are associated with the function of the disks. In general, these disks have two functions: to allow movement between pairs of vertebrae and to act as buffers against shock caused by running, jumping, and other stresses applied to the spine.
If an intervertebral disk were the only joint between a pair of vertebrae, then one of these could move on the other in any direction; but each pair of vertebrae with an intervertebral disk also has a pair of synovial joints, one on each side of the vertebral (neural) arch. These joints limit the kinds of independent movement possible, so that the thoracic vertebrae move in only two directions and the lumbar in only three; only the cervical vertebrae below the atlas have full freedom of movement.

All intervertebral disks allow approximation and separation of their adjacent vertebrae. This is caused partly by movement brought about by muscle action and partly by the weight of the head and the trunk transmitted to the pelvis when a person is upright. The effect of weight is of special importance. The mucoid substance in the centre of the disk behaves like a fluid. It is acted upon by the person's weight and any other pressure forces transmitted along the spine. Therefore, the disk flattens from above downward and expands in all other directions. After arising in the morning and as the day progresses, a person decreases in height because of this compression of the disks. An average decrease of one millimetre in the height of each disk would mean an overall shortening of 2.3 centimetres, or about an inch. The spine lengthens again, of course, during sleep.

In the infant the greater part of the disk consists of the soft centre. Later the fibrous ring becomes relatively thicker in such a way that the soft part is nearer to the back of the disk. As middle age approaches, there is an increase in the fibrous element, the soft centre is reduced in size, and the amount of cartilage is increased. There is a tendency for the posterior part of the fibrous ring to degenerate in such a way that a sudden violent pressure may rupture the disk and allow the central part to protrude backward against the spinal cord; this condition is commonly referred to as slipped disk.

Cartilaginous joints

These joints, also called synchondroses, are the unossified masses between bones or parts of bones that pass through a cartilaginous stage before ossification. Examples are the synchondroses between the occipital and sphenoid bones and between the sphenoid and ethmoid bones of the floor of the skull. As already stated, these permit growth of the adjacent bones and act as virtual hinges at which the ethmoid and occipital bones swing upward upon the sphenoid; this allows backward growth of the nose and jaws during postnatal life. The juxta-epiphyseal plates separating the ossifying parts of a bone are also an example. Growth of the whole bone takes place at these plates when they appear, usually after birth. All synchondroses are transient, and all normally have vanished by the age of 25.

B. DIARTHROSES

Structure and elements of synovial joints

The synovial bursas are closed, thin-walled sacs, lined with synovial membrane. Bursas are found between structures that glide upon each other, and all motion at diarthroses entails some gliding, the amount varying from one joint to another. The bursal fluid, exuded by the synovial membrane, is called synovia, hence the common name for this class of joints. Two or more parts of the bursal wall become cartilage (chondrify) during prenatal life. These are the parts of the bursa that are attached to the articulating bones, and they constitute the articular cartilage of the bones.

A synovial joint consists of a wall enclosing a joint cavity that is wholly filled with synovial fluid. The wall consists of two layers: an outer complete fibrous layer and an inner incomplete synovial layer. Parts of the outer layer are either chondrified as articular cartilages or partly ossified as sesamoid bones (small, flat bones developed in tendons that move over bony surfaces). Parts of the synovial layer project into the cavity to form fatty pads. In a few diarthroses the fibrous layer also projects inward to become intra-articular disks, or menisci. These various structures will be discussed in connection with the layer to which they belong.

The fibrous layer: The fibrous layer is composed of collagen. The part that is visible in an unopened joint cavity is referred to as the investing ligament or joint capsule. At the point where it reaches the articulating bones, it attaches to the periosteum lining the outer surface of the cortex.

Articular cartilage: Articular cartilage (cartilage that covers the articulating part of a bone) is of the type called hyaline (glasslike) because thin sections of it are translucent, even transparent. Unlike bone, it is easily cut by a sharp knife. It is deformable but elastic, and it recovers its shape quickly when the deforming stress is removed. These properties are important for its function.
The surface of articular cartilage is smooth to the finger, like that of a billiard ball. Images obtained by a scanning electron microscope have shown, however, that the surface is actually irregular, more like that of a golf ball. The part of the cartilage nearest to the bone is impregnated with calcium salts. This calcified layer appears to be a barrier to the passage of oxygen and nutrients to the cartilage from the bone, such that the cartilage is largely dependent upon the synovial fluid for its nourishment.

Every articular cartilage has two parts: a central articulating part and a marginal nonarticulating part. The marginal part is much smaller than the central and is covered by a synovial membrane. It will be described later in connection with that membrane.

The central part is either single, if only two bones are included in the joint, or divided into clearly distinct portions by sharp ridges, if more than two bones are included. Thus, the upper articular surface of the arm bone (humerus) is single, for only this bone and the shoulder blade (scapula) are included in the shoulder joint. The lower articular surface of the humerus is subdivided into two parts, one for articulation with the radius and one for articulation with the ulna, both being included in the elbow joint. There is a functional reason for the subdivision, or partition, of articular cartilage when it does occur.

Within a diarthrosis joint, bones articulate in pairs, each pair being distinguished by its own pair of conarticular surfaces. Conarticular surfaces constitute "mating pairs." Each mating pair consists of a "male" surface and a "female" surface; the reasoning for these terms is explained below. As previously stated, there is only one such pair of bones within the shoulder joint; hence, there is only one pair of conarticular surfaces. There are two such pairs within the elbow joint—the humeroradial and humeroulnar. The radius moves on one of the two subdivisions of the lower humeral articular cartilage; the ulna moves on the other subdivision. There are then two pairs of conarticular surfaces within the elbow joint, even though there are only three bones in it.
Articular surfaces are divisible into two primary classes: ovoid and sellar. An ovoid surface is either convex in all directions or concave in all directions; in this respect it is like one or other of the two sides of a piece of eggshell, hence the name (ovum, egg). A sellar surface is convex in one direction and concave in the direction at right angles to the first; in this respect it is like the whole or part of a horse saddle (sella, saddle). There are no flat articular surfaces, although slightly curved ovoid or sellar surfaces may be classified as flat. Following an engineering convention, an ovoid surface is called "male" if it is convex, "female" if it is concave. In any diarthrosis having ovoid conarticular surfaces, the male surface is always of larger area than the female. For this reason the larger of two sellar conarticular surfaces is called male and the smaller female. The larger the difference in size between conarticular surfaces, the greater the possible amount of motion at the joint.

