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The Merck Manual of Medical Information--Home Edition
Section 5. Bone, Joint, and Muscle Disorders
Chapter 46

Bones, Joints, and Muscles

click on thumbnail to see full-size imageBone is a constantly changing bodily tissue that has several functions. All the bones together make up the skeleton. The skeleton, muscles, tendons, ligaments, and other components of joints form the musculoskeletal system. The skeleton provides strength, stability, and a frame for muscles to work against in producing movement. Bones also serve as shields to protect delicate internal organs.

Bones have two main shapes: flat (such as the plates of the skull and the vertebrae) and long (such as the thighbones and arm bones). But their internal structure is essentially the same. The hard outer part consists largely of proteins, such as collagen, and a substance called hydroxyapatite. Composed mainly of calcium and other minerals, hydroxyapatite stores much of the body's calcium and is largely responsible for the strength of bones. The marrow in the center of each bone is softer and less dense than the rest of the bone and contains specialized cells that produce blood cells. Blood vessels run through a bone, and nerves surround it.

Bones come together to form joints. The configuration of a joint determines the degree and direction of possible motion. Some joints, such as those between the plates of the skull, called sutures, don't move in adults. Others allow a range of motion. For example, the shoulder joint, which has a ball-and-socket design, allows inward and outward rotation as well as forward, backward, and sideways motion of the arm. Hinge joints in the elbows, fingers, and toes allow only bending (flexion) and straightening (extension).

Other components of joints provide stability and reduce the risk of damage from constant use. In a joint, the ends of bones are covered with cartilage--a smooth, tough, protective tissue that acts as a shock absorber and reduces friction. Joints also have a lining (synovial tissue) that encloses them to form the joint capsule. Cells in the synovial tissue produce a clear fluid (synovial fluid) that fills the capsule, further reducing friction and facilitating movement.

Muscles are bundles of fibers that can contract. Skeletal muscles, which are responsible for posture and movement, are attached to bones and arranged in opposing groups around joints. For example, muscles that bend the elbow (biceps) are countered by muscles that straighten it (triceps).

Tendons, tough bands of connective tissue, attach each end of a muscle to a bone. Ligaments, which are similar tissues, surround joints and connect one bone to another. They help strengthen and stabilize joints, permitting movement only in certain directions. Bursas are fluid-filled sacs that provide extra cushioning, usually between adjacent structures that otherwise might rub against each other, causing wear and tear--for instance, between a bone and a ligament.

click on thumbnail to see full-size imageThe components of a joint work together to facilitate movement that is balanced and causes no damage. For example, when the knee is bent to take a step, the hamstring muscles on the back of the thigh contract and shorten, pulling the lower leg in and bending the knee. At the same time, the quadriceps muscles on the front of the thigh relax, allowing the knee to bend. Within the knee joint, the cartilage and synovial fluid minimize friction. Five ligaments around the joint help keep the bones properly aligned. Bursas provide cushioning between structures such as the shinbone (tibia) and the tendon attached to the kneecap (patellar tendon).

Musculoskeletal Disorders

Disorders of the musculoskeletal system are major causes of chronic pain and physical disability. Although the components of this system thrive on use, they can become worn, injured, or inflamed.

Injuries to bones, muscles, and joints are very common, ranging in severity from mild pulled muscles to strained ligaments, dislocated joints, and broken bones (fractures). Although these injuries generally are painful and may lead to long-term complications, most of them heal completely.

Inflammation is a natural response to tissue irritation or damage; it causes swelling, redness, heat, and loss of function. Inflammation of a joint is called arthritis; inflammation of a tendon, tendinitis. An inflammation may be confined to a small part of the body (localized), such as a single joint or an injured tendon, or it may be widespread, as occurs in certain inflammatory diseases such as rheumatoid arthritis. An inflammation can become chronic and persistent, sometimes because of continuous movement and mechanical stresses and sometimes because of immune reactions, infections, or deposits of abnormal materials.

Bone and joint infections can be crippling. Immediate treatment can prevent permanent joint damage. Benign tumors and cancers can originate in bone, and cancers can spread to bone from other locations in the body. Metabolic or hormonal imbalances can also affect bones and joints. An example is osteoporosis--a thinning of bone resulting from the excessive loss of minerals in bone. Another example is gout, in which crystals develop in the joints of susceptible people who have an abnormally high uric acid level in the blood.

Laboratory tests may provide helpful information regarding some musculoskeletal disorders, but that information usually isn't enough for a diagnosis. X-rays are taken to evaluate areas of bone pain because often they can detect fractures, tumors, injuries, infections, and deformities. Computed tomography (CT) scans and magnetic resonance imaging (MRI) may be performed to determine the extent and exact location of the damage. Magnetic resonance imaging is especially valuable for imaging tissues such as muscles, ligaments, and tendons. A sample of joint fluid may be examined to identify the bacteria causing an infection or to check for the crystals that confirm a diagnosis of gout or pseudogout. A doctor removes the fluid through a needle--generally a quick, easy, and almost painless office procedure.

Treatment depends on the type of musculoskeletal disorder. Injuries are often treated with rest, warm or cold compresses, perhaps analgesics, and immobilization with splints or bandages. Diseases affecting several joints simultaneously (see page 226 in Chapter 51, Disorders of Joints and Connective Tissue) are often treated with drugs to reduce the inflammation and suppress the body's immune response; however, most chronically damaged joints can't be healed with drugs. Some severely damaged joints can be replaced with artificial ones. Often, treatment requires the combined efforts of doctors and physical therapists, occupational therapists, and physiotherapists. (see box Fractures in this section)

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