Fresh-frozen free-tendon allografts versus autografts in anterior cruciate ligament reconstruction: delayed remodeling and inferior mechanical function during long-term healing in sheep.

The objective of this study was to investigate the biologic healing and restoration of the mechanical function of a free soft tissue autograft and compare these to an identical nonsterilized fresh frozen allograft for anterior cruciate ligament (ACL) reconstruction in an in vivo sheep model. Forty-eight merino sheep received either an allograft or autograft ACL reconstruction with a long flexor tendon. Each group was analyzed at 6, 12, and 52 weeks for descriptive analysis of histologic changes and quantitative analysis of recellularization, revascularization, and mechanical function. Recellularization and revascularization was significantly delayed at 6 and 12 weeks of healing, while at 52 weeks, differences had become less distinct. Overall remodeling had not been completed compared to the intact ACL. Significantly lower structural and mechanical properties and anterior-posterior laxity were found at 52 weeks for allografts, with no differences at the early healing time points. Allograft remodeling is delayed in ACL reconstruction and resulted in reduced long-term stability and mechanical function compared to autologous ACL reconstruction. Caution should be used with early full-weight bearing after allograft ACL reconstruction, because remodeling is delayed and long-term stability might be affected. Clinical studies should be warranted to examine the impact of varying rehabilitation protocols on long-term outcome.