Return to Sport: Does 6-month Functional Testing Predict Second ACL Injuries at Long-term Follow-Up?
Authors: Marigi Erick M. , Bernard Christopher D. , Hale Rena F. , Stuart Michael J. , Levy Bruce A. , Dahm Diane L. , Hewett Timothy E. , Krych Aaron John 
Published: July 2019

Objectives: Following ACL reconstruction, objective measurements of functional testing are often incorporated into the decision-making process with regard to clearance for return to sport (RTS), in order to reduce the risk of second ACL injury. However, there is limited data to assess the predictive value of functional testing to risk of second ACL injury. Therefore, the purpose of this study was to determine whether performance on functional testing following primary ACL reconstruction can predict second ACL injuries. Methods: Patient records were retrospectively analyzed for individuals who presented with an ACL injury at a single institution from 1990-2010. A total of 335 patients who underwent primary ACL reconstruction, had functional testing at the six month ± one month time point postoperatively, and had a minimum of two year post-operative follow-up were included in this study. Functional testing included vertical jump, single leg hop, and triple leg hop. Functional testing is represented as a measure of limb symmetry and was calculated as the involved limb divided by the uninvolved limb providing a percentage relative to the uninvolved limb. Statistical analysis was performed to determine the significance of functional test limb symmetries between those with and without a secondary tear by sex, then by age. Unpaired T-tests with an alpha level less than 0.05 was performed with JMP 13 (SAS Institute Inc., Cary, NC). Group distributions as well as effect sizes were calculated. Results: 335 patients with a mean age of 25 (SD 9.76) at an average time of 9.05 years (SD 3.54) of follow-up after ACL reconstruction met inclusion criteria and underwent statistical analysis (females= 150, male= 185). Of the cohort, 53 patients (18%) experienced second tears (female= 26, male= 27) at an average time of 44.19 months (6.03- 168.4) following ACL reconstruction with 34 occurring on the contralateral side and 19 on the ipsilateral side. Results at six month post ACL reconstruction indicate a significant difference between those who experienced a second ACL injury and those who didn’t with respect to single hop limb symmetry (95% ±7% vs.90% ± 12%, P<0.01). Triple hop limb symmetry was also significantly different between those who suffered a second ACL tear and those who did not (94% ± 6% vs. 90% ±11%, P<0.01). No significant differences were found in limb symmetry of vertical jump. When data was analyzed by sex, significant differences were found in females in the single hop limb symmetry (96% ± 8%, vs. 89% ± 14%, P<0.05) as well as triple hop limb symmetry (95% ± 5% vs. 90% ± 12%, P< 0.05). No significant differences were found in vertical jump for neither females nor males. No significant differences were found when data was categorized by age or sex and age. Conclusion: Overall, at an average of 9.05 years follow-up, 18% of a young and active population of primary ACL reconstructions had a second ACL injury. Contralateral ACL injuries were observed at an increased frequency relative to subsequent ipsilateral ACL tears. Patients with greater limb symmetry on single hop or triple hop functional testing at 6 months are at an increased risk for second ACL tears. When functional testing is used for return-to-sport decisions, physicians should caution patients about the risk of subsequent ACL injury for high performing patients.