In all positions of a diarthrosis, except one, the conarticular surfaces fit imperfectly. This incongruence may not be large and may be lessened by mutual deformation of the opposed parts of the surfaces, a consequence of the deformability of articular cartilage. The exceptional position is called the close-packed position; in it the whole of the articulating portion of the female surface is in complete contact with the apposed part of the male surface, and the joint functionally is no longer a diarthrosis but is instead called a synchondrosis. Every joint has its close-packed position brought about by the action of the main ligaments of the joint. A good example is that of the wrist when the hand is fully bent backward (dorsiflexed) on the forearm. In closed-packed positions two bones in series are converted temporarily into a functionally single, but longer, unit that is more likely to be injured by sudden torsional stresses. Thus, a sprained or even fractured wrist usually occurs when that joint, when close packed, is suddenly and violently bent.

No articular surface is of uniform curvature; neither is it a "surface of revolution" such as a cylinder is. That part of a male conarticular surface that comes into contact with the female in close pack is both wider and of lesser curvature than is the remainder. Inspection of two articulating bones is enough to establish their position of close pack, flexion, extension, or whatever it may be.

Intra-articular fibrocartilages: Intra-articular fibrocartilages are complete or incomplete plates of fibrocartilage that are attached to the joint capsule (the investing ligament) and that stretch across the joint cavity between a pair of conarticular surfaces. When complete they are called disks; when incomplete they are called menisci. Disks are found in the temporomandibular joint of the lower jaw, the sternoclavicular (breastbone and collarbone) joint, and the ulnocarpal (inner forearm bone and wrist) joint. A pair of menisci is found in each knee joint, one between each femoral condyle and its female tibial counterpart. A small meniscus is found in the upper part of the acromioclavicular joint at the top of the shoulder. These fibrocartilages are really parts of the fibrous layer of the diarthrosis in which they occur, and they effect a complete or partial division of the articular bursa into two parts, depending upon whether they are disks or menisci, respectively. When the division is complete, there are really two synovial joints—e.g., the sternodiskal and the discoclavicular.

A disk or meniscus is mostly fibrocartilage, the chondrification being slight and the fibrous element predominating, especially in the part nearest to the investing ligament. Both animal experiments and surgical experience have shown that a meniscus of the knee can regrow if removed. The function of these intra-articular plates is to assist the gliding movements of the bones at the joints that contain them.

The synovial layer: The inner layer of the articular joint capsule is called the synovial layer (stratum synoviale) because it is in contact with the synovial fluid. Unlike the fibrous layer, it is incomplete and does not extend over the articulating parts of the articular cartilages and the central parts of articular disks and menisci.

The layer, commonly called the synovial membrane, is itself divisible into two strata, the intima and the subintima. The intima is smooth and moist on its free (synovial) surface. It could be described as an elastic plastic in which cells are embedded. Its elasticity allows it to stretch when one of the articulating bones either spins or swings to the opposite side and to return to its original size when the movement of the bone is reversed.

The cells of a synovial membrane can be divided into two classes: synovial lining cells and protective cells. The synovial lining cells are responsible for the generation and maintenance of the matrix. Their form depends upon their location. They are flattened and rounded at or near the internal surface of the membrane, more elongated and spindle-shaped elsewhere. They appear to be quite mobile and able to make their way to the free surface of the membrane. Excepting the regions in which the synovial membrane passes from the investing ligament (fibrous capsule) to the synovial periostea, these cells are scattered and do not form a continuous surface layer as do, for example, the cells lining the inner surface of the gut or of a blood vessel. In this respect they resemble the cells of other connective tissues, such as bone and cartilage. Apart from the generation and maintenance of the matrix of the membrane, they also can ingest foreign material and thus have a phagocytic function. They seem to be the only cells capable of secreting hyaluronic acid, the characteristic component of synovial fluid.

The protective cells are scattered through the depths of the membrane. They are of two kinds: mast cells and phagocytes. The mast cells secrete heparin and play the same part in synovial membrane as they do elsewhere—for example, in the skin and the gums. The phagocytes ingest unwanted particles, even such large ones as those of injected India ink; they are, in short, scavengers here as elsewhere.

The subintima is the connective tissue base on which the intima lies; it may be fibrous, fatty, or areolar (loose). In it are found the blood vessels and nerves that have penetrated the fibrous layer. Both the blood vessels and the nerves form plexuses, to be described later. The areolar subintima forms folds (synovial fringes) or minute fingerlike projections (villi) that project into the synovial fluid. The villi become more abundant in middle and old age. The fatty parts of the subintima may be quite thin, but in all joints there are places where they project into the bursal cavity as fatty pads (plicae adiposae); these are wedge-shaped in section, like a meniscus, with the base of the wedge against the fibrous capsule. The fatty pads are large in the elbow, knee, and ankle joints.

The function of fatty pads depends upon the fact that fat is liquid in a living body and that, therefore, a mass of fat cells is easily deformable. When a joint is moved, the synovial fluid is thrown into motion because it is adhesive to the articular cartilages, the motion of the fluid being in the direction of motion of the moving part. The fatty pads project into those parts of the synovial space in which there would be a likelihood of an eddying (vortical) motion of the fluid if those parts were filled with fluid. In short, the pads contribute to the "internal streamlining" of the joint cavity. Their deformability enables them to do this effectively. Of equal importance is the fact that the fatty pads by their very presence keep the synovial fluid between the immediately neighbouring parts of the male and female surfaces sufficiently thin, with proper elasticity as well as viscosity, to lubricate the joint.

Fatty pads are well provided with elastic fibres that bring about recovery from the deformation caused by pressure across a moving joint and that prevent the pads from being squeezed between two conarticular surfaces at rest. Such squeezing can happen, however, as the result of an accident and is very painful because of the large number of pain nerve fibres in these pads.
The synovial fluid: The main features of synovial fluid are: (1) Chemically, it is a dialyzate (a material subjected to dialysis) of blood plasma—that is, the portion of the plasma that has filtered through a membrane—but it contains a larger amount of hyaluronic acid than other plasma dialyzates. (2) Physically, it is a markedly thixotropic fluid—that is, one that is both viscous and elastic. Its viscosity decreases with an increase in the speed of the fluid when it is in motion. Its elasticity, on the other hand, increases with an increase in the speed of the fluid. Its thixotropy is due to the hyaluronic acid in it. (3) Functionally, it has two parts to play: nutrition and lubrication. It has been established that synovial fluid alone, by virtue of its being a blood-plasma dialyzate, can nourish the articulating parts of the articular cartilages. Its thixotropic properties make it suitable for forming what are called elastohydrodynamic lubricant films between the moving and the fixed conarticular surfaces of any mating pair. The motion of the synovial fluid, referred to earlier in connection with the fatty pads, assists its nutritional function by distributing it over the articular surfaces, from which it slowly passes into the interior of the cartilage. The source of the hyaluronic acid is the synovial lining cells.

