Review Question - QID 219224

The advent of conforming polyethylene designs allows for the use of cruciate retaining implant designs without the need for a functional PCL. These TKAs allow for the same biomechanical functionality as that of classic cruciate retaining (CR) and posterior stabilized (PS) implants with equivalent patient-reported outcomes (Answer 5).

As the native knee moves from extension into deep flexion, the femur translates posteriorly secondary to the pull of the taut posterior cruciate ligament (PCL). This “femoral rollback” phenomenon is of critical importance in performing knee flexion beyond 90 degrees; it prevents early abutment between the posterior tibial plateau and the posterior femur (Illustration A). Newer biomechanical studies have shown that it is the lateral femoral condyle that primarily undergoes posterior translation around a medial condyle pivot point (Illustration B). Traditional CR implants retain the PCL to maintain the femoral rollback, whereas traditional PS implants utilize a cam-post design to recreate the same functionality (Illustration C). Importantly, newer “conforming” polyethylene designs are manufactured to have a concave, highly congruent medial condyle dish, and a flat/slightly convex lateral condyle dish. Collectively, this design allows for the re-creation of the femoral rollback phenomenon without the need for a functional PCL (i.e. traditional CR) or cam-post design (i.e. traditional PS). Clinical studies have shown equivalent patient-reported outcomes and complication rates when using these conforming CR implants. Of note, these conforming polyethylene inserts have been given various names in the literature (i.e. anterior stabilized, highly congruent, medial pivot, etc.)

Heckmann and colleagues provided a comprehensive review article regarding conforming polyethylene inserts in total knee arthroplasty. They discuss that traditional CR and traditional PS implants have been exhaustingly shown to display equivalent outcomes in the setting of primary TKA. The authors then discuss conforming polyethylene inserts, their biomechanical rationality, and clinical outcomes compared to traditional TKA designs. They conclude that these conforming polyethylene inserts may restore native kinematics in TKA patients more readily than traditional CR implants and without the need for the cam and post mechanism present in traditional PS implants with equivalent outcomes.

Peters and colleagues performed a retrospective cohort study of their institutional outcomes utilizing traditional CR compared to congruent-CR TKA implants. The decision to use a congruent-CR implant was based on the integrity of the PCL intra-operatively or after it was sacrificed to achieve soft tissue balancing. They reported equivalent Knee Society scores, radiographic alignment, component loosening, manipulation rate, major complications, and time to revision between both groups. The authors concluded that the congruent CR implants have comparable clinical and radiographic results when compared to traditional CR TKA.

Illustration A demonstrates the function of the PCL in femoral rollback to aid in increasing the terminal range of motion with deep flexion. Illustration B further characterizes this femoral rollback as preferentially occurring in the lateral femoral condyle, which rotates around a medial femoral condyle pivoting point. Illustration C demonstrates the differences between traditional CR, conforming CR, and traditional PS designs for TKA.

Incorrect Answers:
Answer 1: The PCL is an important anatomic constraint on the flexion gap during total knee arthroplasty. In traditional CR TKAs with standard polyethylene inserts, it can become deficient over time resulting in an increased flexion gap and resultant flexion instability.
Answer 2: In addition to its constraint on the flexion gap, the PCL also serves to maintain the femoral rollback phenomenon in traditional CR TKA cases. Had the surgeon proceeded to utilize a standard polyethylene with the CR-TKA design after cutting the PCL, there would have been paradoxical anterior femoral translation with flexion. This results in altered biomechanical knee function and significant loss of terminal flexion.
Answer 3: Patellar clunk syndrome occurs when a fibrous nodule of scar tissue forms along the posterior surface of the distal quadriceps tendon. Patients describe feeling a “clunk” as their knee moves from flexion into extension as the nodule catches in the anterior box of the femoral component in traditional PS TKAs. However, it has not been shown to occur in traditional CR or conforming CR design implants, as they do not require a box cut.
Answer 4: The conforming polyethylene inserts are manufactured and sterilized in the same manner as traditional CR and PS polyethylene inserts. For this reason, there is not an increased risk for periprosthetic joint infections with their use.