The two main forms of arthritis affecting the knee are osteoarthritis (OA) and rheumatoid arthritis (RA).

Rheumatoid arthritis is a condition of unknown cause where the lining membrane (the synovium) of joints becomes inflamed. Damage to the joint surfaces follows, resulting in the destruction of the lining cartilage; the joint becomes painful and arthritic. Many joints can be involved, especially those in the hands and feet, but the larger joints such as the hip and knee are also commonly affected.

Osteoarthritis is by far the most common form of arthritis. It is estimated that 25% of females and 16% of males over the age of 60 are symptomatic from osteoarthritis. It accounts for more than 95% of knee replacements undertaken in the UK each year (over 40,000). This page concentrates on osteoarthritis.


OA is a degenerative disorder in which there is progressive loss of articular (surface) cartilage accompanied by new bone formation and capsular fibrosis (stiffening). In effect, this is ‘wear and tear’ arthritis. Many joints can be affected or just one.

There are 2 basic types of osteoarthritis; primary and secondary:

No obvious cause
Many joints involved including fingers, big toe, knees, spinal facet joints
Usually starts in the hands
Mainly postmenopausal women
Familial; i.e. can be inherited
Same pathology as single joint osteoarthritis
Estimated 80% of all OA
Normal cartilage having to cope with an abnormal load
Abnormal cartilage having to cope with a normal load
Cartilage break-up occurs due to defective subchondral bone (bone beneath the articular cartilage)


There is no obvious cause of primary osteoarthritis; causes of secondary OA include:

previous trauma (fracture, ligament rupture and cartilage injuries)
developmental disorders causing abnormal anatomy (e.g. patello-femoral dysplasia and maltracking)
childhood knee conditions causing malalignment of the knee
miscellaneous conditions such as avascular necrosis


Knee osteoarthritis usually starts in one of the knee compartments (medial, lateral or patellofemoral) but with time and disease progression it usually spreads to involve most, if not all of the knee.

The main symptom of knee arthritis is pain. Pain is usually felt at the level of the joint line between the bottom end of the femur and the top of the tibia but can also be a more general type of pain and can even be felt purely at the front of the knee over the patella (kneecap).

The pain from knee OA is usually worse with any sustained activity (e.g. walking for any distance, golf) and also towards the end of the day. The knee may swell as a result of fluid accumulation within the knee joint; this is called an effusion.

Occasionally patients may complain that their knee feels as if it is about to give way. This is known as instability and can be as a result of pain or ligament damage secondary to OA. Frequently the knee becomes stiff, with a reduction of movement, and may on occasion be impossible to straighten. This is called a fixed flexion deformity and makes walking or standing for any period of time very difficult.


Diagnosis of knee OA is made by x-rays. At least 2 and sometimes 3 views are needed to confirm the diagnosis. There are varying degrees of wear and tear visible on x-ray depending on the stage of the condition and the length of time of symptoms.


Many patients with arthritic knees do not need a knee replacement. There are many ways of coping with the pain from knee arthritis; they include:

Simple painkillers
Anti-inflammatory medication
Weight reduction
Activity modification
Using a walking stick (using the stick on the SAME side)
Physiotherapy to strengthen the muscles and maintain knee movement
Injection into the joint (usually local anaesthetic and steroid)
Arthroscopy to ‘clean’ the knee out, resect any meniscal tears and stabilise areas of cartilage damage

However, in the majority of cases there comes a point when these are insufficient and the amount of pain and its impact on lifestyle become intolerable; knee replacement becomes a sensible treatment option.

Am I to young to have a knee replacement?

Traditional knee replacements consist of metal components that are fixed to the bottom end of the femur (femoral component) and top end of the tibia (tibial component); a plastic liner is inserted between the two. This plastic bearing surface wears out with time, probably related to activity levels.

As a consequence, younger patients who are more active and likely to live longer are at greater risk of needing a second, or revision knee replacement. Revision surgery is far more complex and subject to a higher risk of complications. Historically, therefore, knee replacements were rarely performed on patients under the age of 60.

Technological advances have produced bearing surfaces which can withstand higher activity levels and will probably last much longer. For example the Genesis IITM knee replacement made by Smith & Nephew has a femoral component made of OxiniumTM which has been shown in laboratory studies to reduce the wear on the plastic liner by 90%.

It is now far more common for younger patients to be operated on since most patients with significant pain from an arthritic knee prefer to have quality of life rather than continue in pain.

When should you have a knee replacement?

The decision to undergo knee replacement is a lifestyle choice. The criteria for knee replacement are an arthritic knee, as proven by x-ray, which interferes significantly with lifestyle and causes intolerable pain. The level of pain differs from patient to patient. Common complaints are:

Reduction in walking distance
Inability to perform work activities, including housework
Interference with leisure activities
Interference with sleep
Difficulty rising from sitting

Knee replacement is an elective (i.e. planned) procedure and should only be performed when patients are no longer prepared to put up with their pain and disability, and when the benefits and risks have been fully explained by the surgeon and understood by the patient.