Components Of The Artificial Knee Joint

The complexity of the knee joint has presented special challenges in the design and insertion of an artificial joint so that the development of the artificial knee has lagged behind that of the hip.

The knee needs to allow movement from zero degrees (straight) to 90 degrees at least, with complete stability in all other directions, and this is difficult to achieve. The artificial knee joint closely follows the contours of the real joint and has been computer-designed to imitate the natural biomechanics of the normal knee.

There are many designs of total knee replacement, sometimes named after the type of joint or the surgeons who developed them. Examples are Scorpio, AGC, PCA, Duracon, Insall-Burstein and Miller-Galante.

This video shows the components of a Scorpio knee replacement:

There may be four components of a total knee replacement:

• The femoral component

  which fits on the lower end of the thigh bone and looks like a large rounded knuckle.

• The tibial component

  a flat plate-like piece, which fits on the top of the shin bone.

• The plastic insert

  which fits between the femoral and tibial components rather like a thick piece of cheese in a sandwich.

• A plastic button

  which may be fixed to the back of the patella (kneecap).

The metal components (often the femoral and sometimes the tibial) are made of a steel alloy which has cobalt, chromium and molybdenum in it. This material is inert in the body - it does not react with the tissue and fluids in the body to cause problems, and is mechanically very strong. Tibial components can be made of titanium instead which has great strength and is lighter than steel.

The plastic components are made of ultra high molecular weight polyethylene (UHMWP) which is very dense, hard and smooth. The combination of metal and plastic gives the joint very low friction so it moves easily with your weight on it and wears very slowly.

The surgeon decides which type of prosthesis to insert from the variety available. Factors affecting this decision are the his or her preferences and the person’s age, weight, degree of activity, bone quality and specific knee problems.

It is useful to know the success rates of the type of joint prosthesis (artificial joint) used as well as the surgeon’s own success rate. Some type of joint replacement have performed much better than others over ten or fifteen years. All reputable joint replacements will have published results in orthopaedic journals.