Variation in area proportion and mechanical properties between different subregions of ACL insertion: An in vitro biomechanical study in a porcine model.

Purpose: To quantitatively evaluate differences in area proportions and Young's modulus among anatomical subregions of the anterior cruciate ligament (ACL) insertion, including direct and indirect insertions as well as anteromedial (AM) and posterolateral (PL) bundles, and to assess their impact on local stress distribution. Methods: Micro-CT was utilized to quantify the area proportions of four anatomically defined subregions of porcine ACL insertions (n = 6). Mechanical testing was conducted to assess the Young's modulus of each region (n = 6). Finite element analysis was performed to evaluate how variations in regional area proportions (30%, 50% and 70% for the direct insertion) and mechanical properties (homogeneous vs. heterogeneous distributions) influence load distribution at the insertion. Results: The direct region occupied a significantly larger area proportion than the indirect region (54% vs. 46%, p < 0.01), and the AM bundle covered a significantly greater area than the PL bundle (65% vs. 35%, p < 0.01). The indirect region demonstrated a significantly higher Young's modulus than the direct region (12.0 vs. 6.8 MPa, p < 0.01), while no significant difference was observed between AM and PL bundles (8.4 vs. 10.4 MPa). Finite element results indicated that stress distribution at the insertion became more uniform when the direct and indirect regions had comparable area proportions, and incorporating regional heterogeneity in mechanical properties resulted in increased force transmission through the indirect region. Conclusions: Distinct regional differences in area proportions and Young's moduli were found at the ACL insertion, and these characteristics substantially affect local stress distribution. Level of Evidence: Level N/A.