Morphological study of the anterior cruciate ligament and its tibial insertion in the human knee joint

Purpose We investigated whether ligament-derived collagen fibrils traversing the cartilage/fibrocartilage region toward the tibial bone could be visualized using SEM. Methods ACL specimens were obtained from four patients with ACL rupture (aged 16–49 years) and three older adults (aged 60–75 years) with osteoarthritis. Specimens were fixed and examined under light microscopy (LM; sections stained with light green and aldehyde fuchsin), transmission electron microscopy (TEM; modified tannic acid method), and scanning electron microscopy (SEM; NaOH-treated blocks). Results The central portion of the ACL consisted of parallel-arranged collagen fibers, with thread-like elastic fibers stained by aldehyde fuchsin sparsely distributed among them. SEM following NaOH treatment revealed collagen fibrils (diameter ∼120–140 nm). Elastic fibers were also scattered among collagen bundles on SEM. On TEM, electron-dense structures consistent with elastic fibers (diameter ∼600 nm) were identified. At the tibial insertion, LM demonstrated that the ACL branched within the cartilage and attached to the tibia. SEM enabled discrimination between the bundled ACL collagen fibrils and the finer collagen fibrils (diameter ∼80 nm) surrounding chondrocytes. Conclusion We histologically demonstrated that branches of the human ACL pass through the joint and attach to the tibia. Ligament-derived collagen fibrils were observed branching and descending through the cartilage/fibrocartilage region toward the tibial bone.