Information on knee prostheses
What is a knee prosthesis?
A knee prosthesis is an artificial knee joint to replace part or all of the natural knee joint. Artificial knee joints are usually only used when the natural joint is very worn or the joint restricts the patient's freedom of movement due to severe pain. A common indication for a knee endoprosthesis is knee osteoarthritis.
Prostheses are generally only considered when all other conservative measures such as painkillers, cartilage-building substances or joint-sparing therapies no longer work.
A knee joint prosthesis is one of the most common implantations today. Over 150,000 artificial knee joints are used in this country every year.
What types of knee prostheses are there?
The aim and purpose of a knee joint prosthesis is to imitate the natural joint as best as possible in its shape, appearance and, above all, function. Knee prosthesis specialists only focus on the worn parts, mill them out and replace them with a metal surface. Depending on how badly the knee joint has been damaged, there are currently three common types of endoprostheses:
Unicondylar sled prosthesis
The unicondylar sled prosthesis is the simplest and most common form of knee prosthesis. Only one condyle of the femur is provided with a metal surface. This endoprosthesis can only be used if all the ligaments in the knee joint are intact and the damage is limited to the outer or inner condyle, i.e. only the cartilage is arthritic. Many patients with bow legs have this type of damage. Depending on which side is to be replaced, this sled prosthesis is attached either on the outside or inside, whereas the opposite side of the knee joint is provided with a type of metal substructure that also contains a plastic block on which the artificial joint can slide around like a sled.
Bicondylar sled prosthesis
This type of prosthesis is the most common knee endoprosthesis and completely replaces the knee joint: The joint surfaces of the thigh, shinbone and sometimes also the kneecap are replaced. It is important that the axis-stabilizing ligaments of the knee joint, such as the outer and inner meniscus, are still intact and functional. This means that this prosthesis can be implanted even if the anterior cruciate ligament is damaged. The outer and inner ligaments then make a significant contribution to the stability of the knee. If both the anterior and posterior cruciate ligaments are no longer stable, a posterior-stabilized endoprosthesis can also be implanted: It takes over the functions of the cruciate ligament by moving the thigh posteriorly and the shinbone anteriorly when loaded.
Axle-guided knee prosthesis
If the knee ligaments are severely damaged in addition to the bone and cartilage, a pedicled knee endoprosthesis must be used. This type of prosthesis is usually used as a revision prosthesis when all other prosthetic measures have failed. The axis-guided knee prosthesis stabilizes the knee joint in its longitudinal axis. This prevents the lower leg bone from shifting sideways towards the thigh bone. The individual parts of the endoprosthesis then work together like a hinge joint, thus enabling stability. The disadvantage here, however, is that these prostheses have a significantly shorter lifespan and the movement of the knee is generally more limited compared to uni- or bicondylar sled prostheses. There are two types of pedicled total prostheses:
The rotation prosthesis: If the cruciate ligament and the menisci are damaged, but the muscles and joint capsules are still functioning, this rotation prosthesis stabilizes the knee joint anteriorly and posteriorly as well as laterally and medially with a possible external rotation of the ankle.
The axis-guided prosthesis: If the ligaments, joint capsules and muscles are damaged, this prosthesis is implanted. It stabilizes the joint by reducing the range of motion.
Minimally invasive knee endoprosthetics
With the minimally invasive method, the kneecap is not touched and access is gained via a relief incision in the muscles. This method is very gentle on the tissue because the ligament and muscle structures are less traumatized than with the conventional method. This can also prevent movement problems and the patient can be mobilized more quickly postoperatively. However, the minimally invasive method cannot be used in overweight patients because the prosthesis cannot be inserted through the larger subcutaneous fat tissue. Likewise, not every type of knee prosthesis can be implanted minimally invasively, so the situation differs from patient to patient.
What does a knee prosthesis consist of and what are its parts?
As a foreign material, a knee prosthesis must meet certain requirements. It must function painlessly and without disruption. The material must not be incompatible with the organism, and it must also be corrosion-resistant and withstand the natural wear of the joints against each other. The most commonly used materials are therefore ceramic, special plastics (so-called polymers) or metal compounds (for example CoCr alloys). Patients with a nickel allergy can also use titanium alloys. The sliding bearings of the artificial joints are also mainly made of plastics, i.e. polyethylene.
Every knee prosthesis consists of three parts:
- a thigh part
- a lower leg part
- a plastic-based support on the shin part
The thigh component is usually made of a metal connection and replaces the worn surfaces of the femur. The lower leg part is made of titanium and replaces the worn parts of the tibia. The plastic-based sliding bearing made of polyethylene is then placed on top of this, artificially replacing the joint space. Optionally, the back surface of the kneecap can also be removed and replaced with a metal; however, this fourth component is rarely used.
How do you implant an artificial knee joint?
Every knee prosthesis must be anchored in the human bone. This is the only way to ensure it has sufficient stability. Depending on the degree of damage to the bone, its quality, the patient's physical activity, and their health and age, there are three types of anchoring: cemented, cement-free, or hybrid prosthesis.
Cemented prosthesis: The joint is anchored in the human bone using bone cement. The cement is made of polymethyl methacrylate. This type of stabilization is the most common and is used when the patient's bone quality is severely reduced. The advantage here is that the joint can be fully loaded quickly postoperatively.
Cement-free prosthesis: The joint is firmly clamped and pressed into the upper and lower leg bones. The bone is sawn in such a way that the prosthesis has a stable hold. It is assumed that the body's own bone will grow firmly into the prosthesis sooner or later, resulting in stability. The surfaces of the prosthesis are quite rough so that the sprouting bone can anchor itself well. The prerequisite here is of course good bone quality, so this type of implantation is limited to young patients. Here, too, the joint can be fully loaded immediately after the operation.
The hybrid prosthesis: As the name of this prosthesis suggests, it consists of cement-free and cemented parts. The lower leg part is attached using cement while the thigh part remains cement-free. This variant can also be loaded immediately after the operation.
3D knee prosthesis: The 3D knee prosthesis is a custom-made artificial knee joint that is made individually for the patient. There are already clinics in Germany and Switzerland that implant 3D knee joints.
Artificial knee joint surgery
An artificial knee joint can be implanted conventionally, minimally invasively, computer-assisted or robot-assisted. The type of implantation depends on the individual circumstances and possibilities of each patient. While general risks such as infections, impaired wound healing or thrombosis can occur, specific problems with the artificial joint itself can also arise. In some cases, components or the entire joint must then be replaced as part of a prosthesis replacement.
Computer-assisted and robot-assisted knee endoprosthetics
The computer-assisted knee prosthesis implantation enables precise three-dimensional surgical planning. Using a knee CT, the special characteristics of the individual knee joint are determined and reconstructed three-dimensionally on the computer. The precise planning helps the surgeon to fit the artificial joint precisely.
The planning can be implemented even more precisely with a surgical robot. The robot arm defines the cutting plane and the limits at which the bone saw can be activated with millimeter precision. This protects the joint structures such as nerves, vessels or ligaments.
As this question cannot be answered objectively and a reputable doctor would never claim to be the best doctor, one can only rely on the experience of a doctor. The more knee prosthesis operations a doctor performs, the more experienced he becomes in his specialty.
Specialists in an artificial knee joint are therefore orthopedic surgeons who have specialized in the endoprosthetic treatment of the knee joint. Due to their experience and many years of work as orthopedic surgeons with a focus on knee surgery, they are the right people to contact for knee surgery.