Types of synovial joints

Recognition of the bursal nature of synovial joints makes it possible to describe them simply in terms of the bursal wall and to group together a number of types of structures. There are seven types of synovial joints: plane, hinge, pivot, sellar, ellipsoid, spheroidal (ball-and-socket), and bicondylar (two articulating surfaces). This classification is based on the anatomical form of the articular surfaces.

Plane joint: The plane, or arthrodial, joint has mating surfaces that are slightly curved and may be either ovoid or sellar. Only a small amount of gliding movement is found. Examples are the joints between the metacarpal bones of the hand and those between the cuneiform bones of the foot.

Hinge joint: The hinge, or ginglymus, joint is a modified sellar joint with each mating surface ovoid on its right and left sides. This modification reduces movement to a backward-forward swing like that allowed by the hinge of a box or a door. The swing of the joint, however, differs from that of a hinge in that it is accompanied by a slight spin (rotation) of the moving bone around its long axis. This brings the joint either into or out of its close-packed position, which is always that of extension. The joints between the bones of the fingers (phalanges) and that between the ulna (inner bone of the forearm) and the humerus at the elbow are classic examples.

Pivot joint: The pivot, or trochoid, joints are of two forms: in one a pivot rotates within a ring; in the other a ring moves around a pivot. In each case the ring is composed of fibrous tissue, part of which is converted into cartilage to form a female surface; the remainder may be ossified. Similarly, only part of the pivot is covered by a male articular cartilage. Pivot joints are always of the ovoid class; from a functional aspect, they are the ovoid counterparts of hinge joints. The joint between the atlas and the axis (first and second cervical vertebrae), directly under the skull, allows for turning of the head from side to side. Pivot joints also provide for the twisting movement of the bones of the forearm (radius and ulna) against the upper arm, a movement used, for instance, in unscrewing the lid of a jar.

Sellar joint: The sellar joint has already been described in the section Articular cartilage. It has two types of movement, both swings: flexion-extension and abduction-adduction. In addition to these it allows movements combining these two—that is, swings accompanied by rotation of the moving bone. An example of a sellar joint is the carpometacarpal joint of the thumb. The thumb can be swung from side to side or from behind forward, but the most frequent movement is that in which the thumb swings so that it comes "face to face" with one or another of the fingers, as in grasping a needle or a ball. This movement is called opposition (i.e., of thumb to fingers). During opposition the thumb is rotated around its long axis; it has been said that human civilization depends upon the opposition of the thumb.

Ellipsoid joint: The ellipsoid joint also has two types of movement but allows opposition movement only to a small degree. Its surfaces are ovoid and vary in both length and curvature as they are traced from front to back or from side to side, just as the diameter and curvature of an ellipse vary in directions at right angles to each other (hence the name). The joint between the second metacarpal and the first phalanx of the second finger is a good example. It allows the finger to flex and extend, to swing toward or away from its neighbouring finger, and to swing forward with a slight amount of rotation.

Ball and socket joint: The ball-and-socket joint, also known as a spheroidal joint, is the only one with three types of movement. It is an ovoid joint the male element of which could be described as a portion of a slightly deformed sphere. The rounded surface of the bone moves within a depression on another bone, thus allowing greater freedom of movement than any other kind of joint. It is most highly developed in the large hip and shoulder joints of mammals, including humans, in which it provides swing for the arms and legs in various directions and also spin of those limbs upon the more stationary bones.

Bicondylar joint: The condylar joint is better called bicondylar, for in it two distinct surfaces on one bone articulate with corresponding distinct surfaces on another bone. The two male surfaces are on one and the same bone and are of the same type (ovoid or sellar). These joints have two types of movement: one is always a swing, and the other is either another swing or a spin. Bicondylar joints are quite common. The largest is the tibiofemoral joint, in which both pairs of mating surfaces are within a single joint. At this joint, flexion and extension are the main movements; but active rotation of the leg on the femur is possible in most people when the leg and thigh are at right angles to each other. Every vertebra of the cervical, thoracic, and lumbar series is connected to (or separated from) the one below it by a pair of synovial joints as well as by an intervertebral disk. This pair of joints constitutes a bicondylar joint, the shape of whose articular surfaces determines the amount of movement permitted between the vertebra. The atlanto-occipital joint, between the skull and the vertebral column, is also a bicondylar joint. Finally, the right and left temporomandibular joints, between the lower jaw and the skull, are really two parts of a bicondylar joint, not only by definition—if the base of the skull is considered as a single bone—but also functionally, for one mandibular condyle cannot move without the other moving also.

JOINT LIGAMENTS

Any set of collagen fibres joining one bone of an articulating pair to the other is called a ligament. Thus, the articular bursal wall is a ligament, called either the fibrous capsule or the joint capsule.
There are two types of these sets: capsular and noncapsular. Capsular ligaments are simply thickenings of the fibrous capsule itself that take the form of either elongated bands or triangles, the fibres of which radiate from a small area of one articulating bone to a line upon its mating fellow. The iliofemoral ligament of the hip joint is an example of a triangular ligament. Capsular ligaments are found on the outer surface of the capsule. There is one exception to this rule: ligaments of the shoulder joint (glenohumeral ligaments) are found on the inner surface.

Noncapsular ligaments are free from the capsule and are of two kinds: internal and external. The internal type is found in the knee, wrist, and foot. In the knee there are two, both arising from the upper surface of the tibia; each passes to one of the two femoral condyles and lies within the joint cavity, surrounded by synovial membrane. They are called cruciate ligaments because they cross each other X-wise. At the wrist most of the articulations of the carpal bones share a common joint cavity, and neighbouring bones are connected sideways by short internal ligaments. The same is true of the tarsal bones that lie in front of the talus and the calcaneus.

The external noncapsular ligaments are of two kinds: proximate and remote. The proximate ligaments pass over at least two joints and are near the capsules of these joints. They are found only on the outer side of the lower limb. Examples are the outer (fibular) ligament of the knee, which passes from the femur to the upper part of the fibula over both the knee and tibiofibular joints, and the middle part of the outer ligament of the ankle joint, which passes from the lowest part of the fibula to the heel bone. These two ligaments, particularly that passing over the ankle, are especially liable to damage (sprain).

The remote ligaments are so called because they are far from, rather than close to, the joint capsule. A notable example is that of the ligaments that pass between the back parts (spines and laminae) of neighbouring vertebrae in the cervical, thoracic, and lumbar parts of the spinal column. These are the chief ligaments of the pairs of synovial joints between the vertebrae of these regions. Unlike most ligaments, they contain a high proportion of elastic fibres that assist the spinal column to return to its normal shape after it has been bent forward or sideways.

