Osteoarthritis (OA) is a degenerative joint disease characterized by the breakdown and eventual loss of joint cartilage. A normal knee glides smoothly because cartilage covers the ends of the bones that form the joints. With OA, the top layer of cartilage breaks down and wears away, allowing bones under the cartilage to rub together.
OA generally manifests itself in three progressive stages and most often begins in the inside, or medial compartment of the knee, but may begin in the outside, or lateral compartment (early-stage). Untreated, OA in one compartment may progress into a second compartment of the knee causing increased pain and reduced mobility (mid-stage). When the disease ultimately affects all three compartments of the knee (late-stage), pain is often severe and treatment is generally limited to total knee replacement, an invasive procedure which removes the natural knee joint and replaces it with an artificial joint.
Read more about treatment options for knee arthritis.