Osteoarthritis (OA) is the most common form of arthritis, affecting over 30 million people in the United States. Also called degenerative joint disease (DJD), OA is associated with joint injury and with the aging process ("wear and tear"). The joints most commonly affected by OA are those of the hips, knees, and spine (neck and lower back) as well as the fingers and thumbs.
OA is a chronic, progressive disease that causes deterioration of the joint cartilage and the formation of new bone (bone spurs) at the edges of the joints. Cartilage and synovial fluid are meant to provide a smooth, low-friction transition between the ends of bones. When cartilage loses its elasticity and wears down, joint movement becomes less smooth. Eventually, cartilage can completely erode and the opposing bone ends rub together. This leads to joint pain that may be intermittent or chronic, to stiffness in the morning and after rest, to small pieces of bone and fragments of cartilage in the remaining synovial fluid, and to a loss of coordination, posture, and mobility.
It is uncommon to see OA in people under 40 years of age. More men than women have OA before age 45 while it is more common in women after the age of 45. Overall, women, particularly elderly women, are more likely to develop OA than men. This is thought to be possibly related to menopausal hormone loss, though not fully understood why, or to long-term stress on the knees from women's broader hips.
Athletes who sustain multiple joint injuries over time also can be affected by OA. The primary cause is mechanical, such as joint damage caused by running or excess weight-bearing activities. For example, recreational runners may be prone to hip OA but less so to knee OA while professional runners (e.g., Olympic athletes) tend to be at greater risk of both hip and knee OA. However, any major injury to bone, joints, tendons, or ligaments (e.g., anterior cruciate ligament – ACL, meniscal tears) can increase the risk of OA. Repetitive motion or heavy weight-bearing activities as seen in certain occupations or with leisure activities can also contribute to developing OA.
More rarely, OA may be metabolic, genetic, or chemical in nature. In some studies, up to 50% of hand and hip cases were attributed to a positive family history of OA. Muscle weakness and anatomic misalignment (e.g., knock-kneed, bowlegged) can increase the risk of developing OA.
It is expected that the number of OA cases will almost double by the year 2020 due to increased longevity of the general population and the prevalence of obesity. Increased body weight can cause significant stress on the knees; three to six times a person's body weight is placed on knee joints during a normal, single stride. Excess body weight exacerbates this stress, increasing the risk of OA within the joint area.