Contrary to the opinion of earlier anatomists, ligaments are not normally responsible for holding joint surfaces together. This is because a set of collagen fibres, like a string, can exert a reactive force only if stretched and tightened by some tensile stress. Normally, the bones at a joint are pressed together (when at rest) by the action of muscles or by gravity. An individual ligament can stop a movement that tightens it. Such a movement will loosen the ligaments that would be tightened by the opposite movement. The one exception to this case is the movement that brings a joint into the close-packed position. This movement is brought about by a combination of a swing with a spin of the moving bone. Experiments show that the combination of movement screws the articular surfaces firmly together so that they cannot be separated by traction and that the capsule and most of the ligaments are in simultaneous maximal tautness.

NERVE SUPPLY AND BLOOD SUPPLY OF JOINT

The nerve and blood supply of synovial joints follows the general rule for the body: "Ubi nervus ibi arteria" ("Where there is a nerve, there also is an artery").

Articular nerves

The sources of nerve fibres to a joint conform well to Hilton's law—the nerves to the muscles acting on a joint give branches to that joint as well as to the skin over the area of action of these muscles. Thus, the knee joint is supplied by branches from the femoral, sciatic, and obturator nerves, which among them supply the various muscles moving the joint. Some of these nerves go to the fibrous capsule and ligaments; others innervate this capsule and reach the synovial membrane. Some of these nerves are sensory; others give both motor and sensory fibres to the arteries that accompany them.

The sensory fibres to the fibrous capsule are of two kinds: (1) algesic, responsible for painful sensation, particularly when the capsule or other ligaments are overstretched or torn, and (2) proprioceptive, which terminate in various forms of specialized structures and convey information to all parts of the central nervous system, including the cerebellum and the cerebrum. It has been established that this information includes the posture of a resting joint and both the rate and extent of motion at a moving joint. The latter is supplemented by impulses conveyed by the nerves from the muscles acting and the skin affected by the movement.

The sensory fibres to the synovial membrane reach it by innervating the fibrous capsule at various points and form wide-meshed networks in the subsynovial layer. They are mainly algesic in function, and stimulation of them gives rise to diffused rather than localized pain (unlike the corresponding fibres to the fibrous capsule). They are found wherever the synovial membrane is, being especially abundant in the fatty pads, and are also present over the peripheral (nonarticulating) parts of the articular cartilage, disks, and menisci. This fact accounts for the excruciating pain that accompanies injury of these latter structures. The articulating part of the articular cartilage has no nerve supply.

Articular blood and lymph vessels

The joints are surrounded by a rich network of arteries and veins. The arteries in the vicinity of a synovial joint give off subdivisions that join (anastomose) freely on its outer surface. From the network of vessels so formed, branches lead to the fibrous capsule and ligaments and to the synovial membrane. Blood vessels to the synovial membrane are accompanied by nerves, and, when these vessels reach the subsynovial membrane, they proliferate to form another anastomotic network from which capillaries go to all parts of the membrane. These subsynovial arteries also ramify to the fatty pads and the nonarticulating parts of the articular cartilage, disks, and menisci. None, however, go to the articulating part of an articular cartilage, which therefore depends upon the synovial fluid for its nourishment.

Veins align with the arteries. In addition, a joint has a well-developed set of lymphatic vessels, the ultimate channels of which join those of the neighbouring parts of the limb or body wall.

NUTRITION AND METABOLISM OF JOINT

The metabolism and nutrition of the fibrous capsule and ligaments are similar to that of fibrous tissues elsewhere. Their blood supply is relatively small, indicating a low rate of metabolic changes. Unlike skin, for example, they heal slowly if injured.

The metabolism of articular cartilage is primarily dependent upon that of its cells (chondrocytes). Carbohydrate metabolism in these cells is similar to that of cells elsewhere and is unaffected by age. The oxygen consumption of the chondrocytes, on the other hand, decreases with age once the cells have matured. All the evidence suggests that the intracellular combustion is of glucose and protein, in that order of preference, rather than of fat. Sulfur passes from the blood to the synovial fluid and from there to the chondrocytes. From these it is transferred to the matrix to help to form chondroitin sulfate and keratosulfate molecules, the main constituents of the cartilaginous material. Chondroitin sulfate could be described as a sulfonated form of hyaluronic acid, the characteristic constituent of synovial fluid. Its presence in the matrix of the cartilage, but not in the synovial fluid, shows that the chondrocytes are necessary for its formation. After the second decade of life, the proportion of chondroitin sulfate falls and that of keratosulfate rises, as would be expected in view of the corresponding diminution of metabolic activity of the cells.

Excepting the articular cartilages, disks, and menisci, all other tissues of synovial joints are nourished directly by the blood vessels. The excepted parts are nourished indirectly by the synovial fluid. This is distributed over the surface of the articulating cartilage by the movements of the joint. The need for keeping joints healthy by frequent exercise of all of them is thus apparent.

SKELETAL SYSTEM

Posted by Prakash

Friday, October 2, 2009

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Skeletal system forms the internal skeleton that serves as a framework for the body. This framework consists of many individual bones and cartilages. There also are bands of fibrous connective tissue—the ligaments and the tendons—in intimate relationship with the parts of the skeleton. This article is concerned primarily with the gross structure and the function of the skeleton of the normal human adult.

The human skeleton, like that of other vertebrates, consists of two principal subdivisions, each with origins distinct from the others and each presenting certain individual features. These are (1) the axial, comprising the vertebral column—the spine—and much of the skull, and (2) the appendicular, to which the pelvic (hip) and pectoral (shoulder) girdles and the bones and cartilages of the limbs belong. Discussed in this article as part of the axial skeleton is a third subdivision, the visceral, comprising the lower jaw, some elements of the upper jaw, and the branchial arches, including the hyoid bone.

When one considers the relation of these subdivisions of the skeleton to the soft parts of the human body—such as the nervous system, the digestive system, the respiratory system, the cardiovascular system, and the voluntary muscles of the muscle system—it is clear that the functions of the skeleton are of three different types: support, protection, and motion. Of these functions, support is the most primitive and the oldest; likewise, the axial part of the skeleton was the first to evolve. The vertebral column, corresponding to the notochord in lower organisms, is the main support of the trunk.

