Biomechanics of the Bone-Ligament Interface: The Importance of the Indirect Origins and Insertions on Anterior Cruciate Ligament

After anterior cruciate ligament (ACL) reconstruction, the graft-bone interface is the mechanically weakest point, linked to complications like graft wear, rupture, and bone tunnel enlargement resulting from compression and sliding forces between the graft and the bone tunnel. A comprehensive understanding of both the macro- and microstructural characteristics, as well as the associated mechanical properties and functional behavior at the ACL-bone interface, is essential for improving surgical techniques and developing artificial grafts that restore normal biomechanical function and minimize postoperative complications in this complex system. Different functional regions can be identified at the ACL-bone interface, including the division between the anteromedial (AM) and posterolateral (PL) bundles and the differentiation between direct and indirect interfaces. While reconstruction efforts often focus on restoring the AM and PL bundles in the direct interface, the indirect interface is often overlooked, as it is often perceived to lack a direct connection to the ligament's mid-substance. Interestingly, our study revealed minimal microstructural differences between the AM and PL bundles, except for more significant bone anisotropy at the PL bundle interface. In contrast, more pronounced quantitative differences were evident between the direct and indirect interfaces. Specifically, the cortical tissue thickness and bone volume fraction (BVF) were significantly higher in the indirect region compared to the direct region. Moreover, the indirect region exhibited a sharper fiber angle and lower bone anisotropy. Tensile testing of the different interface regions demonstrated a significantly higher tensile modulus of the indirect region compared with the direct region. In contrast, no significant differences were detected between the AM and PL bundles. Finite element analysis simulating knee loading during a gait cycle revealed that the maximum stress in the indirect entheses remained consistently and significantly higher than that in the direct region throughout the gait cycle. Our findings reveal higher BVF, thicker cortical tissue layer, greater tensile modulus, and higher stress levels in the indirect interface, suggesting that it may play a more critical role in load-bearing and injury prevention. Elevated stress may promote bone regeneration to enhance the interface's resilience. Thicker and denser bone tissue and sharper fiber angles may contribute to its higher tensile modulus and better accommodation of shear forces, potentially protecting distal bone tissues from ligament tension and reducing failure risk. Based on these findings, we suggest re-evaluating reconstruction techniques to incorporate indirect ligament-bone interface reconstruction. This approach may improve the biomechanical function of the entheses and mitigate postoperative complications.