Increased posterior tibial slope is associated with failure after anterior cruciate ligament primary repair

PURPOSE: To determine whether preoperative posterior tibial slope (PTS) and intercondylar notch morphology are associated with arthroscopic anterior cruciate ligament primary repair (ACLPR) failure 2-5 years postoperatively, and to explore the association of the difference between medial and lateral PTS (MPTS and LPTS), and magnetic resonance imaging (MRI) versus radiograph slope agreement. METHODS: This retrospective secondary analysis included a prospectively collected, single-surgeon cohort of patients aged ≥18 years who underwent ACLPR for selected proximal anterior cruciate ligament (ACL) tears between 2008 and 2021 with 2-5 years of follow-up. MPTS, LPTS and anterior/posterior notch width (aNW and pNW) were quantified on preoperative MRI; PTS was additionally measured in a radiograph subset. Failure was defined as objective knee laxity (side-to-side difference ≥ 3 mm, Lachman ≥ 2 or pivot-shift ≥ 2) and/or patient-reported symptomatic instability, including telemedicine follow-up. Associations with failure were assessed using univariable and multivariable logistic regression and exploratory analyses included receiver operating characteristic/area under the curve and time-to-failure analyses. RESULTS: Among 249 patients (51.4% female; mean age 39.4 ± 10.8 years) with median follow-up of 3.2 years (interquartile range 2.4-5.0), 24 (9.6%) met failure definition. After adjustment for age and suture augmentation, increased PTS was associated with failure per 1° increase (MPTS: odds ratio [OR] 1.62; 95% confidence interval [CI] 1.35-1.96; LPTS: OR 1.35; 95% CI 1.17-1.55; both p < 0.001), while older age was associated with lower odds of failure. In exploratory analysis, directed ΔPTS (MPTS-LPTS) was associated with failure (adjusted OR 1.27; 95% CI 1.07-1.51; p = 0.008). Intercondylar notch width was not associated with failure. MRI and radiographic-based slope measurements correlated (MPTS r = 0.706; LPTS r = 0.590; both p < 0.001). CONCLUSION: In this selected cohort of patients undergoing arthroscopic ACLPR, greater MPTS and LPTS and younger age were associated with higher odds of failure at 2-5 years, while notch width was not. These findings suggest that PTS may help contextualise risk when counselling patients considered for ACLPR. LEVEL OF EVIDENCE: Level IV, case series.