The central nervous system lies largely within the axial skeleton, the brain being well protected by the cranium and the spinal cord by the vertebral column, by means of the bony neural arches (the arches of bone that encircle the spinal cord) and the intervening ligaments.
A distinctive characteristic of humans as compared with other mammals is erect posture. The human body is to some extent like a walking tower that moves on pillars, represented by the legs. Tremendous advantages have been gained from this erect posture, the chief among which has been the freeing of the arms for a great variety of uses. Nevertheless, erect posture has created a number of mechanical problems—in particular, weight bearing. These problems have had to be met by adaptations of the skeletal system.

Protection of the heart, lungs, and other organs and structures in the chest creates a problem somewhat different from that of the central nervous system. These organs, the function of which involves motion, expansion, and contraction, must have a flexible and elastic protective covering. Such a covering is provided by the bony thoracic basket, or rib cage, which forms the skeleton of the wall of the chest, or thorax. The connection of the ribs to the breastbone—the sternum—is in all cases a secondary one, brought about by the relatively pliable rib (costal) cartilages. The small joints between the ribs and the vertebrae permit a gliding motion of the ribs on the vertebrae during breathing and other activities. The motion is limited by the ligamentous attachments between ribs and vertebrae.

The third general function of the skeleton is that of motion. The great majority of the skeletal muscles are firmly anchored to the skeleton, usually to at least two bones and in some cases to many bones. Thus, the motions of the body and its parts, all the way from the lunge of the football player to the delicate manipulations of a handicraft artist or of the use of complicated instruments by a scientist, are made possible by separate and individual engineering arrangements between muscle and bone.
In this article the parts of the skeleton are described in terms of their sharing in these functions. The disorders and injuries that can affect the human skeleton are described in the article bone disease.

AXIAL AND VISCERAL SKELETON

THE CRANIUM
The cranium—the part of the skull that encloses the brain—is sometimes called the braincase, but its intimate relation to the sense organs for sight, sound, smell, and taste and to other structures makes such a designation somewhat misleading.

Development of cranial bones

The cranium is formed of bones of two different types of developmental origin—the cartilaginous, or substitution, bones, which replace cartilages preformed in the general shape of the bone; and membrane bones, which are laid down within layers of connective tissue. For the most part, the substitution bones form the floor of the cranium, while membrane bones form the sides and roof.
The range in the capacity of the cranial cavity is wide but is not directly proportional to the size of the skull, because there are variations also in the thickness of the bones and in the size of the air pockets, or sinuses. The cranial cavity has a rough, uneven floor, but its landmarks and details of structure generally are consistent from one skull to another.

The cranium forms all the upper portion of the skull, with the bones of the face situated beneath its forward part. It consists of a relatively few large bones, the frontal bone, the sphenoid bone, two temporal bones, two parietal bones, and the occipital bone. The frontal bone underlies the forehead region and extends back to the coronal suture, an arching line that separates the frontal bone from the two parietal bones, on the sides of the cranium. In front, the frontal bone forms a joint with the two small bones of the bridge of the nose and with the zygomatic bone (which forms part of the cheekbone; see below The facial bones and their complex functions), the sphenoid, and the maxillary bones. Between the nasal and zygomatic bones, the horizontal portion of the frontal bone extends back to form a part of the roof of the eye socket, or orbit; it thus serves an important protective function for the eye and its accessory structures.

Each parietal bone has a generally four-sided outline. Together they form a large portion of the side walls of the cranium. Each adjoins the frontal, the sphenoid, the temporal, and the occipital bones and its fellow of the opposite side. They are almost exclusively cranial bones, having less relation to other structures than the other bones that help to form the cranium.

Interior of the cranium

The interior of the cranium shows a multitude of details, reflecting the shapes of the softer structures that are in contact with the bones.

The internal surface of the vault is relatively uncomplicated. In the midline front to back, along the sagittal suture, the seam between the two parietal bones, is a shallow depression—the groove for the superior longitudinal venous sinus, a large channel for venous blood. A number of depressions on either side of it mark the sites of the pacchionian bodies, structures that permit the venous system to absorb cerebrospinal fluid. The large thin-walled venous sinuses all lie within the cranial cavity. While they are thus protected by the cranium, in many places they are so close beneath the bones that a fracture or a penetrating wound may tear the sinus wall and lead to bleeding. The blood frequently is trapped beneath the outermost and toughest brain covering, the dura mater, in a mass called a subdural hematoma.

Conspicuous markings on the internal surface of the projection of the sphenoid, called the greater wing, and on the internal surfaces of the parietal and temporal bones are formed by the middle meningeal artery and its branches, which supply blood to the brain coverings. Injury to these vessels may lead to extradural hematoma, a mass of blood between the dura mater and the bone.

In contrast to the vault and sides of the cranium, the base presents an extremely complicated aspect. It is divided into three major depressions, or fossae, in a descending stair-step arrangement from front to back. The fossae are divided strictly according to the borders of the bones of the cranium but are related to major portions of the brain. The anterior cranial fossa serves as the bed in which rest the frontal lobes of the cerebrum, the large forward part of the brain. The middle cranial fossa, sharply divided into two lateral halves by a central eminence of bone, contains the temporal lobes of the cerebrum. The posterior cranial fossa serves as a bed for the hemispheres of the cerebellum (a mass of brain tissue behind the brain stem and beneath the rear portion of the cerebrum) and for the front and middle portion of the brain stem. Major portions of the brain are thus partially enfolded by the bones of the cranial wall.

There are openings in the three fossae for the passage of nerves and blood vessels, and the markings on the internal surface of the bones are from the attachments of the brain coverings—the meninges—and venous sinuses and other blood vessels.

The anterior cranial fossa shows a crestlike projection in the midline, the crista galli ("crest of the cock"). This is a place of firm attachment for the falx cerebri, a subdivision of dura mater that separates the right and left cerebral hemispheres. On either side of the crest is the cribriform (pierced with small holes) plate of the ethmoid bone, a midline bone important as a part both of the cranium and of the nose. Through the perforations of the plate run many divisions of the olfactory, or first cranial, nerve, coming from the mucous membrane of the nose. At the sides of the plate are the orbital plates of the frontal bone, which form the roofs of the eye sockets. Their inner surfaces are relatively smooth but have a number of sharp irregularities more obvious to the touch than to the sight. These irregularities mark attachments of dura mater to bone.

The rear part of the anterior cranial fossa is formed by those portions of the sphenoid bone called its body and lesser wings. Projections from the lesser wings, the anterior clinoid (bedlike) processes, extend back to a point beside each optic foramen, an opening through which important optic nerves, or tracts, enter into the protection of the cranial cavity after a relatively short course within the eye socket.

The central eminence of the middle cranial fossa is specialized as a saddlelike seat for the pituitary gland. The posterior portion of this seat, or sella turcica ("Turk's saddle"), is actually wall-like and is called the dorsum sellae. The pituitary gland is thus situated in almost the centre of the cranial cavity. It is covered also by the brain coverings and has no connection with the exterior of the cranium except by blood vessels.

