Arthritis means inflammation of a joint and therefore is a description of many different pathologies that can affect the knee. However the most common arthritis, and the one discussed below, is OSTEOARTHRITIS.
Osteoarthritis is the destruction of the smooth cartilage that overlies the ends of the bones and produces the near frictionless movement of the knee. Osteoarthritis can follow major trauma to the knee but more commonly is the result of "wear and tear" of the joint over many years, that is due to repetitive minor injuries to the knee. It can also be hereditary.
Osteoarthritis often starts in middle age and gets worse as the person gets older. The affected joint may develop a deformity due to there being more wear on one side of the joint than on the other. The most common deformity in the knee is that the person becomes bow-legged but one can also become knock kneed. Arthritic knees are painful, especially after activity and may swell and feel stiff towards the end of the day.
The symptoms of early arthritis can be alleviated by injections directly into the knee of a substance called hyaluronic acid. Some doctors use steroid injections that can relieve pain but may be counter-productive as the steroid softens the remaining cartilage further and may accelerate the rate of deterioration. Steroids are best reserved for patients with severe arthritis who are unfit or unwilling to have a joint replacement. Operations for arthritis include arthroscopic debridement ("keyhole clean-up"), osteotomies and joint replacements both partial and total.