The effect of contralateral limb strength variation on limb symmetry index targets following anterior cruciate ligament reconstruction

Objectives: Limb symmetry index (LSI) strength targets and thresholds are commonly utilised within return to sport test batteries after ACL reconstruction. Contralateral limb performance may be an important factor to consider when interpreting these targets. The aim of this study was to determine how varying contralateral limb strength during rehabilitation influences limb symmetry strength targets after ACL reconstruction. Methods: This was a prospective longitudinal study of 381 included patients who had a primary ipsilateral hamstring autograft ACL reconstruction. Isokinetic strength testing was performed at the 6 and 12-month post operative timepoints. Peak extensor and flexor mechanism torque of both the operated and the contralateral limbs were calculated. Weight and adjusted LSI scores were recorded. Statistical analysis of contralateral strength and its effect on LSI and an LSI threshold of >85% was undertaken. Subgroup analysis was undertaken within groups that demonstrated no change, >10% increase and >10% decrease in contralateral strength scores over 6-12 month follow up. Results: Strength targets to achieve a Limb Symmetry Index (LSI) threshold of >85% were found to be dependent upon the variable strength of the contralateral limb. Both the operated and contralateral limb strength scores reflected a mean strength gain from 6-12 months. Over this time, the operated limb gains were, on average, greater than the contralateral limb. Consequently, the likelihood of passing an LSI threshold of 85% improved with time. Subgroup analysis revealed that despite mean cohort strength gains, a significant percentage of patients (23.1% for knee extensors and 9.4% for knee flexors) lost >10% of their contralateral strength from 6-12 months during rehabilitation. This group was the most likely to pass an LSI threshold of 85% despite the recording the lowest operative limb strengths. Conclusion: Contralateral limb strength is variable during rehabilitation from 6-12 months after ACL reconstruction. Strength scores for the contralateral limb demonstrated variation of a magnitude of >10% more commonly than <10%. In cases where strength deficits of >10% of the contralateral limb were identified, there was an aberrantly high likelihood of passing an 85% LSI threshold.