What are the main differences between cruciate-retaining (CR) and posterior-stabilized (PS) knee implants?

• A. CR uses cam-post; PS preserves PCL.
• B. CR has no ligaments; PS uses ligaments.
• C. CR and PS are identical.
• D. CR preserves PCL with different stability features; PS substitutes PCL using a cam-post mechanism; affects ligament balancing and rollback.

Answer: (D)

The key idea is how the posterior cruciate ligament (PCL) and a cam-post mechanism shape stability and knee motion in these implants.

In cruciate-retaining knees, the PCL is kept intact. This ligament provides posterior stability and guides rollback of the femur during flexion, so the knee relies on natural ligament tension and soft-tissue balance rather than a mechanical substitute. The design aims for more natural knee kinematics because the PCL remains a stabilizing and guiding structure.

In posterior-stabilized knees, the PCL is removed and a mechanical substitute takes over. A tibial post and a femoral cam engage during flexion to produce posterior translation of the femur relative to the tibia, providing stability and enabling rollback without the PCL. This changes how the knee is balanced during surgery and tends to produce a more constrained, predictable rollback pattern due to the cam-post mechanism.

So the best description is that CR preserves the PCL with different stability features, while PS substitutes the PCL with a cam-post mechanism, and this distinction directly affects ligament balancing and tibiofemoral rollback.