Histomorphometric analysis of anterior cruciate ligament bundles and anatomical insights into injury mechanisms

The anterior cruciate ligament consists of two bundles, the anteromedial and posterolateral bundles, which are frequently associated with meniscal dysfunction. Despite previous studies investigating the relationship between biomechanical instability and injury, a comprehensive histological analysis of the anatomical aspects contributing to injury and degenerative changes and the structural connectivity between the anterior cruciate ligament and the meniscus is lacking. Masson's trichrome, pentachrome, Safranin-O, and modified Verhoeff-Van Gieson histological stains and microcomputed tomography were used in this analysis. The anteromedial bundle of the anterior cruciate ligament is tightly connected to the medial meniscus via articular cartilage, whereas the posterolateral bundle is loosely connected to the transition zone of the lateral meniscus via connective tissue. Due to the differences in the structural connectivity between the meniscus and each anterior cruciate ligament bundle, the degree of deformation of the space between the two bundles varies significantly with knee flexion angle. Furthermore, the two bundles exhibit histological differences in the ratio of elastic fibers to collagen at regions. Specifically, the ratios of the upper and lower parts were 11.36 +/- 0.90% and 4.87 +/- 0.34%, respectively, for the anteromedial bundle, and 10.33 +/- 0.37% and 5.32 +/- 0.78%, respectively, for the posterolateral bundle.