Spatially resolved structure-function mapping of the anterior cruciate ligament-bone interface : regional variation in microarchitecture, tensile properties, and stress distribution.

Aims: To investigate the spatial distribution of structural features at the anterior cruciate ligament (ACL)-bone interface and examine their relationship with local tensile properties and stress distribution, thereby elucidating the structure-function relationship at this critical soft-hard tissue junction. Methods: High-resolution micro-CT was employed to obtain 3D imaging of 17 porcine ACL-femur interface specimens. Structural features were analyzed across different functional regions, specifically comparing the anteromedial (AM) and posterolateral (PL) bundles, as well as the direct and indirect insertion sites. Parameters assessed included macroscopic area, cortical tissue thickness, fibre angle, bone volume fraction (BVF), and degree of anisotropy (DA) in trabecular bone. Additionally, tensile tests were conducted on six porcine specimens to determine the elastic moduli of different interface regions. Finite element analysis was conducted to investigate stress distribution across the ACL-femur interface during the gait cycle. Spatial variations in structural features were then compared with local tensile properties and stress levels to elucidate the structure-function relationship. Results: Significant differences in structural features, tensile moduli, and peak stress were observed among the functional regions of the ACL-femur interface. The spatial distribution of structural features closely mirrored the patterns of mechanical properties and stress. Regions exhibiting higher tensile moduli and experiencing greater peak stress demonstrated increased cortical tissue thickness and BVF (indirect > direct). Furthermore, regions with larger fibre angles showed higher trabecular DA (PL > AM; direct > indirect). Overall, the differences between the direct and indirect regions were more pronounced than those between the AM and PL bundles. Conclusion: The mechanical function of the ACL-bone interface is strongly associated with the spatial organization of its structural features. The indirect region differs significantly from the direct region in both structural and mechanical characteristics, highlighting region-specific adaptations for load transfer at the ligament-bone junction.