Changes in Cross-Sectional Area and Signal Intensity of Healing ACLs and ACL Grafts in the First Two Years After Surgery

Abstract

Objectives: The quality of a repaired anterior cruciate ligament (ACL) or reconstructed graft is typically quantified by evaluating knee, lower extremity or subject performance. However, magnetic resonance (MR) imaging of the healing ACL or graft provides a more direct measure of the tissue quality (i.e., signal intensity) and quantity (i.e., cross-sectional area). Here we used a quantitative MR imaging approach to evaluate longitudinal changes in graft size and signal intensity over the first 2 years after ACL reconstruction (ACLR). We also studied the structural changes in ACLs treated with a novel surgical repair procedure, Bridge-Enhanced ACL Repair (BEAR). We hypothesized that there would be differences in the cross-sectional area or signal intensity of the repaired ACL or reconstructed graft within 2 years after surgery, and that these structural properties correlate with knee anatomy. Methods: MR images of the patients recruited in the BEAR first-in-human trial (10 BEAR and 10 ACLR) were used to measure ACL/graft cross-sectional area and signal intensity at 3, 6, 12 and 24 months. ACL stump length, posterior slope of the medial and lateral tibial plateau, maximum depth of the medial tibial plateau, and sagittal ACL/graft elevation angle and femoral notch width were also measured from MRI. Notchplasty was defined as the difference between the post- and preoperative notch width. Results: Cross-sectional area of the grafts was 48-98% larger than the contralateral intact ACL at all time points (p<.01). The BEAR ACLs were 23-28% greater than the contralateral intact ACL at 3 and 6 months (p<.02), but similar at 12 and 24 months (Fig 1A). Both the BEAR and ACLR groups had a slightly higher signal intensity (indicative of lower tissue quality) at 3 months compared to the contralateral intact ACL, which then trended to a maximum at 6 months before reduction to the levels seen in the contralateral intact ACL at 24 months (p>.1; Fig 1B). The BEAR ACL had the same sagittal elevation angle as the contralateral intact ACL and coursed from insertion site to insertion site rather than from the femoral to tibial tunnel. The ACLR grafts coursed from tibial to femoral tunnels and were 6.5 º more vertical (p=.005) than the contralateral intact ACL. For the BEAR ACLs, the bigger notch correlated with bigger cross-sectional area, while a shorter ACL femoral stump, steeper lateral tibial slope and shallower medial tibial depth were associated with higher signal intensity (R2>.40; p<.05). Performance of notchplasty resulted in increased ACL cross-sectional area after BEAR procedure (p=.007; Fig 1). No anatomic features were correlated with ACL graft size or signal intensity. Conclusion: ACLR grafts continue to remodel over the first 2 years after surgery, whereas the remodeling of ACLs treated with the BEAR procedure occur predominantly in the first year. The BEAR procedure more closely restores the cross-sectional area and orientation in the sagittal plane than an ACL graft placed using a transtibial technique. The cross-sectional area of the repaired ACLs may be increased by performing a notchplasty at the time of surgery. [Figure: see text]