Questioning the rules of engagement: a critical analysis of the use of limb symmetry index for safe return to sport after anterior cruciate ligament reconstruction

Objective To evaluate the association between limb symmetry index (LSI) in quadriceps and hamstrings strength together with hop tests, as a proxy of recovery, and the deviation from being symmetrical (LSI 100%), with a safe return to sport (RTS) after anterior cruciate ligament reconstruction (ACL-R). Methods Athletes between 15 and 30 years old with a preinjury Tegner activity level ≥6 were eligible for inclusion. Data were extracted from a rehabilitation-specific registry, Project ACL (Gothenburg, Sweden) at the time of or after RTS for each athlete. The outcome of interest in this study was a safe RTS—not suffering a second ACL injury within 2 years from RTS—addressed by using the LSI in five different ways with results from the test battery which include five tests of muscle function at or after the time of RTS. Logistic regression analyses were performed with safe RTS as a dependent variable and presented with ORs and 95% CIs. Results In total, 233 athletes (51.1% women) were included. The best-differentiating cut-offs for strength and hop tests showed non-significantly poor discriminatory ability between athletes who had a safe RTS and those who did not (Youden J 0.09–0.24 and area under the curve 0.50–0.59). Athletes who had ≥80% or ≥85% LSI had significantly lower odds of safe RTS compared with athletes who did not meet the cut-offs of safe RTS (OR=0.32 (95% CI 0.12 to 0.87) and OR=0.39 (95% CI 0.18 to 0.84), respectively). There was no effect of a 1% increase in LSI or deviation from 100% symmetry on safe RTS. Conclusion The use of LSI from tests of muscle function to determine safe RTS after ACL-R, that is, RTS without sustaining a second ACL injury within 2 years, cannot differentiate between athletes who had a safe RTS and those who did not, regardless of whether LSI was used as cut-offs, incremental or as deviation from symmetry. Thus, it is of clinical importance that clinicians do not solely rely on the LSI to clear athletes for RTS. Data are available on reasonable request. Data can be obtained on request due to privacy reasons and ethical statements.