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

BACKGROUND Bio-enhanced 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. HYPOTHESIS/PURPOSE The purpose of the study was to determine which histological features best correlated with the mechanical properties of the healing ACL repairs and ACL grafts in a porcine model at six and twelve months after injury. STUDY DESIGN Controlled laboratory study. METHODS Forty-eight Yucatan mini-pigs underwent ACL transection followed by: 1) conventional ACL reconstruction with bone-patellar tendon-bone (BPTB) allograft, 2) bio-enhanced ACL reconstruction with BPTB allograft using a bioactive scaffold, or 3) bio-enhanced ACL repair using the same bioactive scaffold. After 6 and 12 months of healing, structural properties of the ACL or graft (yield & 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 We found that after six months of healing, the cellular organization sub-score was most predictive of yield load (r2=0.98), maximum load (r2=0.89) and linear stiffness (r2=0.95) of the healing ACL, while at 12 months, the collagen sub-score (r2=0.68) became the best predictor of maximum load. For ACL grafts, the reverse was true, with the collagen sub-score predictive of yield and maximum loads at six months (r2=0.55), and graft cellularity predictive of maximum load of the graft at 12 months (r2=0.50). CONCLUSIONS These findings suggest there may be key biologic 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.