The knee joint is one of the largest and the most complex joints in the body. Apart from bending and straightening, it allows other intricate movements such as rotation and gliding. The natural knee consists of three bones: the thigh bone (femur), the shin bone (tibia) and the kneecap (patella). Refer to figure 1.
The lower end of the femur, the upper end of the tibia and the under surface of the patella are all covered by articular cartilage, or menisci - a tough and very smooth material that ensures that movement of the knee is both effortless and pain free.
The knee joint is enclosed in a capsule. The inner surface of the joint capsule is lined with the synovial membrane that produces synovial fluid (a gluey liquid that provides friction-free sliding).
Working together, the synovial fluid and menisci, act as a shock absorber. They absorb the powerful forces that impact on the joint during exercise and activity. Envelopes of tough ligaments connect the femur and tibia. These cover the joint, providing stability and allowing controlled movement. The knee’s movements are initiated and controlled by the strong muscles of the thigh and lower leg.
A healthy knee joint will allow the leg to move freely within its range of motion, while absorbing the impact that results from activities such as walking and running.
Knee Joint Replacement
Approximately 32,000 knee joint replacements are performed each year in Australia.¹ Knee joint replacement surgery involves relining the ends of the femur, the tibia and, if necessary, the underside of the patella with man-made components called prostheses.
Components of a Knee Replacement
Each knee prosthesis is made up of several parts comprising femoral, patella and tibial components. Refer to figure 2.
- The femoral component is used to reline the femur and is made of a metal alloy.
- Although the patella is not always relined, the patella component replaces the under-surface of the patella that rubs against the femur. The patella component is either made completely of plastic or plastic with a metal-alloy back.
- The tibial component can be a single or two piece design. The single piece is made of plastic while the two piece design consists of a metal tray that is secured to the bone and a plastic insert that provides a smooth surface on which the femur moves. The plastic insert may be attached firmly to the tibial tray or may be designed to move relative to the tibial tray.
Your surgeon may choose to use one of two implant procedures:
- Cemented joint replacement is a procedure in which bone cement is used to fix the prosthesis in place in the bone.
- Cementless joint replacement does not involve bone cement to fix the prosthesis in place. The prosthesis is designed in such a manner that natural bone growing into the surface of the implant leading to stable fixation.
Procedure Options
There are several procedures for knee replacement. The right one for you will be decided according to your requirements and your surgeons’ preference.
Procedure options include:
Total Knee Replacement
This operation involves relining the femur and tibia. The patella may not necessarily be resurfaced. Refer to figure 3.
Partial Knee Replacement (Unicompartmental Knee Replacement)
This procedure may be recommended when only one side (compartment) of the knee is diseased and needs to be resurfaced. The compartments of the knee that are not diseased are not resurfaced. Refer to figure 4.
Revision of Total Knee Replacement
A re-operation on a previously performed total knee replacement that has worn out, failed, or become loose. Part or all of the previous components are removed and replaced with new ones.
Remember that whilst the majority of these procedures are successful, they are not guaranteed to last forever. Artificial joints can become loose and unstable, requiring a revision (repeat) surgery. Whilst improvements in surgical techniques and instrumentation will help your replacement last longer, it still remains difficult to estimate the life expectancy of joint replacement.
Bilateral-Partial Knee Replacements
Where both knees are replaced at the same time.
¹ Australian Orthopaedic Association Joint Replacement Registry, see www.aoa.org.au/docs/njrrrep06.pdf
