OA affects about eight million people in the United Kingdom, and about 27 million people in the United States, where it accounts for 25% of visits to primary care physicians and half of all NSAID (Non-Steroidal Anti-Inflammatory Drugs) prescriptions. It is estimated that 80% of the US population will have radiographic evidence of OA by age 65, although only 60% of those will be symptomatic.
Primary
This type of OA is a chronic degenerative disorder related to but not caused by aging, as there are people well into their nineties who have no clinical or functional signs of the disease. As a person ages, the water content of the cartilage decreases as a result of a reduced proteoglycan content, thus causing the cartilage to be less resilient. Without the protective effects of the proteoglycans, the collagen fibers of the cartilage can become susceptible to degradation and thus exacerbate the degeneration. Inflammation of the surrounding joint capsule can also occur, though often mild (compared to that which occurs in rheumatoid arthritis). This can happen as breakdown products from the cartilage are released into the synovial space, and the cells lining the joint attempt to remove them. New bone outgrowths, called "spurs" or osteophytes, can form on the margins of the joints, possibly in an attempt to improve the congruence of the articular cartilage surfaces. These bone changes, together with the inflammation, can be both painful and debilitating.
Secondary
This type of OA is caused by other factors but the resulting pathology is the same as for primary OA:
- Congenital disorders, such as:
- Congenital hip luxation
- People with abnormally-formed joints (e.g. hip dysplasia) are more vulnerable to OA, as added stress is specifically placed on the joints whenever they move. However, recent studies have shown that double-jointedness may actually protect the fingers and hand from osteoarthritis.
- Diabetes.
- Inflammatory diseases (such as Perthes' disease), (Lyme disease), and all chronic forms of arthritis (e.g. costochondritis, gout, and rheumatoid arthritis). In gout, uric acid crystals cause the cartilage to degenerate at a faster pace.
- Injury to joints, as a result of an accident.
- A joint infection, e.g. from an injury.
- Hormonal disorders.
- Ligamentous deterioration or instability may be a factor.
- Marfan syndrome
- Obesity. Obesity puts added weight on the joints, especially the knees.
- Sports injuries, or similar injuries from exercise or work. Certain sports, such as running or football, put undue pressure on the knee joints. Injuries resulting in broken ligaments can lead to instability of the joint and over time to wear on the cartilage and eventually osteoarthritis.
- Pregnancy
- Alkaptonuria
- Hemochromatosis and Wilson's disease
Further Reading
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"Osteoarthritis"
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