Medial compartment osteoarthritis involves the knee joint, whereby the cushioning layer between your knee bones deteriorates over time. Deterioration of this cushion layer, also known as cartilage, results in your knee bones rubbing together. Over time, this leads to knee pain, knee joint stiffness and swelling [source: FamilyDoctor.org].

Severe OA involves extensive damage in the joint, with most of the cartilage worn away. This increases the severity of symptoms, and people may experience more constant pain, limited range of motion, and decreased function. Severe OA may affect everyday activities, sleep, and quality of life.