Persistent isokinetic knee flexion strength deficits at the time of return to sport are not associated with a second ACL injury.

Abstract

Purpose: To investigate the rate of a second anterior cruciate ligament (ACL) injury based on different levels of knee flexion strength limb symmetry index (LSI) at the time of return to sport (RTS) after ACL reconstruction with hamstring tendon autograft.

Methods: Data was extracted from a rehabilitation registry for patients aged 15-40 years, who participated in knee-strenuous sports pre-injury (Tegner ≥ 6) and underwent ACL reconstruction with hamstring tendon autograft. Isokinetic knee flexion strength was analysed and reported as LSI. Patients were categorised into three groups (≥90%, 80%-89.9% and <80%) based on their LSI at reported time of RTS. Patients were followed for 2 years after ACL reconstruction to record a second ACL injury, and hazard ratios (HR) were calculated using a Cox proportional hazards model.

Results: A total of 526 patients (48% female, mean age 22 ± 6) were included, with 51 (9.7%) second ACL injuries recorded within 2 years after ACL reconstruction. Among patients with LSI ≥ 90% (71%), 43 second ACL injuries (11.0%) occurred. The LSI 80%-89.9% group had 4 second ACL injuries (4.0%), and the LSI < 80% group had four injuries (8.2%). Persistent knee flexion strength asymmetry did not significantly influence the hazard of a second ACL injury. The LSI 80%-89.9% group had a lower hazard (HR 0.34, confidence interval [CI]: 0.12-0.94), while the LSI < 80% group showed no significant difference (HR 0.70, CI: 0.25-1.97) compared with the LSI ≥ 90% group.

Conclusion: Persistent isokinetic concentric knee flexion strength asymmetry at RTS were not associated with a second ACL injury.

Level of evidence: Level III.