The deep lateral portions of the middle cranial fossa contain the temporal lobes of the cerebrum. In the forward part of the fossa are two openings: the superior orbital fissure, opening into the eye cavity; and the foramen rotundum, for the passage of the maxillary nerve, which serves the upper jaw and adjacent structures. Farther back are the conspicuous foramen ovale, an opening for the mandibular nerve to the lower jaw, and the foramen spinosum, for the middle meningeal artery, which brings blood to the dura mater.

Also in the middle fossa, near the apex of that part of the temporal bone called the petrous (stonelike) temporal bone, is the jagged opening called the foramen lacerum. The lower part of the foramen lacerum is blocked by fibrocartilage, but through its upper part passes the internal carotid artery, surrounded by a network of autonomic nerves, as it makes its way to the interior of the cranial cavity.

The delicate structures of the internal ear are not entrusted to the cranial cavity as such but lie within the petrous portion of the temporal bone in a bony labyrinth, into which the thin-walled membranous labyrinth, with its areas of sensory cells, is more or less accurately fitted but with an adequate space for protective fluid, the perilymph, between bone and membrane.
The posterior cranial fossa is above the vertebral column and the muscles of the back of the neck. The foramen magnum, the opening through which the brain and the spinal cord make connection, is in the lowest part of the fossa. Between its forward margin and the base of the dorsum sellae is a broad, smooth, bony surface called the clivus (Latin for "hill"). The bridgelike pons and the pyramid-like medulla oblongata of the brain stem lie upon the clivus and are separated from the bone only by their coverings. Near the foramen magnum are ridges for attachment of folds of the dura mater.

In the sides of the posterior cranial fossa are two transverse grooves, each of which, in part of its course, is separated by extremely thin bone from the mastoid air cells in back of the ear. Through other openings, the jugular foramina, pass the large blood channels called the sigmoid sinuses and also the 9th (glossopharyngeal), 10th (vagus), and 11th (spinal accessory) cranial nerves as they leave the cranial cavity.

The vessels, as well as the cranial nerves, are subject to injury at the openings into or from the cranial cavity and in special areas, such as close to the mastoid air cells. In the latter location, mastoiditis may lead to enough breakdown of bone to allow disease-bearing organisms to reach the other structures within the cranial cavity.

The hyoid: example of the anchoring function

The primary function of the hyoid bone is to serve as an anchoring structure for the tongue. The bone is situated at the root of the tongue in the front of the neck and between the lower jaw and the largest cartilage of the larynx, or voice box. It has no articulation with other bones and thus has a purely anchoring function.

The hyoid consists of a body, a pair of larger horns, called the greater cornua, and a pair of smaller horns, called the lesser cornua. The bone is more or less in the shape of a U, with the body forming the central part, or base, of the letter. In the act of swallowing, the hyoid bone, tongue, and larynx all move upward rapidly.

The greater cornua are the limbs of the U. Their outer ends generally are overlapped by the large sternocleidomastoid muscles. The lesser cornua are small projections from the places called, somewhat arbitrarily, the junctions of the body and the greater cornua. The hyoid bone has certain muscles of the tongue attached to it. The hyoglossus muscles originate on each side from the whole length of the greater cornua and also from the body of the hyoid. They are inserted into the posterior half or more of the sides of the tongue. The hyoid bone anchors them when they contract to depress the tongue and widen the oral cavity. The two geniohyoid muscles originate close to the point at which the two halves of the lower jaw meet; the fibres of the muscles extend downward and backward, close to the central line, to be inserted into the body of the hyoid bone. Contraction of the muscles pulls the hyoid bone upward and forward.
Inserting into the middle part of the lower border of the hyoid bone are the sternohyoids, long muscles arising from the breastbone and collarbone and running upward and toward each other in the neck.

Other muscles attached to the hyoid bone are the two mylohyoid muscles, which form a sort of diaphragm for the floor of the mouth; the thyrohyoid, arising from the thyroid cartilage, the largest cartilage of the larynx; and the omohyoid, which originates from the upper margin of the shoulder blade and from a ligament, the suprascapular ligament.

The position of the hyoid bone with relation to the muscles attached to it has been likened to that of a ship steadied as it rides when anchored "fore and aft." Through the muscle attachments, the hyoid plays an important role in mastication, in swallowing, and in voice production.
At the beginning of a swallowing motion, the geniohyoid and mylohyoid muscles elevate the bone and the floor of the mouth simultaneously. These muscles are assisted by the stylohyoid and digastric muscles. The tongue is pressed upward against the palate, and the food is forced backward.

THE FACIAL BONES AND THEIR COMPLEX FUNCTIONS

The upper jaws

The larger part of the skeleton of the face is formed by the maxillae. Though they are called the upper jaws, the extent and functions of the maxillae include much more than serving as complements to the lower jaw, or mandible. They form the middle and lower portion of the eye socket. They have the opening for the nose between them, beneath the lower borders of the small nasal bones. A sharp projection, the anterior nasal spine, is formed by them at the centre of the lower margin of the opening for the nose, the nasal aperture.

The infraorbital foramen, an opening into the floor of the eye socket, is the forward end of a canal through which passes the infraorbital branch of the maxillary nerve, the second division of the fifth cranial nerve. It lies slightly below the lower margin of the socket.

The alveolar margin, containing the alveoli, or sockets, in which all the upper teeth are set, forms the lower part of each maxilla, while a lateral projection from each forms the zygomatic process, forming a joint with the zygomatic, or malar, bone (cheekbone).

The lower jaws

The left and right halves of the lower jaw, or mandible, begin originally as two distinct bones, but in the second year of life the two bones fuse at the midline to form one. The horizontal central part on each side is the body of the mandible. The upper portion of the body is the alveolar margin, corresponding to the alveolar margins of the maxillae. The projecting chin, at the lower part of the body in the midline, is said to be a distinctive characteristic of the human skull. On either side of the chin is the mental foramen, an opening for the mental branch of the mandibular nerve, the third division of the fifth cranial nerve.

The ascending parts of the mandible at the side are called rami (branches). The joints by means of which the lower jaw is able to make all its varied movements are between a rounded knob, or condyle, at the upper back corner of each ramus and a depression, called a glenoid fossa, in each temporal bone. Another, rather sharp projection at the top of each ramus and in front, called a coronoid process, does not form part of a joint. Attached to it is the temporalis muscle, which serves with other muscles in shutting the jaws. On the inner side of the ramus of either side is a large, obliquely placed opening into a channel, the mandibular canal, for nerves, arteries, and veins.

