Description
Background
Knee flexion muscle strength are considered important to protect the anterior cruciate ligament (ACL) from excessive anterior translation and rotation of the tibia.
Aim
To investigate the risk of a second ACL injury in patients with a persistent knee flexion strength deficit at the time of return to sport (RTS) after ACL reconstruction with hamstring tendon autograft.
Methods
Cross-sectional data from athletes aged between 15 and 40 years, participating in knee-strenuous sports (Tegner activity scale ≥6), with a primary ACL reconstruction with hamstring tendon autograft were extracted from a rehabilitation outcome registry at the time of RTS. Results from seated isokinetic flexion strength tests (Biodex) were analyzed for limb symmetry index (LSI) and patients were divided into groups (≥90%, 80–89%, <80%) based on LSI. Athletes were subsequently followed for 24 months after ACL reconstruction to record eventual second ACL injuries. Risk hazard ratio (HR) for second ACL injury based on knee flexion strength LSI and a possible protective effect of achieving a symmetrical knee extension strength and hop performance (vertical hop, hop for distance, 30-second side hop) were calculated with a Cox proportional hazard regression model.
Results
In total 526 patients (48% women, mean age 22.4±6.2) were included and 51 (9.8%) second ACL injuries were recorded. The majority of the study population, 376 patients (72.8%) achieved LSI ≥90%, with 43 second ACL injuries (11.4%) recorded. The LSI 80–89% group included 101 patients and 4 second ACL injuries (4.0%), while the LSI <80% group included 49 patients and recorded 4 second ACL injuries (8.2%). Patients with a persistent knee flexion strength asymmetry (LSI <80%) at RTS did not demonstrate an increased risk for a second ACL injury compared to the symmetrical group (LSI ≥90) (HR = 1.06; 95% confidence interval 0.42–2.67). A Cox regression analysis adjusted for achieving symmetrical knee extension strength and hop test performance, did not reveal any difference in the risk of second ACL injury (HR = 0.90; 95% CI 0.34–2.36).
Conclusion
Persistent knee flexor strength deficits at the time of RTS are not associated with a second ACL injury.
