The knee joint can be thought of as a hinge joint with the primary motion of straightening and bending. In reality, it is more complex than a simple hinge, as the surfaces actually glide and roll upon one another. It is composed of the end of the thigh bone (femur), the top of the leg bone (tibia), and the kneecap (patella). The ends of the bone are covered with a smooth, glistening layer called articular cartilage. The articular cartilage is what allows the bones to glide smoothly with less resistance than ice sliding on ice. The articular cartilage can be seen on x-ray as the space in between the bones. Knee replacement, or knee arthroplasty, is a surgical procedure to replace the weight-bearing surfaces of the knee joint to relieve pain and disability. Knee replacement surgery can be performed as a partial or a total knee replacement.[2] In general, the surgery consists of replacing the diseased or damaged joint surfaces of the knee with metal and plastic components shaped to allow continued motion of the knee.
Who should undergo knee replacement?
Today, a knee replacement surgery is considered a routine operation. Below are the three most common reasons for the procedure:

Osteoarthritis : This type of arthritis is age related – caused by the normal long wear and tear of the knee joint. The majority of patients are over 50; however, younger people may be affected.
Rheumatoid Arthritis : also called inflammatory arthritis, occurs when the membrane surrounding the knee joint is inflamated and thick. This inflammation becomes chronic and will damage the cartilage causing soreness and stiffness.
Post-traumatic Arthritis : This type of arthritis is due to a severe knee injury. When the bones around the knee break or the ligaments tear, this will affect the knee cartilage. Sometimes, surgery is the best option.

When is knee replacement surgery recommended?
When considering the option of knee replacement surgery, doctors take into account a number of symptoms:

Severe knee pain or stiffness : Do symptoms seriously undermine the patient’s ability to carry out everyday tasks and activities, such as walking, going upstairs, getting in and out of cars, getting up from a chair, etc?
Moderate but continuous knee pain : Is pain present while sleeping or resting?
Chronic knee inflammation and swelling : Does the swelling not improve after taking medications or resting? Do drugs cause unpleasant side effects?
Knee deformity : Is there is a noticeable arch in the inside or outside of the knee?
Nothing else worked : The doctor has prescribed medications and physical therapy without any substantial improvement.
Depression : Depression can be a serious, debilitating and devastating illness. Chronic pain and problems with mobility can eventually lead to depression, especially if the patient cannot do normal daily or social activities properly. Replacement knee surgery may prevent depression from occurring, or help get rid of it.