Association Between Early Postoperative Limb Symmetry Index Scores and 2-Year Patient-Reported Outcomes After Quadriceps Tendon Autograft ACL Reconstruction

Background: The limb symmetry index (LSI) is a measure comparing strength and functional movement between the surgical and nonsurgical limbs, used after anterior cruciate ligament reconstruction (ACLR) to guide rehabilitation and assess return-to-sport readiness. With increasing use of the quadriceps tendon autograft (QT-ACLR), limited data exist on whether the LSI score correlates with patient-reported outcome measures (PROMs). Hypothesis/Purpose: To evaluate the predictive value of postoperative strength and functional LSI tests at 3, 6, and 9 months for short-term PROMs, determine Patient Acceptable Symptom State (PASS) thresholds, and assess differences based on PASS achievement. It was hypothesized that quadriceps strength LSI score would more strongly predict 2-year PROMs than hamstring strength LSI score, and that higher 9-month isokinetic quadriceps LSI scores would be associated with higher International Knee Documentation Committee (IKDC) and Lysholm scores. Study Design: Case series; Level of evidence, 4. Methods: Patients undergoing primary QT-ACLR from 2021 to 2024 with LSI testing during rehabilitation (isometric and isokinetic lower extremity strength, Y-Balance, hop testing) and no revision or additional knee surgery were included. The LSI score was assessed at 3, 6, and 9 months, and PROMs, including Lysholm and IKDC scores, were collected at a mean follow-up of 2.0 ± 0.7 years. Multivariable linear regression was used to evaluate associations between LSI measures and PROMs after adjusting for age, sex, and body mass index (BMI). Receiver operating characteristic analysis was used to determine PASS thresholds. Results: A total of 100 knees in 100 patients (mean age, 25.3 ± 11.2 years; BMI, 25.9 ± 6.5 kg/m 2 ; 52% male) were included. Quadriceps and hamstring strength and functional performance progressively improved from 3 to 9 months, with the LSI score reaching 77% to 81% for quadriceps strength and 98% to 100% for hamstring strength by 9 months postoperatively. At a mean follow-up of 2.0 ± 0.7 years, the mean satisfaction score was 8.4 of 10, the Lysholm score was 90.7, and the IKDC score was 77.8. Linear regression, adjusted for age, sex, and BMI, demonstrated that 3-month isometric quadriceps LSI score had a strong effect on IKDC score, explaining substantial variance (β = 3.9; adjusted R 2 [Adj R 2 ] = 0.36; P < .03); 6-month isokinetic quadriceps and hamstrings showed moderate but significant effects (β = 2.9-3.7; Adj R 2 = 0.13-0.17; P < .05) on IKDC score; and 9-month fast quadriceps had a strong effect explaining substantial variance in Lysholm and IKDC scores (β = 3.9-4.7; Adj R 2 = 0.34-0.35; P ≤ .003). Lysholm and IKDC scores correlated with satisfaction and accurately discriminated PASS thresholds (area under the curve, 0.927-0.937; P < .001). Patients achieving the IKDC PASS demonstrated higher LSI scores across time points and all PROMs. Conclusion: After QT-ACLR, strength-associated LSIs were independently associated with PROMs at a mean 2-year follow-up, after adjusting for age, sex, and BMI. Early isometric quadriceps strength at 3 months predicted higher 2-year IKDC scores, with PASS achievers generally attaining a 60% to 70% LSI score by 3 months. Isokinetic quadriceps strength at 9 months predicted both IKDC and Lysholm scores. These findings highlight the importance of early quadriceps strength recovery and the value of LSI measures in predicting PROMs and informing early rehabilitation.