Simulated Partial and Complete Resection of the Posterior Cruciate Ligament in Medially Conforming Total Knee Arthroplasty Causes a Graded Reduction in Femoral Rollback and Increase in Posterior Tibial Sag: A Computational Study.

Background: Medially conforming (MC) total knee arthroplasty (TKA) has seen increased clinical utilization. This design allows for either retention or resection of the posterior cruciate ligament (PCL); however, the impact of the PCL on femoral rollback and posterior tibial sag is unknown. Therefore, we developed a computational model to quantify how the PCL affects femoral rollback and posterior sag in MC-TKA.

Methods: Computational models of ten cadaver knees were virtually implanted with MC-TKAs. Clinical tests of passive flexion and posterior sag were simulated, and femoral rollback and posterior tibial translation (PTT) were quantified. These tests were simulated in MC-TKA with the PCL retained, partially resected, and completely resected. We then assessed how increasing the tibial insert thickness in PCL-resected MC-TKA and switching to posterior-stabilized (PS)-TKA impacted posterior sag.

Results: Femoral rollback decreased medially by a median of 2.4 mm (P ≤ 0.001) and laterally by a median of 3.3 mm (P < 0.001) with simulated PCL resection. For the simulated sag test, PTT increased by a median of 4.2 mm (P < 0.05) and 7.4 mm (P ≤ 0.001) with partial and complete PCL resection, respectively. Moreover, PTT was reduced by a median of 7.1 mm (P ≤ 0.01) when converting a PCL-resected MC-TKA to a (PS)-TKA.

Conclusion: In a computational model, MC-TKA does not fully compensate for the function of the PCL, which facilitates femoral rollback in passive flexion and resists PTT during a posterior sag test. Resecting the antero-lateral (AL) bundle, with preservation of the postero-medial (PM) bundle of the PCL, yields more femoral rollback and less PTT than complete PCL resection in MC-TKA. Increasing tibial insert thickness in MC-TKA also does not account for the role of the PCL in reducing PTT during a sag test, while a PS-TKA does.