The zygomatic arch, forming the cheekbone, consists of portions of three bones: the maxilla, in front; the zygomatic bone, centrally in the arch; and a projection from the temporal bone to form the rear part. The zygomatic arch actually serves as a firm bony origin for the powerful masseter muscle, which descends from it to insert on the outer side of the mandible. The masseter muscle shares with the temporalis muscle and lateral and medial pterygoid muscles the function of elevating the mandible in order to bring the lower against the upper teeth, thus achieving bite.

THE SPINE

The assumption of erect posture during the development of the human species has led to a need for adaptation and changes in the human skeletal system. The very form of the human vertebral column is due to such adaptations and changes.

The vertebral column

The vertebral column is not actually a column but rather a sort of spiral spring in the form of the letter S. The newborn child has a relatively straight backbone. The development of the curvatures occurs as the supporting functions of the vertebral column in humans—i.e., holding up the trunk, keeping the head erect, serving as an anchor for the extremities—are developed.
The S-curvature enables the vertebral column to absorb the shocks of walking on hard surfaces; a straight column would conduct the jarring shocks directly from the pelvic girdle to the head. The curvature meets the problem of the weight of the viscera. In an erect animal with a straight column, the column would be pulled forward by the viscera. Additional space for the viscera is provided by the concavities of the thoracic and pelvic regions.

Weight distribution of the entire body is also effected by the S-curvature. The upper sector to a large extent carries the head; the central sector carries the thoracic viscera, the organs and structures in the chest; and the lower sector carries the abdominal viscera. If the column were straight, the weight load would increase from the head downward and be relatively great at the base. Lastly, the S-curvature protects the vertebral column from breakage. The doubly bent spring arrangement is far less vulnerable to fracture than would be a straight column.

The protective function of the skeleton is perhaps most conspicuous in relation to the central nervous system, although it is equally important for the heart and lungs and some other organs. A high degree of protection for the nervous system is made possible by the relatively small amount of motion and expansion needed by the component parts of this system and by certain physiological adaptations relating to circulation, to the cerebrospinal fluid, and to the meninges, the coverings of the brain and spinal cord. The brain itself is snugly enclosed within the boxlike cranium. Sharing in the protection afforded by the cranium is the pituitary gland, or hypophysis.

The spinal cord

For the spinal cord, with its tracts of nerve fibres traveling to and from the brain, the placement in relation to the spinal column is somewhat like that of a candle in a lantern. Normally, there is considerable space between the nervous and the bony tissue, space occupied by the meninges, by the cerebrospinal fluid, and by a certain amount of fat and connective tissue. In front are the heavy centrums, or bodies, of the vertebrae and the intervertebral disks—the tough, resilient pads between the vertebral bodies—while in back and on the sides the cord is enclosed and protected by the portion of each vertebra called the neural arch. Between the neural arches are sheets of elastic connective tissue, the interlaminar ligaments, or ligamenta flava. Here some protective function has to be sacrificed for the sake of motion, because a forward bending of part of the column leads to separation between the laminae and between the spines of the neural arches of adjoining vertebrae. It is through the ligamenta flava of the lower lumbar region (the small of the back) that the needle enters the subarachnoid space in the procedure of lumbar puncture (spinal tap).

Besides its role in support and protection, the vertebral column is important in the anchoring of muscles. Many of the muscles attached to it are so arranged, in fact, as to move either the column itself or various segments of it. Some are relatively superficial, and others are deep-lying. The large and important erector spinae, as the name implies, holds the spine erect. It begins on the sacrum (the large triangular bone at the base of the spinal column) and passes upward, forming a mass of muscle on either side of the spines of the lumbar vertebrae. It then divides into three columns, ascending over the back of the chest. Although slips (narrow strips) of the muscle are inserted into the vertebrae and ribs, it does not terminate thus; fresh slips arise from these same bones and continue on up into the neck until one of the divisions, known as the longissimus capitis, finally reaches the skull.

Small muscles run between the transverse processes (projections from the sides of the neural rings) of adjacent vertebrae, between the vertebral spines (projections from the centres of the rings), and from transverse process to spine, giving great mobility to the segmented bony column.

The anchoring function of the spinal column is of great importance for the muscles that arise on the trunk, in whole or part from the column or from ligaments attached to it, and that are inserted on the bones of the arms and legs. Of these muscles, the most important for the arms are the latissimus dorsi (drawing the arm backward and downward and rotating it inward), the trapezius (rotating the shoulder blade), the rhomboideus, and the levator scapulae (raising and lowering the shoulder blade); for the legs, the psoas (loin) muscles.

The rib case

The rib cage, or thoracic basket, consists of the 12 thoracic (chest) vertebrae, the 24 ribs, and the breastbone, or sternum. The ribs are curved, compressed bars of bone, with each succeeding rib, from the first, or uppermost, becoming more open in curvature. The place of greatest change in curvature of a rib, called its angle, is found several inches from the head of the rib, the end that forms a joint with the vertebrae.

The first seven ribs are attached to the breastbone by cartilages called costal cartilages; these ribs are called true ribs. Of the remaining five ribs, which are called false, the first three have their costal cartilages connected to the cartilage above them. The last two, the floating ribs, have their cartilages ending in the muscle in the abdominal wall.

Through the action of a number of muscles, the rib cage, which is semirigid but expansile, increases its size. The pressure of the air in the lungs thus is reduced below that of the outside air, which moves into the lungs quickly to restore equilibrium. These events constitute inspiration (breathing in). Expiration (breathing out) is a result of relaxation of the respiratory muscles and of the elastic recoil of the lungs and of the fibrous ligaments and tendons attached to the skeleton of the thorax. A major respiratory muscle is the diaphragm, which separates the chest and abdomen and has an extensive origin from the rib cage and the vertebral column. The configuration of the lower five ribs gives freedom for the expansion of the lower part of the rib cage and for the movements of the diaphragm.

APPENDICULAR SKELETON

Pectoral girdle and pelvic girdle

The upper and lower extremities of humans offer many interesting points of comparison and of contrast. They and their individual components are homologous—i.e., of a common origin and patterned on the same basic plan. A long evolutionary history and profound changes in the function of these two pairs of extremities have led, however, to considerable differences between them.

