Osteoarthritis affects approximately 12% of the population (approximately 21 million adults); it is the most common type of knee arthritis. It is a progressive musculoskeletal, degenerative disease that wears away the articular cartilage and underlying bone in your knee joint, triggering chemical reactions within the knee joint that cause destruction. It can be very painful, weakening and eventually deforming.
If you have osteoarthritis of the knee, the cartilage over your knee joint bones gets scraped and worn down, becoming very thin (sometimes resulting in small fragments that float into the joint cavity and cause more aggravation). This makes your knee feel very stiff. As a result, your joint cavity produces extra fluid in the joint which is comprised of destructive proteins and enzymes that cause the cartilage and bones to deteriorate further. This causes your knee to swell up (known as “water on the knee”). The surrounding knee bones react by growing thicker. The bone at the edge of your knee joint eventually grows outward and develops into bony spurs (osteophytes), which affects your femur, tibia and patella. Your knee joint tries to repair itself by creating these bony spurs however they generally just make your knee more painful and difficult to move; leaving thin parts on some areas of the knee and thick parts on other areas.
Your joint capsule and ligaments then slowly thicken and shrink, and your leg muscles weaken (atrophy), which in turn creates instability in your knee joint (it gives way when you walk or put weight on it). Osteoarthritis of the knee usually develops slowly over the years.
A study published by the Louisiana State University Health Sciences Center reviewed radiographic evidence and estimated that osteoarthritis of the knee affects 25% � 30% of people between 45 – 64 years of age, 60% of people older than 65 years of age, and 80% of people older than 75 years of age.