Types of Knee Implants
Each knee implant system is made up of three components:
- Femoral component
- Polyethylene spacer
- Tibial component
The femoral component replaces the lower ends of the femur (femoral condyles). The implant curves around the end of the femur and has an interior groove so the patella can smoothly move against the bone as the knee bends and straightens.
The polyethylene spacer paired with the tibial component replaces the top surface of the tibia. The femur glides along this surface.
The tibial component is a flat metal platform that holds the polyethylene insert. The components are designed so that metal articulates against plastic, providing smooth movement and minimal wear.
The patellar component is a dome-shaped piece of polyethylene that is identical to the shape of the patella and glides in the groove of the femur.
Fixed-bearing Prosthesis vs. Mobile-bearing Prosthesis
The difference between a Fixed-bearing Prosthesis and a Mobile-bearing Prosthesis is in the bearing surface. In a mobile-bearing prosthesis, the femoral component and tibial plate move across a polyethylene insert to create a dual surface articulation. A mobile-bearing prosthesis allows greater rotation of the knee and helps reduce the amount of wear to the bearing to help prevent loosening where the prosthesis attaches to bone.
Cruciate Retaining and Posterior Stabilized
The femoral component in a posterior stabilized design has an internal post with a center cam. The corresponding tibial component for this design has a raised surface with a center cam. The posterior stabilized design prevents the femur from sliding forward too far on the tibia when the knee bends. Cruciate retaining designs do not have a center cam and the tibia component has a flat surface.
Cemented fixation uses a fast-curing bone cement to hold the prostheses in place.
Cementless fixation relies on bone growing into the surface of the implant for fixation.
Hybrid fixation is when the femoral component is inserted without bone cement and the tibial and patellar components are inserted with bone cement.