Increased residual anterior knee laxity at one year is associated with a dose-dependent increase in graft re-rupture risk following hamstring autograft ACL reconstruction in athletes

Abstract

Purpose: Residual anterior knee laxity following anterior cruciate ligament reconstruction (ACLR) has been associated with inferior subjective outcomes and an increased risk of revision surgery; however, its prognostic value for predicting subsequent graft re-rupture remains unclear. This study aimed to assess the association between KT-1000-measured 1-year post-operative anterior knee laxity and graft re-rupture after ACLR. We hypothesised that increased residual anterior knee laxity at 1 year post-operatively would be associated with a higher risk of graft re-rupture in a dose-dependent manner. Methods: This retrospective cohort study included 1011 amateur and professional athletes who underwent primary ACLR with hamstring tendon autograft (HTA) between 2005 and 2024 by a single surgeon using a standardised surgical technique. Patients undergoing revision ACLR, multiligament reconstruction, or any lateral extra-articular augmentation procedure were excluded. Anterior knee laxity was quantified using the KT-1000 arthrometer at a mean of 12.1 +/- 1.3 months post-operatively. A landmark time-to-event design was applied, with follow-up starting from the KT-1000 assessment to minimise immortal time bias and to focus on anterior knee laxity after biological graft maturation. The primary exposure was KT-1000 side-to-side difference (SSD), analysed as both a continuous variable and using clinically relevant thresholds (<3 mm, 3-5 mm, and >5 mm). Associations between post-operative laxity and graft re-rupture, adjusting for demographic, surgical, and activity-related factors were assessed using a multivariable Cox proportional hazards model. Secondary analyses evaluated the relationship between KT-1000 laxity, return-to-play status, and post-operative activity level. Results: Patients who experienced graft re-rupture had significantly greater post-operative KT-1000 SSD compared with those without re-rupture (3.5 +/- 1.3 mm vs. 2.1 +/- 0.9 mm; p < 0.001). Increased KT-1000 SSD was independently associated with higher graft re-rupture risk (adjusted hazard ratio, 2.97 per 1-mm increase; 95% confidence interval [CI], 2.5-3.5). Higher laxity categories were associated with progressively increased re-rupture risk with adjusted hazard ratios of 1.76 (95% CI, 1.3-2.3; p < 0.001) for 3-5 mm and 8.85 (95% CI, 4.7-16.4; p < 0.001) for >5 mm compared with <3 mm. Post-operative 1-year KT-1000 anterior knee laxity was not significantly associated with return-to-play status (odds ratio, 0.84 per 1-mm increase; 95% CI, 0.6-1.1; p = 0.28), but was modestly associated with lower post-operative Tegner activity level (p < 0.001). Conclusion: KT-1000-measured post-operative anterior knee laxity at 1 year post-operatively was independently associated with the risk of subsequent graft re-rupture after HTA ACLR in athletic patients. These findings suggest that residual laxity may serve as a useful risk stratification tool, rather than a deterministic predictor of failure, and should be interpreted in the context of overall clinical and biomechanical assessment.