Estimated ACL Mechanics During A Drop Vertical Jump Preceding A Secondary ACL Injury

Following an anterior cruciate ligament reconstruction (ACLR), a secondary ACL injury in either the ipsilateral or contralateral ACL has been reported in as many as 30% of patients. However, ACL length measurements during movement in these cases have not been well studied. Recent advances in kinematics-based ACL modeling can provide detailed estimates of these mechanics post-ACLR which could inform secondary injury prevention strategies. PURPOSE: To estimate ACL mechanics after ACLR during a drop vertical jump (DVJ) preceding a secondary ACL injury. METHODS: Three subjects (2 male, 1 female; 1 ipsilateral, 2 contralateral second tear; 16.1 ± 0.2 yrs; 58.1 ± 9.2 kg; 1.7 ± 0.1 m) out of a large registry of ACLR subjects experienced a second ACL tear. These subjects performed three DVJs at 6 and 9 months post-ACLR, with the second tear occurring within 12 months post-ACLR. Knee kinematics and kinetics were calculated and applied to a knee model to estimate ACL length and slack length. Maximum knee abduction moment and ACL strain were evaluated during the initial landing phase of the DVJ. RESULTS: Prior to a secondary ACL injury, all subjects increased their maximum knee abduction moment (0.15 ± 0.20 Nm/kg) and ACL strain (0.34 ± 0.33%) on the side of the second tear from 6 to 9 months post-ACLR. When the tear occurred ipsilateral to the one involved in the ACLR, there was a lesser increase in knee abduction moment (0.02 Nm/kg) but a greater increase in ACL strain (0.72%) relative to those torn contralateral to the involved knee (0.21 Nm/kg and 0.16%). CONCLUSION: Increased knee abduction moment and ACL strain post-ACLR may be early indicators of a secondary ACL injury, demonstrating the greater insight provided with individualized knee modeling. Future work will compare these subjects to those with successful ACLR recoveries beyond 9 months to better identify early predictors of secondary injuries and establish prevention measures.