Causes
The biologic factors leading to the deterioration of cartilage in osteoarthritis are not entirely understood. Many experts believe that osteoarthritis results from a genetic susceptibility that causes some biologic response to injuries to the joint, which in turn leads to progressive deterioration of cartilage. In addition, the ability to make repairs becomes progressively limited as cartilage cells age.
Aging Cells
Although osteoarthritis generally accompanies aging, osteoarthritic cartilage is chemically different from normal aged cartilage. As chondrocytes (the cells that make up cartilage) age, they lose their ability to make repairs and produce more cartilage. This process may play an important role in the development and progression of osteoarthritis.
Genetic Factors
Researchers report a high correlation of osteoarthritis between parents and children or between siblings. Genetic factors are thought to be involved in about half of osteoarthritis cases in the hands and hips and a somewhat lower percentage of cases in the knee. A number of genes are under investigation that might contribute to an inherited risk.
For example, mutations in the ank gene may be important in some cases. The ank gene regulates pyrophosphate, a chemical that inhibits the formation of mineral deposits, and may protect the cartilage in joints. Mutations in the ank gene then may result in lower pyrophosphate levels in the joint, leading to accumulation of mineral deposits and arthritis. (About 60% of people with osteoarthritis have mineral deposits in their cartilage.)
Another gene, called the osteoprotegerin gene, is important in regulating bone and cartilage formation. Mutations in this gene may play a role in osteoarthritis.
Inflammatory Response and Matrix Metalloproteinases
The inflammatory response is an overreaction of the immune system to an injury or other assault in the body, such as an infection. This response causes specific immune factors, called cytokines, to gather in injured areas and cause inflammation and damage to body tissue and cells. The inflammatory response plays an important role in rheumatoid arthritis and other muscle and joint problems associated with autoimmune diseases. It has generally been believed that inflammation plays at most a minor role in osteoarthritis and is more likely to be a result -- not a cause -- of the disease.
However, recent studies suggest that inflammation may play an important role in the progression of osteoarthritis and its chronic nature. For example, a 2003 study found evidence of severe inflammation in the lining of the joints in 30% of patients with osteoarthritis. Still, the effects of the inflammatory response in osteoarthritis are likely to be different from those in rheumatoid arthritis and less severe.
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