Knee Replacement Implants
Total Knee Implants
A total knee replacement is basically a curved metal cap on the end of the femur (thigh bone) and a flat metal tray on the top of the tibia (shin bone) with a piece of polyethylene (i.e. plastic) that sits between the two pieces of metal.
Implant fixation is mostly done with bone cement (95% of the market), but press fit implants have been gaining in popularity.
Knee replacement kinematics is a balance between letting the knee ligaments dictate the knee motion versus allowing the knee implant to force the knee motion to do what the surgeon thinks is best. Surgeons who believe the native ligaments should guide the knee motion consider a knee replacement as a knee resurfacing operation. Surgeons who believe the implant should dictate the knee motion consider a knee replacement as a knee reconstruction operation. Are we striving to mimic the original knee kinematics or create a new type of knee kinematics? These questions remain openly debated and still unanswered in the orthopedic community today
Total knee replacements can be divided into 5 groups.
Cruciate Retaining (CR)
Cruciate Sacrificing (CS)
Cruciate Substituting - Posterior Stabilized (PS)
Constrained devices
Hinged Knee Replacements
Cruciate Retaining (CR) Knee Implants
CR knee implants are thought of more as of a knee resurfacing than a knee reconstruction. CR knee implants rely on the posterior cruciate ligament to help stabilize the knee joint and help with femoral roll back. CR knee implants typically require more precise ligament balancing, feel more natural with less mechanical noises, but on average obtain slightly less knee flexion.
Comparison of CR vs. PS
The CR implant on the left has a simpler design and relies on the posterior cruciate ligament (shown in pink) to help with femoral roll back. The PS implant on the right has a cam (blue) and post (yellow) design that engages in deep flexion and drives femoral roll back.
Press Fit vs. Cemented fixation
Press Fit Fixation
Press fit implants have a porous coating on the bone facing side such that the patient’s bone can grow into the implant. These implants have a slightly higher early failure rate and a lower late failure rate. Once the bone is ingrown, the implant should be well attached to the bone for life. Surgeons must be meticulously when preparing the bone for a press fit implant, so that the saw blade does not burn the bone and kill the bone cells.
Cemented Fixation
Bone cement is typically used to bond the implant to the bone. When the monomer and polymer are mixed, bone cement feels like cookie dough and hardens in about 5-10 minutes. The cement fills in the cracks and crevices of the bone and firmly fixes the implant to the bone. These implants have little pockets on the back of the implants where the bone cement can reside to help improve fixation. Bone cement does not have any adhesive properties. If a knee replacement is mal-aligned, or grossly unstable, the implant can loosen from the bone cement.