One of the major features of osteoarthritis (OA) is the gradual deterioration of articular cartilage in the joints.
Articular cartilage covers the end of your bones where they come together to form joints. This tough but flexible tissue cushions the bones from weight-bearing stress, acting as the body’s shock absorber. Its slick surface allows the bones to glide smoothly over one another. Without these protective effects, the pressure and friction associated with joint movement would cause the bones to grind together and rapidly wear down.
How cartilage functions
Cartilage is composed of water, collagen and proteoglycans.
Collagen is a protein that forms strong, interweaving fibers that expand and contract as the body moves. Proteoglycans are large water-attracting molecules that help nourish the collagen fibers and hold them in place. As a joint is compressed, the proteoglycans expel water; when the pressure is released, they reabsorb water and nutrients.
Together, these components provide the joints with both strength and elasticity: The proteoglycans allow cartilage to change shape—and thus absorb some of the impact—as the body moves. The collagen imparts the tensile strength needed to maintain structural integrity.
Scattered sparsely throughout this matrix are chondrocytes. These tiny cells produce new collagen and proteoglycans. They also release enzymes that break down collagen and proteoglycans when they become old and weak.
What happens in OA
In OA, the cartilage gradually loses proteoglycans, and collagen fibers weaken and become more brittle.
In addition, the chondrocytes secrete even more cartilage-degrading enzymes. Initially, these changes cause only minor roughening of cartilage, accompanied by mild pain and stiffness. In some people, the disease gets no worse. In others it may progress, eventually causing cracked, ulcerated cartilage. Bones may develop outgrowths (osteophytes), and small pieces of bone or cartilage may break off, float in the joint space and irritate the synovial membrane (joint lining). Ultimately, cartilage may wear away completely and bones may rub against each other, causing severe pain and loss of joint function.