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Our study was aimed to advance the currently limited knowledge about differences in the biological remodeling of free soft-tissue tendon allografts and autografts for ACL reconstruction. Allogenic and autologous ACL reconstructions were performed in a sheep model using the flexor digitalis superficialis tendon. After 6, 12 and 52 weeks the animals were sacrificed. We analyzed the collagen crimp formation and its relationship to expression of contractile myofibroblasts in both graft types. Additionally, structural properties and ap-laxity were compared during biomechanical testing. At 6 weeks only descriptive differences were found between autografts and allografts with a more organized crimp pattern and myofibroblast distribution in autografts. Significant differences in myofibroblast density and crimp formation were found after 12 weeks. At these early stages, the progress of remodeling in autografts was more advanced toward the central areas than in allografts. At 1 year, grafts in both study groups returned to an ACL-similar structure. Structural properties and ap-laxity did not vary significantly between auto- and allografts at early healing stages. However, at 52 weeks, failure loads, stiffness and ap-drawer test showed superior values for autograft ACL reconstruction. Extracellular remodeling of allografts develops slower than in autografts. Therefore, rehabilitation procedures will have to be adapted according to graft and patient selection. Postoperative treatment regimens from autograft primary ACL reconstruction should not be directly transferred to allograft ACL reconstructions.

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