Coordinating Institutions

KNEE ENDOPROSTHESIS DESCRIPTION


The healthy knee provides optimum mobility combined with optimum stability

With its ligaments and muscles, the knee is the largest joint in the human body. It enables us to stand, to get up, and above all to walk.

Our knee combines the greatest possible mobility with the best possible stability. It forms the connection between the longest bones in the body, the thigh bone and the lower leg bones (shin and calf bone).

Between the bones of the upper and lower leg lies the meniscus. The ligament apparatus consists of cruciate and collateral ligaments, which provide the knee with stability. The kneecap, or patella, with its ligament apparatus, is essential for transferring the strength of the thigh muscle to the muscles of the lower leg.

How does the human knee joint work?

The entire knee is surrounded by a joint capsule, in which all the various components of the joint must work together perfectly. If one component in the knee joint is diseased, the entire functioning unit is destroyed. This makes itself apparent in a very wide range of symptoms.

The movement mechanism of the healthy knee is more complicated than that of all the other joints in the human body. The knee has a sliding joint axis which can be moved in five different directions. It essentially consists of rolling and sliding movements of the thigh bone on the shin bone, but slight external or internal rotational movements are also possible as the knee is bent. When the leg is fully extended, this mobility is held in check by the stabilising function of the ligaments.

Diseases of the knee joint leading to knee replacement

Replacement with an artificial knee is essential when the damage to the knee is so extensive that surgery to preserve the natural joint is no longer a sensible option.

Deterioration of the knee joint, or arthrosis as it is called, can be the result of age-related wear and tear to the joint cartilage. If this wear and tear exceeds that normally caused by the ageing process, pain and restricted mobility are the result. Causes of this abnormal deterioration can be incorrect loading on the knee joint caused by deformities of the leg (bow legs or knock knees) or old injuries or inflammations in the knee. The layers of cartilage become thinner and rub against each other. When the cartilage has been worn away to such an extent that the bone is exposed, every movement causes pain. The inner skin of the joint then produces a large quantity of viscous joint fluid and an effusion forms in the joint, causing the patient severe pain.

When the joint cartilage is so badly damaged through arthrosis, only an artificial knee replacement can bring relief from pain and more mobility.

Knee joint prostheses

The artificial knee joint replaces the damaged parts of the knee. It is designed to be similar in shape to the human knee, and the size also matches the patient's damaged knee. An x-ray taken before the operation and trial prostheses placed on the bones during surgery give the surgeon information about the correct size for the artificial knee. The knee prosthesis is normally fixed with bone cement, which binds the prosthesis firmly onto the bone. It is also possible to fix the knee prosthesis without cement, although this method is normally used on younger patients. Your surgeon will decide which is the right method for fixing the prosthesis in your particular case.

According to the extent of arthrotic damage to the natural knee joint, one of three different types of knee prosthesis can be chosen for:

Partial surface replacement

In this case, one side of the knee joint is damaged (frequently the inner side), but the ligaments, the opposite side of the joint and the patella are still functioning properly. A surface prosthesis is fixed onto one of the condyles of the thigh bone. When the knee is moved, this prosthesis slides on the synthetic disc which, together with a metal base plate, is fixed onto the top surface of the shin bone on the corresponding side. Stability in the knee is ensured by the cruciate and collateral ligaments which are preserved.

Total surface replacement

In this case, several elements of the knee joint (cartilage and possibly also cruciate ligaments) are damaged by arthrosis, but the collateral ligaments must still be intact.

A surface prosthesis is fixed onto the end of the thigh bone. Because the outer condyle on the thigh bone is bigger than the inner condyle and different in shape, there is a right and left leg version of the total surface prosthesis for the thigh bone. The shin bone is once again fitted with a metal base plate and a synthetic disc which acts as a sliding surface. Because the human thigh bone comes in such a variety of shapes and sizes, it is possible to choose a combination of implant sizes to achieve the best fit in terms of both length and width. How the metal base plate is fixed into position depends on the condition of the shin bone.

If there is additional damage to the cartilage layer on the patella, which slides up and down in front of the thigh bone, this damaged face is also replaced by a synthetic plate.

Complete axial joint replacement

In this case, the entire knee joint, cartilage and ligaments are damaged, or the upper and lower leg bones deviate from their correct axis by more than 30 degree. Stability is no longer guaranteed, and the prosthesis must now also take on the function originally performed by the ligaments. A hinged prosthesis is implanted, linking the thigh and shin bones together via a movable axis. Larger areas of bone are removed and a stem prosthesis is fixed into position in both thigh and shin bones.

What happens during the operation?

The deciding factor determining which type of replacement is necessary is the extent of damage to the various components of the knee joint.

Remaining cartilage, misshapen bone and the meniscus are removed during the operation. Using suitable stencils, the bone is prepared to receive the type of prosthesis chosen, and the fixing points for the prostheses are drilled or cut into the bone at the same time. A trial prosthesis is used to check the correct position and mobility of the knee joint, and then the permanent prosthesis is inserted and fixed into place. The blood supply to the knee is stopped while the operation is being performed. When the operation is complete, tubes are placed in the joint to allow any build-up of blood to drain away. The various layers of the incision on the knee are then stitched back together.