ACL Graft Signal Characteristics Measured by Quantitative MRI Are Significantly Correlated with Patient Reported Outcomes Two-Years after Hamstring Autograft ACL Reconstruction

Abstract

Objectives: Following surgical reconstruction of the anterior cruciate ligament (ACL), the tendon graft undergoes a remodeling process of ligamentization. Collagen within the ACL graft becomes organized along the long-axis of the ligament and the proteoglycan content increases. Quantitative imaging sequences, specifically T2 and T1rho, allow for a non-invasive assessment of collagen orientation and proteoglycan content, respectively. The purpose of this study was to investigate the longitudinal progression of T2 and T1rho relaxation times of the graft after ACL reconstruction and the relationship between these quantitative imaging markers and patient-reported outcome measures. We hypothesized that T2 and T1rho would decrease significantly over time, reflecting improved collagen organization and ncreased proteoglycan content, and that T2 and T1rho relaxation times would be inversely correlated with Knee Osteoarthritis Outcome Scores (KOOS). Methods: Thirty-two patients (mean age 29.4±8.2 years, 12 females) were followed prospectively after arthroscopic ACL reconstruction with hamstring autograft. Patients provided informed consent and all procedures were IRB-approved. Exclusion criteria included allograft reconstruction, other ligamentous injury, meniscal tear requiring repair, prior knee surgery, or history of arthritis. Post-operative MR imaging was obtained for 31 patients at 6 months, 30 patients at 12 months, 26 patients at 24 months, and 23 patients at 36 months after surgery. T2 and T1rho relaxation times were simultaneously acquired with a combined 3D sagittal sequence using a 3 T MR system (GE Healthcare) with a 1Tx/8Rx knee coil. The intra-articular ACL graft was identified on a fat-suppressed sagittal high-resolution 3D fast spin echo images and manually segmented. The T2 and T1rho relaxation times for the graft were recorded. Patients completed the KOOS at each imaging time point. Repeated measures analysis of variance (ANOVA) tests with Tukey corrections were used to compare T2 and T1rho relaxation times between time points. The relationship between KOOS and T2 and T1rho values at 2 years post-operative was investigated with Spearman’s rank correlation. Significance was defined as p<0.05. Results: The T2 relaxation times of the ACL graft were significantly higher at 6 months relative to 12 months (p<0.001), 24 months (p<0.001,) and 36 months (p<0.001) after ACL reconstruction, as well as significantly higher at 12 months relative to 36 months (p<0.001) (Figure 1A). The T1rho relaxation times of the ACL graft were significantly higher at 6 months relative to 12 months (p<0.001), 24 months (p<0.001), and 36 months (p<0.001) (Figure 1B). The two-year T2 relaxation times and T1rho relaxation times were significantly correlated with the KOOS Sports, Pain, Symptoms, and Activities of Daily Living sub-scores. Conclusion: We observed significant changes in the tendon graft over time following ACL reconstruction with hamstring autograft, reflecting a higher concentration of proteoglycan and more longitudinally-organized collagen structure over time. Improved collagen organization (lower T2 values) and increased proteoglycan content (lower T1rho values) were correlated with better outcomes based on KOOS scores. Quantitative T2 and T1rho relaxation times of ACL graft may offer a non-invasive method for monitoring graft maturation that correlates with patient-reported knee function after ACL reconstruction. [Figure: see text][Table: see text]