The Human Anterior Cruciate Ligament: Sex Differences in Ultrastructure and Correlation with Biomechanical Properties

The purpose of this study was to investigate the existence of sex-based differences in the ultrastructural characteristics of the human anterior cruciate ligament (ACL) as the underlying cause of differences in the structural and mechanical properties between sexes. The ACL of six male and six female cadaveric donors were randomly chosen from a pool of 10 male and 10 female ACLs that had previously been tested for their structural and mechanical properties. Eighteen tissue samples from the distal, proximal, and middle sections of the anteromedial and posterolateral bundles were analyzed by transmission electron microscopy. Female ACLs exhibited both lower fibril concentration and lower percent area occupied by collagen fibrils (p < 0.05) compared to males. There was also a difference in the fibril diameters (p < 0.05); donor age, height, body mass, and body mass index contributed significantly to this difference. In females, ACL stiffness and modulus of elasticity were highly correlated to fibril concentration (r = 0.96 and 0.97, respectively); in males ACL failure load and strength were highly correlated to percent area occupied by collagen (r = 0.96 and 0.96, respectively). These differences in ultrastructure may underlie differences in ACL properties between sexes.