Large magnetic resonance imaging-based medial posterior tibial slope is associated with time-dependent worsening of rotational knee laxity after anterior cruciate ligament reconstruction.

Purpose: To examine the relationship between posterior tibial slope (PTS) and knee laxity before, during and after anterior cruciate ligament (ACL) reconstruction, using anterior tibial translation (ATT) measured as an arthrometer-based side-to-side difference and rotational laxity assessed during the pivot shift test via clinical grade and inertial sensor-based measurements of tibial acceleration and external rotational angular velocity (ERAV). Methods: This retrospective cohort study assessed patients who underwent primary ACL reconstruction with subsequent hardware removal. Medial PTS (MPTS), lateral PTS (LPTS) and slope asymmetry ( | MPTS-LPTS | ) were measured using magnetic resonance imaging. Based on a recent review, binary strata were defined as MPTS ≥ 9.05° versus <9.05° and LPTS ≥ 9.55° versus <9.55°, and comparisons were performed accordingly. Knee laxity was evaluated at three time points: preoperatively, at time-zero (intraoperatively during temporary graft fixation), and postoperatively (at the time of hardware removal), using ATT, pivot shift grade, acceleration and ERAV. Variables with p < 0.1 in univariate analysis, along with key PTS factors, were entered into multivariate linear or ordinal logistic regression models. Cohen's d was calculated for binary predictors of continuous outcomes, with |d | ≥0.5 indicating a moderate or greater effect size; odds ratios (ORs) with 95% confidence intervals were reported. Results: A total of 106 patients (24.3 ± 10.3 years; 37.7% male; 22.4 ± 2.7 kg/m2) were analysed. Large MPTS (≥9.05°) was significantly associated with increased acceleration (p = 0.017, |d | = 0.62) and ERAV (p = 0.004, |d | = 0.59) at the postoperative time point (hardware removal, 1.6 ± 0.6 years after ACL reconstruction). No significant associations with PTS were observed for ATT or pivot shift grade. Conclusion: Large MPTS was associated with increased postoperative rotational laxity, although no laxity was noted intraoperatively during temporary fixation. These results indicate that certain tibial slope morphologies may drive the onset of laxity. Preoperative assessment may help identify at-risk patients and optimize surgical strategy. Level of Evidence: Level IV.