Microstructure Variations in the Soft-Hard Tissue Junction of the Human Anterior Cruciate Ligament

The role of the sub-bundles in the anterior cruciate ligament (ACL) has been defined, such that the anterior-medial bundle directly resists anterior tibial translation while the posterior lateral bundle is involved in rotational stability. With regards to this biomechanical function, much of the previous work on bundle-specific morphology has been carried out on the macroscale, with much less attention given to the micro-to-ultrastructural scalar levels. This is especially true of the enthesis and its microstructure, a biomechanically significant region that has been largely neglected in the published literature dealing with ACL sub-bundle anatomy. In this study, the human ACL tibial enthesis was investigated at multiple scalar levels using differential interference contrast and scanning electron microscopies with the aim of determining whether the sub-bundle ligament structure, and its known macroscale function, is consistent with its micro-architecture at the ligament-bone junction. The investigation found that different ligament insertion morphologies exist between the two bundles, where the AM bundle has more intense interdigitation with the bone matrix than that of the PL bundle. The results suggest that such structure-function relationships, especially across scalar-levels, provide new insight into the significance of the sub-bundle anatomy of the ACL. Anat Rec, 300:1547-1559, 2017. © 2017 Wiley Periodicals, Inc.