The artificial
knee joint

The structure of the knee joint

The knee joint connects the bottom of the thighbone (or femur) with the top of the shinbone (or tibia) and enables flexion and extension of the leg. In a healthy knee, the surfaces of these two bones (i.e. where they meet and actually form the joint) are covered in a layer of cartilage which allows for smooth, pain-free sliding. The menisci are crescent-shaped pieces of cartilage that are firmly attached to the top of the tibia but which can move around a little allowing for a small amount of rotation and toggling at the knee. They also absorb both progressive and sudden weight loads. The back of the knee cap (or patella) is also covered with cartilage and slides in a groove on the front of the femur. The joint is surrounded by a soft tissue envelope called the capsule which seals the joint and produces a fluid which nourishes, lubricates as well as enables smooth movement. The knee joint is stabilized by various ligaments and tendons (across the joint and alongside it). The knee cap also provides further stabilizing protection.

Arthritis or arthrosis of the knee

While wear and tear of the cartilage is by far the most common reason for knee replacement surgery, rheumatic conditions (rheumatoid arthritis), metabolic diseases, congenital or acquired malformations as well as injuries are also quite common. The wear mentioned earlier is progressive and as such the joint becomes increasingly painful. Putting weight on it hurts more and walking distance reduces over time. Eventually even resting provides little relief from the pain.

The replacement of the knee joint

Since the worn or damaged cartilage cannot be restored an arthritic knee is often replaced by artificial components. During the surgery the surfaces that should be covered with cartilage are completely replaced with a femoral component on the thighbone and a tibial component on the shinbone. Depending on bone quality, body weight and physical activity of the patient the components are either fixed with or without cement to the bone. A sliding surface composed of polyethylene is inserted between the tibial and femoral components. In order to increase the durability of the components, special metal alloys are used, which are provided with a ceramic coating in some systems. This ceramic coating also allows for treatment of patients suffering from certain metal sensitivities.

The operation

The extent of the arthritis and suitability for replacement is determined via radiographs (or x-rays) beforehand. These images are also used for planning the surgery and any special equipment that may be needed. The procedure itself is totally pain-free whether it is done under a general anaesthetic (where the patient is unconscious) or under a spinal anaesthesia (where the patient is sedated but awake). Surgery times vary but typically range from 60 minutes to 140 minutes. Once in the operating room, the joint is opened (or exposed) and the cartilage is completely removed from the top of the tibia and the bottom of the femur. Small bony deformities can also be removed at this stage. The bones are prepared very precisely using special instruments and templates. Subsequently, the metal femoral and tibial components are accurately implanted as well as the polyethylene inlay which separates them and tensions the joint appropriately. The components are usually fixed with bone cement but sometimes cementless devices (with special coatings) are used. If necessary the back of the kneecap (patella) is also replaced with a polyethylene component. The mobility of the new joint is tested and examined, before the tissue is closed and a drain (which is removed 2-4 days later) for the wound secretions may also be inserted. The skin stitches or staples are removed after 12 to 14 days.

Possible complications

Apart from the general operation risks such as bleeding, postoperative haemorraging and infection, there are also special complications that can occur during and after the operation. Even with most meticulous surgical techniques the risk of damage to large blood vessels and nerves must not be ignored. The endoprostheses can loosen, the leg length can differ, and in very rare cases there is also the possibilty of dislocation. Sometimes blood transfusions become necessary. However, usually blood saving techniques and systems are implemented for re-infusion (Cellsaver method). Any lost blood is collected, cleaned and returned to the patient after the operation. In this way, foreign blood transfusion, and thus the dangers of becoming infected with hepatitis or HIV (AIDS) can be significantly decreased. For patients who initially cannot be fully mobilized after an operation, an increased risk of deep vein thrombosis (DVT) exists. This risk is reduced with the implementation of patient physiotherapy, as well as with the regulation of antithrombotic stockings and blood thinning medication.

Postoperative care

Exercise is introduced on the first day after the operation as the risk of complications is lowered by this early mobilisation. If a cemented prosthesis was implanted, the leg can immediately accept the patient’s full weight; in the case of a cementless implantation, the operated leg may only take partial body weight for a period of approximately six weeks. Mobility is regained within a few months. All sports,which are joint gentle, are allowed, for example swimming, cycling, dancing, cross-country skiing, golf and gymnastics. Extreme sports such as alpine skiing, squash and heavy lifting must be renounced. Obesity and being overweight can impair the lifespan of the implant. Intensive physiotherapy promotes the mobility and structure of the muscles. The rehabilitation measures co-ordinated by the surgeon and the physiotherapist are of great importance for a successful operation and a rapid recovery of the patient. The active cooperation of the patient greatly assists in the fast achievement of this goal.

Patient information

Info about the ACS® Knee System