Osteoarthritis is the most common form of arthritis affecting almost 30 million Americans. It is a disease that affects articular cartilage, the tough cushiony material that caps the ends of long bones within a joint.
In addition to gradual loss of articular cartilage, there is thickening of the bone underlying the cartilage and growth of spurs, called osteophytes. Also, chronic inflammation of the synovium the lining of the joint also occurs.
Cartilage consists of a matrix that is made up of proteoglycans and collagen. Within this matrix is a second component, chondrocytes, which are cartilage cells.
While osteoarthritis is not absolutely due to aging, it is definitely associated with it. Over time, cartilage begins to develop small cracks, called "fissure." These are basically stress fractures of the cartilage. The development of fissures is accompanied by an alteration in matrix chemistry so that the strength and load bearing properties of cartilage start to wane. Essentially, the cartilage loses its water content, the physical properties of cartilage change, and fewer chondrocytes are present, and therefore less matrix is created.
The development of osteoarthritis is frequently broken up into three major phases. The first phase consists of edema or swelling of the matrix. Small cracks appear, and there is disorganization of the chondrocyte population. This happens because the chondrocytes no longer manufacture good quality matrix. The strength and elasticity of cartilage begins to fail. There is a pronounced imbalance between cartilage synthesis and cartilage degradation. This is caused by the chondrocyte behaving abnormally and manufacturing destructive enzymes called proteinases. These proteinases degrade matrix.
The second phase is characterized by extension of these fissures. They become deep cracks and the underlying bone also starts to crack.
The third phase is when cartilage pieces begin to flake and break off. Underlying bone is exposed and the bone develops cysts, and becomes harder- the term used is "sclerosis." Osteophytes- bony spurs- form.
In addition, the lining of the joint capsule, the synovium, becomes inflamed. The inflammation of the lining is due to small pieces of cartilage being gobbled up and recruiting inflammatory cells to help break down these pieces. The inflammatory cells cause the production of cytokines. These are protein messengers that promote inflammation.
Another feature that complicates the picture more is that with cartilage loss, there are biomechanical factors involved. Forces involved in cartilage compression, shearing, and other stresses cause further cartilage abnormalities.
So... there is a complex interaction involving multiple "players" that causes osteoarthritis.
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