Histological Predictors of Maximum Failure Loads Differ Between the Healing ACL and ACL Grafts After 6 and 12 Months In Vivo

BACKGROUND: Bioenhanced anterior cruciate ligament (ACL) repair, where the suture repair is supplemented with a biological scaffold, is a promising novel technique to stimulate healing after ACL rupture. However, the histological properties of a successfully healing ACL and how they relate to the mechanical properties have not been fully described. PURPOSE: To determine which histological features best correlate with the mechanical properties of the healing ACL repairs and ACL grafts in a porcine model at 6 and 12 months after injury. STUDY DESIGN: Controlled laboratory study. METHODS: A total of 48 Yucatan mini-pigs underwent ACL transection followed by: (1) conventional ACL reconstruction with bone–patellar tendon–bone (BPTB) allograft, (2) bioenhanced ACL reconstruction with BPTB allograft using a bioactive scaffold, or (3) bioenhanced ACL repair using the same bioactive scaffold. After 6 and 12 months of healing, structural properties of the ACL or graft (yield and failure load, linear stiffness) were measured. Following mechanical testing, ACL specimens were histologically analyzed for cell and vascular density and qualitatively assessed using the advanced Ligament Maturity Index. RESULTS: After 6 months of healing, the cellular organization subscore was most predictive of yield load (r (2) = 0.98), maximum load (r (2) = 0.89), and linear stiffness (r (2) = 0.95) of the healing ACL, while at 12 months, the collagen subscore (r (2) = 0.68) became the best predictor of maximum load. For ACL grafts, the reverse was true, with the collagen subscore predictive of yield and maximum loads at 6 months (r (2) = 0.55) and graft cellularity predictive of maximum load of the graft at 12 months (r (2) = 0.50). CONCLUSION: These findings suggest there may be key biological differences in development and maintenance of ACL tissue after repair or reconstruction, with early ligament function dependent on cellular population of the repair but early graft function dependent on the maintenance of organized collagen. CLINICAL RELEVANCE: Bioenhanced ACL repair shows promising potential as an alternative clinical treatment for ACL injury. This study contributes to the understanding of the cellular contribution to mechanical characteristics of the healing ACL in both repaired and reconstructed ACLs.