Limb Stiffness Is Lower In Those Who Rupture Contralateral Non-ACLR Knee Post Primary ACL Surgery

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PURPOSE: The aim of this study was to identify biomechanical differences during jump and CoD testing 9 months after ACL reconstruction (ACLR) between those that went on to injure their non-ACLR limb and those that did not at 2 year follow up. METHODS: Nine months after ACLR, 105 males athletes who were returning to pre-injury sport participation (55 going on to suffer contralateral ACL injury/60 matched subjects with no second injury at 2 years post op) were prospectively assessed using isokinetic strength testing and 3D biomechanical analysis of double leg drop jump (DLDJ), single leg drop jump (SLDJ) and planned and unplanned 90° change of direction (CoD) as well as an IKDC questionnaire. Differences in IKDC, strength and jump height measures on the non-ACLR side and in limb symmetry index between groups were analysed with statistical parametric mapping (SPM, 0D unpaired t-test). Biomechanical differences in the jump and CoD tests on the non-ACLR side and in symmetry between groups was analysed with SPM (1d, unpaired t-test). Effect size was calculated using Cohen’s D for all analyses. RESULTS: There was no difference in IKDC score between groups. There was a small effect size difference in quadriceps strength on the non-ACLR side in the group that went on to injure that limb (ES 0.39) with no difference in LSI or across the hamstring strength and jump height tests. There was no difference in measures of biomechanical symmetry between groups for any of the tests and no difference on the non-ACLR side for the CoD tests. There were large effect size differences on the non-ACLR side in the SLDJ and DLDJ for COM stiffness (ES 0.78 & 0.82), ground contact time (ES 0.73 & 0.85), lowering of COM to ankle and knee (ES 0.7 to 0.81), vertical ground reaction force (ES 0.63 to 0.78) representing less stiffness on the non-ACLR side in the group that went on to injure non-ACLR limb after return to sport. CONCLUSION: This study demonstrated clear biomechanical differences on the non-ACLR side in those that went on to injure the ACL in the limb despite little difference in strength and jump measures and measures of symmetry. These results suggest plyometric exercises to improve limb stiffness may be important to reduced injury risk to the non-ACLR limb after primary ACLR.