A total knee replacement replaces your diseased knee joint with man-made components, eliminating the damaged bearing surfaces that are causing pain. A diagnosis of advanced osteoarthritis of the knee will indicate the need for total replacement of the knee joint.
While you may think of the knee as a hinge, it is really much more complex. The various surface components of the knee roll and glide as the joint bends. The different knee replacement implants on the market try to replicate these motions.
Based upon your age and lifestyle, there are several design options to choose from that will help you return to an active, enjoyable life. Ultimately, your surgeon will make an implant recommendation (both design and brand) based upon his or her skill and experience with a particular device as well as your specific circumstances and needs. It is, however, important to understand the differences between implants so that you can engage intelligently with your surgeon on the subject.
Implant Components
There are three components to a knee implant, replacing three bone surfaces – femoral, tibial and patellar implants. The components weigh between 10 to 15 ounces in total.
Femoral Component: In a knee implant, the femoral component, made of metal, curves up around the end of the femur (or thighbone). It has a central groove allowing the patella (or kneecap) to move up and down smoothly as the knee joint bends and straightens.
Tibial Component: The tibial component of a knee implant is a flat metal platform with a polyethylene insert or spacer. These have a double dish configuration for the femoral condyles and also either a notch to accommodate the cruciate ligaments (cruciate sparing) or a cam structure to take their place (cruciate sacrificing).
Patellar Component: The patellar ‘button’ is a dome-shaped piece of ultrahigh molecular weight polyethylene that replicates the surface of the kneecap.
Implant Materials
These are designed such that a metal component always moves against plastic or other synthetic material. This promotes smooth movement and minimal future wear. The metal parts of the implant are usually created from titanium or cobalt/chromium-based alloys. The plastic parts are an ultrahigh-molecular weight polyethylene.
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Types of Implants
Fixed Bearing: A fixed-bearing prosthesis is the most common knee replacement implant in use today. The components are as described above but the polyethylene cushion of the tibial component is fixed to the metal platform base.
Mobile Bearing: The difference between a fixed-bearing implant and a mobile bearing implant is in the bearing surface. They allow patients a few degrees of greater rotation to the medial and lateral sides of their knee.
Medial Pivot (also known as Rotating Platform): In a rotating platform, the polyethylene insert can rotate slightly around a conical post, thereby copying the activity of the natural knee joint.
Posterior Cruciate Ligament (PCL)-Retaining or Substituting: Another important aspect of a total knee replacement is the treatment of the Posterior Cruciate Ligament which prevents the femur from shunting back on the top of the tibia when it is flexed – sometimes referred to as ‘roll back’. Depending upon its condition, this ligament can be kept (retained) or removed (substituted).
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Customized and Personalized Knee Implants
Because people come in every shape and size, and because improperly sized implants can contribute to irritation and/or instability, surgeons and implant manufacturers endeavor to provide patients with knee implants to fit their specific frame. People with smaller skeletons present challenges that require the adoption of implants constructed for their needs. Relatively new implants are designed to accommodate those male or female femora which are smaller from side-to-side than from front-to-back. This helps ensure the a patient will receive the best implant fit possible.
As the total number of knee replacements has grown significantly over the years, the number of women having knee replacement surgery has likewise increased. Leading orthopedic surgeons have reported they were frequently making adjustments during surgery to make traditional implants fit female patients. Many researchers believe that women’s knees differ from men’s not only in size but also in shape whilst other researchers believe that the differences have less to do with gender and more to do with stature in general. Nevertheless, some manufacturers have developed gender-specific implants to fit the unique contours of women’s knees.