The girdles are those portions of the extremities that are in closest relation to the axis of the body and that serve to connect the free extremity (the arm or the leg) with that axis, either directly, by way of the skeleton, or indirectly, by muscular attachments. The connection of the pelvic girdle to the body axis, or vertebral column, is by means of the sacroiliac joint. On the contiguous surfaces of the ilium (the rear and upper part of the hip bone) and of the sacrum (the part of the vertebral column directly connected with the hip bone) are thin plates of cartilage. The bones are closely fitted together in this way, and there are irregular masses of softer fibrocartilage in places joining the articular cartilages; at the upper and posterior parts of the joint there are fibrous attachments between the bones. In the joint cavity there is a small amount of synovial fluid. Strong ligaments, known as anterior and posterior sacroiliac and interosseous ligaments, bind the pelvic girdle to the vertebral column. These fibrous attachments are the chief factors limiting motion of the joint, but the condition, or tone, of the muscles in this region is important in preventing or correcting the sacroiliac problems that are of common occurrence.

The pelvic girdle consists originally of three bones, which become fused in early adulthood and each of which contributes a part of the acetabulum, the deep cavity into which the head of the thighbone, or femur, is fitted. The flaring upper part of the girdle is the ilium; the lower anterior part, meeting with its fellow at the midline, is the pubis; and the lower posterior part is the ischium. Each ischial bone has a prominence, or tuberosity, and it is upon these tuberosities that the body rests when seated.

The components of the girdle of the upper extremity, the pectoral girdle, are the shoulder blade, or scapula, and the collarbone, or clavicle. The head of the humerus, the long bone of the upper arm, fits into the glenoid cavity, a depression in the scapula. The pectoral girdle is not connected with the vertebral column by ligamentous attachments, nor is there any joint between it and any part of the axis of the body. The connection is by means of muscles only, including the trapezius, rhomboids, and levator scapulae, while the serratus anterior connects the scapula to the rib cage. The range of motion of the pectoral girdle and in particular of the scapula is enormously greater than that of the pelvic girdle.

Another contrast, in terms of function, is seen in the shallowness of the glenoid fossa, as contrasted with the depth of the acetabulum. It is true that the receptacle for the head of the humerus is deepened to some degree by a lip of fibrocartilage known as the glenoid labrum, which, like the corresponding structure for the acetabulum, aids in grasping the head of the long bone. The range of motion of the free upper extremity is, however, far greater than that of the lower extremity. With this greater facility of motion goes a greater risk of dislocation. For this reason, of all joints of the body, the shoulder is most often the site of dislocation.

Long bones of arms and legs

The humerus and the femur are corresponding bones of the arms and legs, respectively. While their parts are similar in general, their structure has been adapted to differing functions. The head of the humerus is almost hemispherical, while that of the femur forms about two-thirds of a sphere. There is a strong ligament passing from the head of the femur to further strengthen and ensure its position in the acetabulum.

The anatomical neck of the humerus is only a slight constriction, while the neck of the femur is a very distinct portion, running from the head to meet the shaft at an angle of about 125°. Actually, the femoral neck is developmentally and functionally a part of the shaft. The entire weight of the body is directed through the femoral heads along their necks and to the shaft. The structure of the bone within the head and neck and the upper part of the shaft of the femur would do credit to an engineer who had worked out the weight-bearing problems involved in the maintenance of upright posture.

The forearm and the lower leg have two long bones each. In the forearm are the radius—on the thumb side of the forearm—and the ulna; in the lower leg are the tibia (the shinbone) and the fibula. The radius corresponds to the tibia and the ulna to the fibula. The knee joint not only is the largest joint in the body but also is perhaps the most complicated one. The bones involved in it, however, are only the femur and the tibia, although the smaller bone of the leg, the fibula, is carried along in the movements of flexion, extension, and slight rotation that this joint permits. The very thin fibula is at one time in fetal development far thicker relative to the tibia than it is in the adult skeleton.

At the elbow, the ulna forms with the humerus a true hinge joint, in which the actions are flexion and extension. In this joint a large projection of the ulna, the olecranon, fits into the well-defined olecranon fossa, a depression of the humerus.

The radius is shorter than the ulna. Its most distinctive feature is the thick disk-shaped head, which has a smoothly concave superior surface to articulate with the head, or capitulum, of the humerus. The head of the radius is held against the notch in the side of the ulna by means of a strong annular, or ring-shaped, ligament. Although thus attached to the ulna, the head of the radius is free to rotate. As the head rotates, the shaft and outer end of the radius are swung in an arc. In the position of the arm called supination, the radius and ulna are parallel, the palm of the hand faces forward, and the thumb is away from the body. In the position called pronation, the radius and ulna are crossed, the palm faces to the rear, and the thumb is next to the body. There are no actions of the leg comparable to the supination and pronation of the arm.

Hands and feet

The skeleton of the wrist, or carpus, consists of eight small carpal bones, which are arranged in two rows of four each. The skeleton of the ankle, or tarsus, has seven bones, but, because of the angle of the foot to the leg and the weight-bearing function, they are arranged in a more complicated way. The bone of the heel, directed downward and backward, is the calcaneus, while the "keystone" of the tarsus is the talus, the superior surface of which articulates with the tibia.
In the skeleton of the arms and legs, the outer portion is specialized and consists of elongated portions made up of chains, or linear series, of small bones. In an evolutionary sense, these outer portions appear to have had a complex history and, within the human mammalian ancestry, to have passed first through a stage when all four would have been "feet," serving as the weight-bearing ends of extremities, as in quadrupeds in general. Second, all four appear to have become adapted for arboreal life, as in the lower primates, the "four-handed folk." Third, and finally, the assumption of an upright posture has brought the distal portions of the hind, now lower, extremities back into the role of feet, while those of the front, now upper, extremities have developed remarkable manipulative powers and are called hands. At what place in the primates a foot becomes a hand is difficult to say, and one might in fact be justified in speaking of hands in raccoons, squirrels, and some other nonprimates.

In humans the metatarsal bones, those of the foot proper, are larger than the corresponding bones of the hands, the metacarpal bones. The tarsals and metatarsals form the arches of the foot, which give it strength and enable it to act as a lever. The shape of each bone and its relations to its fellows are such as to adapt it for this function.

The phalanges—the toe bones—of the foot have bases relatively large compared with the corresponding bones in the hand, while the shafts are much thinner. The middle and outer phalanges in the foot are short in comparison with those of the fingers. The phalanges of the big toe have special features.

The hand is an instrument for fine and varied movements. In these, the thumb with its skeleton, the first metacarpal bone and the two phalanges, is extremely important. Its free movements include—besides flexion, extension, abduction (ability to draw away from the first finger), and adduction (ability to move forward of the fingers), which are exercised in varying degrees by the big toe also—a unique action, that of opposition, by which the thumb can be brought across, or opposed to, the palm and to the tips of the slightly flexed fingers. This motion forms the basis for the handling of tools, weapons, and instruments.

The Human Body

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