The knee joint is the biggest and strongest joint in the body. The knee joint is located in the place where the thigh bone meets the upper end of the tibia. The kneecap is at the front of the joint and protects it. A healthy knee joint allows you to move the lower part of your leg back and forth and slightly twist it, pointing your toes in and out. Ligaments and cartilage stabilise and support the joint and stop it from moving too far from side to side.
- Knee joint implants
- Types of prostheses
- The right implant for you
- Fixed-bearing prostheses and mobile-bearing prostheses
- Materials used to make prostheses
- Criteria regarding the materials
- Cemented and cementless fixation
- Revision components
Your doctor can recommend knee replacement surgery if you are suffering from severe knee pain or your mobility is limited because of rheumatoid arthritis, degenerative joint disease or injury. Knee replacement surgery will relieve your pain and allow you to live a full life.
During the operation the orthopaedic surgeon will replace your damaged joint with an artificial implant. A complete knee joint replacement is the most common procedure. However, some patients can benefit from a partial knee joint replacement.
Implants are made from metal alloys, ceramic materials or artificial materials and they are attached to the bones with the use of acrylic cements. There are different kinds of implants. Your surgeon will talk to you about the kind of implant that meets your needs.
For simplicity, the knee is considered a "hinge" joint because of its ability to bend and straighten like a hinged door. In reality, the knee is much more complex because the bone surfaces actually roll and glide as the knee bends.
The first designs of a knee joint prosthesis we based on the idea of a hinge and they involved inserting a hinge that joins together two elements. Current implant designs recognize the complexity of the joint and more closely mimic the motion of a normal knee. Some implant designs preserve the patient's own ligaments, while others substitute for them. Several manufacturers make knee implants and there are more than 150 designs on the market today.
Recent progress in the design of prostheses involves gender-specific implants. Many studies suggest that the knee's shape and proportions different in men and women. Many manufacturers have developed components for the end of the thighbone which more closely match the average woman's knee. However, at this time there is no research to show that gender specific implants last longer or provide better function than standard implants.
The brand and design used by your doctor or hospital depends on many factors such as your needs (which depend on your age, weight, health and activity), the surgeon’s experience and knowledge of a given prosthesis, its cost and performance record. You should discuss this with your doctor.
Elements of the knee joint prosthesis
Up to three bone surfaces may be replaced in complete knee replacement surgery:
• The lower end of the femur. The metal femoral component curves around the end of the femur (thighbone). It is grooved so that the kneecap can move up and down smoothly against the bone as the knee bends and straightens.
• The top surface of the tibia. The tibial component is typically a flat metal platform with a cushion of strong, durable plastic, called polyethylene. Some designs do not have the metal portion and attach the polyethylene directly to the bone. For additional stability, the metal portion of the component may have a stem that inserts into the centre of the tibia bone.
• The back surface of the patella.. The patellar component is a dome-shaped piece of polyethylene that duplicates the shape of the patella (kneecap).
Components are designed so that metal always borders with plastic, which provides for smoother movement and results in less wear of the implant.
The tibial component has a raised surface with an internal post that fits into a special bar (called a cam) in the femoral component. The cruciate ligament is removed to fit the elements to the bone. These components work together to do what the PCL does: prevent the thighbone from sliding forward too far on the shinbone when you bend your knee.
As the name implies, the posterior cruciate ligament is kept with this implant design. Cruciate-retaining implants do not have the centre post and cam design. This implant may be appropriate for a patient whose posterior cruciate ligament is healthy enough to continue stabilizing the knee joint.
Prosthesis saving the ACL
Unicompartmental knee implants
During complete knee replacement surgery, large implants are used to resurface the ends of the femur and tibia bones. If only one side of the knee joint is damaged, smaller implants can be used (unicompartmental knee replacement) to resurface just that side.
In a unicompartmental (partial) knee replacement, only the damaged part of the knee is replaced.
Most patients get a fixed-bearing prosthesis. In this design, the polyethylene of the tibial component is attached firmly to the metal implant beneath. The femoral component then rolls on this cushioned surface.
In some cases, excessive activity and/or extra weight can cause a fixed-bearing prosthesis to wear down more quickly. Worn components can loosen from the bone and cause pain. Loosening is a major reason some artificial joints fail.
If you are younger, more active, and/or overweight, your doctor may recommend a rotating platform/mobile-bearing knee replacement. These implants have been designed for potentially longer performance with less wear.
Just like the fixed-bearing prostheses, mobile-bearing implants have three parts in order to replicate the natural joint. However, in a mobile-bearing knee the polyethylene insert can rotate short distances inside the metal tibial tray. This is designed to allow patients a few degrees of greater rotation to the medial and lateral sides of their knee.
In comparison with the fixed-bearing prostheses, mobile-bearing knee implants require more support from soft tissues, such as the ligaments surrounding the knee. If the soft tissues are not strong enough, mobile-bearing knees are more likely to dislocate. They can also be more expensive that fixed-bearing implants.
What is more, research has not yet established improved durability or function with mobile-bearing implants.
The metal elements of the implants are made from titanium or cobalt-chrome alloys. The elements are made out of ultra-high-molecular-weight polyethylene (UHMWPE). All together the elements way around 400-500 grams depends on the size.
The materials used to make prostheses must meet several requirements:
• They must be biocompatible; that is, they can be placed in the body without creating a rejection response.
• They must be able to duplicate the knee structures they are intended to replace. For example, they are strong enough to take weightbearing loads, flexible enough to bear stress without breaking, and able to move smoothly against each other as required.
• They must be able to retain their strength and shape for a long time.
There are different types of fixation used to connect knee implants to the bone. Cemented fixation holds implants in place with a fast-curing bone cement (polymethylmethacrylate). Cementless fixation relies on new bone growing into the surface of the implant.
There are also hybrid fixations. In hybrid fixation for total knee replacement, the femoral component is inserted without cement, and the tibial and patellar components are inserted with cement.
Your surgeon will evaluate your situation carefully before making any decisions about components and fixation. Do not hesitate to ask what type of fixation will be used in your situation and why that choice is appropriate for you.
Revision components typically have longer stems which fit into the bones.
How long a knee replacement lasts depends on several factors, including activity level, weight, and general health.
Just as wear in the natural joint contributed to the need for a replacement, wear in the implant may eventually require a second surgery (called a joint revision).
A revision procedure may require special components. They typically have longer stems which fit into the femur and tibia. They may also have attached metal pieces called augments which substitute for missing bone.
Revision components often have a cam in the centre of the knee similar to a posterior stabilized component. In revision components, though, the cam is larger to give the knee more stability.
In cases where the knee is very unstable and a large amount of bone is missing, it may be necessary to join the femur and tibia with a metal "hinge" in the centre.