The study design involved here is experimental in nature. The resection of the anterior cruciate ligament (ACL) during conventional total knee arthroplasty (TKA) has been considered a potential factor leading to abnormal in vivo knee kinematics. Bi-cruciate retaining (BCR) TKA designs allow the preservation of the ACL with the potential to restore native knee kinematics. This study aimed to investigate the effect of posterior tibial slope (PTS) on stress experienced by the ACL during weight bearing sit-to-stand (STS) and single-leg deep lunge. The ACL elongation patterns were measured in 30 unilateral BCR TKA patients during weight-bearing STS and single-leg deep lunge using a validated dual fluoroscopic tracking technique. The minimum normalized stress within the anteromedial (AM) and posterolateral (PL) bundle of the ACL during weight-bearing STS and single-leg deep lunge was found at a PTS of 3.7 degrees. The maximum AM and PL bundle stresses were observed at a PTS of 8.5 and 9.3 degrees, respectively during STS and at 8.4, and 9.1 degrees, respectively during single-leg deep lunge. There was a significant positive correlation between PTS and stress observed within the AM and PL bundle of the ACL during weight-bearing STS (R 2 = 0.37; p < 0.01; R2 = 0.36; p = 0.01) and single-leg deep lunge (R 2 = 0.42; p < 0.01; R 2 = 0.40; p < 0.01). The study demonstrates that PTS of operated BCR TKA knees has a significant impact on the stress experienced by the preserved ACL during weight-bearing STS and single-leg deep lunge. This suggests that avoiding excessive PTS may be one of the surgical implant alignment factors to consider during surgery to minimize increased loading of the preserved ACL.
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