Joint Function
Restoration of joint function and stability is the other major purpose of the procedure. It is helpful to have a brief outline of the major structures in the joint in order to understand the types of disorders and injuries that can make joint replacement necessary as well as to understand the operation itself.
The knee is the largest joint in the human body, as well as one of the most vulnerable. Unlike the hip joint, which is partly protected by the bony structures of the pelvis, the knee joint is not shielded by any other parts of the skeleton. In addition, the joint must bear the weight of the upper body as well as the stresses and shocks carried upward through the feet when a person walks or runs. Moreover, the joint is essentially a hinge, designed to move primarily backwards and forwards; it is not a ball-and-socket joint like the hip, which can swivel and rotate in a variety of directions. Many injuries result from stresses caused by twisting or turning movements, particularly when the foot remains in one position while the upper body changes direction rapidly, as in basketball, tennis, or skiing.
The normal joint consists of a bone, the patella or kneecap, and a set of tendons, ligaments, and cartilage disks that connect the femur, or thighbone, to the lower leg. There are two bones in the lower leg, the tibia, which is sometimes called the shinbone; and the fibula, a smaller bone on the outside of the lower leg.
There are two collateral ligaments on the outside of the joint that connect the femur to the tibia and fibula respectively. These ligaments help to control the stresses of side-to-side movements on the knee. The patella—a triangular bone at the front of the knee—is attached by tendons to the quadriceps muscles of the thigh. This tendon allows a person to straighten the joint. Two additional tendons inside the knee stretch between the femur and the tibia to prevent the tibia from moving out of alignment with the femur. Cartilage, which is a whitish elastic tissue that allows bones to glide smoothly against each other, covers the ends of the femur, tibia, and fibula as well as the surfaces of the patella. In addition to the cartilage that covers the bones, the joint also contains two crescent-shaped disks of cartilage known as menisci (singular, meniscus), which lie between the lower end of the femur and the upper end of the tibia and act as shock absorbers or cushions. The entire joint is surrounded by a thick layer of protective tissue known as the joint capsule.
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Disorders and conditions that may lead to replacement surgery include:
- Osteoarthritis(OA). Osteoarthritis is a disorder in which the cartilage in the knee joint gradually breaks down, allowing the surfaces of the bones to rub directly against each other. The patient experiences swelling, pain, inflammation, and increasing loss of mobility. OA most often affects adults over age 45, and is thought to result from a combination of wear and tear on the joint,lifestyle, and genetic factors. As of 2003, OA is the most common cause of joint damage requiring replacement.
- Rheumatoid arthritis RA. Rheumatoid arthritis is a disease that begins earlier in life than OA and affects the whole body. Women are three times as likely as men to develop RA. Its symptoms are caused by the immune system's attacks on the body's own cells and tissues. Patients with RA often suffer intense pain even when they are not putting weight on the affected joints.
- Trauma. Damage to the joint from a fall, automobile accident, or workplace or athletic injury may trigger the process of cartilage breakdown inside the joint. Trauma is a common cause of damage to the joint. Some traumatic injuries are caused by repetitive motion or overuse of the joint; these types of injury include bursitis, or housemaid's knee's, and so-called runner's knee's. Other traumatic injuries are caused by sudden twisting of the joint, a direct blow to a bent joint, or being tackled from the side in football.
There are several factors that increase a person's risk of eventually requiring joint replacement surgery. While some of these factors cannot be avoided, others can be corrected through lifestyle changes:
- Genetic. Both OA and RA tend to run in families. One study done in France reported that the genetic factors affecting osteoarthritis in the joint can be traced back almost 8,000 years. Both OA and RA, however, are polygenic disorders, which means that more than one gene is involved in transmitting susceptibility to these forms of arthritis.
- Age. Joint cartilage becomes thinner and weaker with age, even in people who have no family history of arthritis.
- Sex. Women athletes have three times as many knee injuries as men. At present, orthopedic specialists are conducting studies to determine the cause(s) of this difference. Some doctors think it is related to the fact that most women have wider hips than most men, which results in a different pattern of stresses on the critical joint. Others think that the ligaments in women's joints tend to loosen more easily.
- Biomechanical. Biomechanics refers to the study of body structures in terms of the laws of mechanics, such as measuring the forces that affect the operation of a joint. Biomechanical studies have shown that people with certain types of leg or foot deformities, such as bowlegs or difference in leg length, are at increased risk of knee disorders because the stresses on the joint are not distributed normally.
- Gait-related factors. Gait refers to a person's pattern of motion when walking or running. Some people walk with their feet turned noticeably outward or inward; others tend to favor either the heel or the toe when they walk, which makes their gait irregular. Any of these factors can increase strain on the joint.
- Shoes. Poorly fitted or worn-out shoes contribute to joint strain by increasing the force transmitted upward to the joint when the foot strikes the sidewalk or other hard surface. They also introduce or increase irregularities in gait. Women's high-heeled shoes are particularly harmful to the joint because they do not cushion the foot; and they cause prolonged tightening and fatigue of the leg muscles.
- Work or other activities that involve jumping, jogging, or squatting. Jogging tends to loosen the ligaments that hold the parts of the joint in alignment, while jumping increases the shock on the joint and the risk of twisting or tearing the joint when the person lands. Squatting can increase the forces on the joint as much as eight times body weight.