Relationship Between Quantitative MRI UTE T2* of ACL Autografts and BMI-Normalized Knee Laxity within the First Year After ACL Reconstruction

Background: The anterior cruciate ligament reconstruction (ACLR) graft undergoes a remodeling process that affects its structural properties. Ultrashort echo time T 2* (UTE-T 2* ) imaging has been instrumental in examining this process. However, more research is needed on the postoperative relationship between UTE-T 2* of the graft and its mechanical properties. Purpose: To longitudinally examine ACL graft changes after ACLR using UTE-T 2* decay coefficients and knee laxity and explore their relationship. Study Design: Case series; Level of evidence, 4. Methods: A total of 31 patients who underwent ACLR had magnetic resonance imaging of their knees at 1, 6, and 12 months after surgery using a UTE-T 2* sequence. Bilateral knee laxity was measured at 6 and 12 months using a GNRB arthrometer (force = 200 N). UTE-T 2* coefficients of the graft were calculated using mono- (T 2m* ) and biexponential (short T 2s* and long T 2l* ) analyses, and outcomes were normalized to body mass index. Linear mixed models were used to determine longitudinal changes in UTE-T 2* and laxity; the Pearson correlation was used to explore the correlations between these outcomes. Results: T 2m* of the graft increased from 1 to 6 months (Δ = 0.092; P = .008), followed by a decrease from 6 to 12 months (Δ = −0.079; P = .021). Regardless of the limb side, a decrease in laxity was detected between 6 and 12 months after surgery (Δ = −0.033; P = .046). Positive correlations between laxity and UTE-T 2* were detected at 6 months (T 2s* : R = 0.285; P = .025) and 12 months (T 2m* : R = 0.532; P < .01; T 2s* : R = 0.669; P < .001; T 2l* : R = 0.354; P = .034). Conclusion: Biexponential analysis of UTE-T 2* MRI provides a sensitive tool for detecting structural changes in the graft after ACL reconstruction, reflecting the dynamic process of graft remodeling. Among the decay coefficients assessed, T 2s* demonstrates a stronger correlation with postoperative laxity, highlighting its potential as a critical biomarker for monitoring graft integrity over time.