Hip rotation strength after anterior cruciate ligament injury: relationship to functional, symptomatic and structural outcomes

Abstract

Introduction: Hip muscle strength is often implicated in pathoaetiology and management of knee related conditions, including as part of holistic rehabilitation after anterior cruciate ligament (ACL) injury. Despite this, no studies have investigated how hip rotation strength relates to outcomes after ACL reconstruction (ACLR). This study aimed to assess the associations of hip rotation strength at 1 year post-ACLR, to function, patient reported outcomes (PROMs) and imaging outcomes at 1 and 5 years. A secondary aim was to compare hip rotation strength between the injured and non-injured sides. Methods: 111 participants 1 year following ACLR completed hip external-rotation (ER) and internal-rotation (IR) muscle strength assessment with a hand-held dynamometer (peak torque normalised to bodyweight). At 1- and 5-years post-ACLR, participants completed functional (hop testing battery, one leg rise), self-reported (Anterior Knee Pain Scale (AKPS), Knee Osteoarthritis Outcome Score) and MRI and radiographic assessment. Cartilage health of the patellofemoral and tibiofemoral compartments was assessed using the semiquantitative MRI Osteoarthritis Knee Score. Regression models (controlling for age and sex) assessed the relationships between hip rotation strength at 1 year post-ACLR and functional tests, patient reported outcomes and cartilage findings at 1 and 5 years. Results: The injured side had significantly weaker hip ER strength compared to the non-injured side (p=0.014, effect size (ES) =-0.33 (95%CI -0.60 to -0.07). There was no difference between sides for hip IR strength (p=0.411, ES = -0.11 (95%CI -0.37 to 0.15). Weaker hip ER and IR strength was significantly associated with lower performance on all functional tests and worse AKPS scores at 1 and 5 years. Lower hip ER strength was significantly associated with worsening tibiofemoral cartilage lesions at 5 years (p=0.017, Odds Ratio 0.01 (95%CI 0.00 to 0.41). Hip strength was not associated with any patellofemoral imaging outcomes. Discussion: Hip rotation strength may play a role in worsening knee function, symptoms and tibiofemoral cartilage health after ACLR, though further prognostic studies are needed. Hip ER strength of 0.1 Nm/kg higher resulted in 0.63 times lower odds of tibiofemoral cartilage worsening at 5 years. The hip rotator muscles may influence outcomes given their vital role in single leg propulsion and force absorption. Associations to PROMs and structure may also reflect global reductions in lower extremity strength, or reduced activity levels and increased BMI observed after ACL injury. A link between hip strength and patellofemoral outcomes is less clear from our findings. Conflict of interest statement: My co-authors and I acknowledge that we have no conflicts of interest of relevance to the submission of this abstract.