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Should we rely on the limb symmetry index to evaluate functional performance after anterior cruciate ligament injury and reconstruction?

      Introduction: A limb symmetry index (LSI) >90% on hop tests is a widely advocated benchmark for functional recovery and return-to-sport clearance after ACL injury. Yet, the LSI assumes the contralateral limb is the “gold standard” and immune to decline. Due to the bilateral functional deficits evident after ACL injury and ACLR, other assessment methods such as comparing performance of both limbs to age, sex and activity-level matched uninjured peers may be required. Our primary aim was to describe the functional performance changes in the ACL-injured and contralateral limbs 1- to 5-years post-ACL reconstruction (ACLR) to determine the influence on LSI. Our secondary aim was to compare the ACLR group results to uninjured healthy controls.
      Methods: 59 participants (38 men) aged 29±16 years completed three hop-tests and a one-leg rise 1- and 5-years post-ACLR. Mean(±SD) scores for the ACLR and contralateral limbs, and LSI at 1- and 5-years were reported. Linear mixed-effects models evaluated the difference in change between the ACLR and contralateral limbs. Participants were classified with stable, improving or worsening function relative to previously published minimal detectable change thresholds. Healthy controls completed the hop-tests (n=41) and one-leg rise (n=31) at a single time-point to provide reference data. Linear regression models (adjusted for age and body mass index), assessed the differences in functional performance at 1- and 5-years between the ACLR group and the healthy control group.
      Results: The contralateral limb had a significantly (p<0.05) greater decrease in functional performance between 1- and 5-years for the three hop-tests, compared to the ACLR limb. Worsening was more common in the contralateral limb than the ACLR limb; resulting in significant improvements in the LSI for the single-hop (mean 87% at 1-year to 95% at 5-years), side hop (77% to 86%) and one-leg rise (76% to 85%). The LSI on all four tests at 1-year post-ACLR was significantly lower than healthy controls, but did not generally differ between groups at 5-years.
      Discussion: Although the LSI significantly improved between 1- and 5-years post-ACLR, this was mostly due to worsening function in the contralateral limb. The LSI should not be used in isolation to evaluate functional performance changes after ACLR, as it may overestimate functional improvement. Exercise-based interventions may need to continue beyond the typical rehabilitation period of 6-12 months to improve or maintain function in both limbs, considering the deficits at 1-year post-ACLR compared to healthy controls, and minimal improvement over the proceeding 4-years.
      Conflict of Interest: My co-authors and I acknowledge that we have no conflict of interest of relevance to the submission of this abstract