Absolute Risk of Anterior Cruciate Ligament Graft Reruptures From the Posterior Tibial Slope Falls With Increasing Age: A Risk Stratification Analysis

Background: Posterior tibial slope (PTS) and young age are well-established independent predictors of an anterior cruciate ligament (ACL) graft rerupture. Although both factors have been studied individually, whether age changes the relative effect of PTS, its absolute clinical impact, or both has not been formally examined across the full age range of patients undergoing ACL reconstruction (ACLR). Purpose: To determine whether age modifies the relative or absolute rerupture risk associated with PTS after ACLR, and to identify age thresholds at which clinically relevant absolute risk levels are reached for given slope values. Study Design: Cohort study; Level of evidence, 3. Methods: A retrospective cohort of 585 patients who underwent primary ACLR with hamstring tendon autografts was analyzed, comprising a low-risk group (isolated ACLR) and a high-risk group (ACLR with lateral extra-articular tenodesis), with a minimum follow-up of 6 years. The Firth penalized logistic regression was used to model the joint effects of PTS and age on graft reruptures in pooled and group-specific analyses. A PTS×age interaction term was evaluated via the likelihood ratio test to assess whether the relative effect of PTS differed across age strata. Predicted absolute rerupture probabilities were derived from additive models, and age thresholds below which the risk fell under predefined clinically relevant levels (5% and 10%) were estimated for PTS values of 12° and 15°, with stability assessed by bootstrap resampling (1000 iterations). Results: After excluding 4 patients with missing covariate values, 581 patients were included in the final sample (low risk: n = 431; high risk: n = 150). In the pooled cohort (n = 581; 38 reruptures), PTS was independently associated with reruptures (odds ratio per degree, 1.28 [95% CI, 1.10-1.47]; P = .001), and increasing age was independently protective (odds ratio per year, 0.91 [95% CI, 0.87-0.95]; P < .001). No statistically significant PTS×age interaction was identified in any analysis (pooled cohort: P = .685), indicating that the relative effect of each degree of PTS remained approximately stable across ages. However, the predicted absolute rerupture risk decreased markedly with increasing age. In the pooled cohort, a patient with a PTS of 15° had an estimated rerupture probability of 36.6% at age 18 years versus 3.9% at age 45 years. Age thresholds at which the predicted risk fell below 10% were approximately 27 years for a PTS of 12° and approximately 35 years for a PTS of 15°. Conclusion: Age modulated the absolute, but not the relative, risk associated with PTS after ACLR. Younger patients with elevated slope values had the highest absolute rerupture risk, reinforcing the clinical value of integrating age into preoperative slope-based risk stratification.