There are three categories of knee replacements:
1. Partial, unicompartmental replacements.
2. Total knee replacements.
3. Constrained total knee replacement.
Knee replacement surgery originated in the 1960's with the development of the hinged implant. This consisted of two pieces connected by a hinge and assumed that the knee motion consisted of simple bending and straightening. Unfortunately, the knee is more complex than that and these early implants failed. The poor results led to great suspicion with regards to knee replacement surgery and many doctors were reluctant for a long time to recommend this type of procedure to their patients. With time, implants were modified to remove as little bone as possible, with partial knee replacements representing the ultimate in conservative bone cuts. Twenty-four years ago Dr. Marmor introduced the concept of the partial replacement whereby the upper tibia was covered with a small piece of plastic and the opposing femur covered with a relatively small piece of metal. These unicompartmental replacements have stood the test of time and are now widely utilized. The original femoral tibial unicompartmental replacements have been used as a model for the patello-femoral unicompartmental replacements, whereby the patella is resurfaced with plastic and the opposing trochlea is resurfaced with metal. Such an implant is of course only used when the femoral tibial compartments are healthy.
With a better appreciation of knee kinematics, the total condylar knee replacements appeared in the early 1970's. These are similar to the unicompartmental replacements, except that they cover the entire tibia and the entire end of the femur (and occasionally the patella). The amount of bone removed corresponds the thickness of the eventual metal and plastic. Long stems into the tibia or femur are not necessary in standard cases. Many sizes are available so that each patient will have an implant which fits perfectly regardless of his or her size. The good function of these implants is predicated on the integrity of the collateral ligaments. The role of the cruciate ligaments remains more controversial. Most surgeons currently agree that the anterior cruciate ligament can be sacrificed. Surgeons are split with regards to whether the posterior cruciate ligament should be saved or not. The fixation of the implant can be carried out with or without cement.
One of the improvements in knee replacement design is the use of a rotating plastic component. This can be one solid piece of plastic or two separate components which are designed to mimic the menisci. The rotation of the plastic improves the wear characteristics of the entire construct. The freedom of the plastic to rotate leads to a greater uncoupling of the femoral and tibial motion. These decrease the stresses at the bone implant interface and minimize the chances of implant loosening.
The surgery is difficult and precise. The bony cuts must be carried out with great care, lest the limb be placed in so-called varus or valgus (bow-legged or knock-knee’d). This is particular important for unicompartmental replacements: an excessively thick implant may put excessive pressure on the opposite femoral tibial compartment and lead to arthritis in that compartment. Occasionally, there has been significant bone loss associated with the patients deformity. This bone loss precludes adequate seating of the implant. In such situations, bone is commonly used to fill these defects. The bone is obtained from the bony cuts of the other compartments or more rarely is a "allograft" procured from a bone bank. There must be just the right amount of bone resected and just the right amount of ligament tension to ensure smooth flexion and extension of the knee. Errors in technique can lead to limitation of motion and/or instability. Proper instrumentation and adequate experience on the part of the surgeon are critical.
The last group of implants that we will discuss consist of hinged implants with rotating platforms. These are related to the hinged implants of the 60's with the major difference that they feature a rotating plastic component. These allow the implant to rotate as the knee flexes and extends. These implants require greater fixation and usually feature a stem which goes up into the femur and down into the tibia. These implants are used for major ligamentous insufficiencies.
The guiding principles are to minimize the amount of bone removed, facilitate any eventual re-operation and perform the amount of bony and ligamentous work which precisely allows for smooth, painless function of the knee.
When the ligaments are intact and two out of three compartments of the knee are healthy, it is reasonable to consider a unicompartmental replacement for the sole compartment which is unhealthy.
If more than one compartment is involved, we prefer the standard total condylar knee replacement, and in particular, we prefer the models which feature a rotating plastic component. We use this type of implant very frequently. Only rarely do we use the hinged implant described above.