Student Saint Louis University St. Louis, Missouri, United States
Abstract Body : The human anterior cruciate ligament (ACL) plays an important role in knee flexion and knee rotation. Most commonly, the ACL is described as having two distinct anatomical bundles: anteromedial and posterolateral. These bundles have been observed in a gross anatomical setting, but not as closely observed histologically. In this study, we sought to determine whether gross anatomical observations of bundle number in humans are supported by histological observations.
Nineteen knees were isolated via transverse sections through the thigh and leg. The knees were obtained from cadavers received through the Saint Louis University Gift Body Program of the Center for Anatomical Science and Education (CASE) with signed, informed consent from the donor. The knee joints were dissected to expose in the intracapsular ligaments, and menisci. The ACL was further cleaned to remove the synovial sheath allowing for clear visualization of fiber orientation. To establish bundle number visible via gross observation, three individuals assessed bundle number and an inter-rater agreement was calculated. ACLs were then isolated from the knees, processed in paraffin wax, sectioned at 10 micrometers, and stained with picrosirius red. Sections were visualized under light microscopy and polarized light microscopy.
Prevalence of a double-bundle ACL under gross observation was 60%. Inter-rater agreement was fair (Fleiss’s kappa = 0.202). Under histological observation, we observed that numerous connective tissue septa were present, but few could be said to divide the ligament into distinct bundles. This was especially true proximally. Additionally, individual collagen fiber projections could be appreciated that corresponded with bundle divisions as visualized grossly, but did not correspond to connective tissue septa.
These results suggest that the human ACL may exhibit more anatomical variation than has been previously appreciated in human cadaveric studies. Inter-rater agreement was fair, indicating that the demarcation of ACL bundles is not an obvious feature, even under gross observation. Additionally, our histological observations support the idea that ACL bundles exhibit substantial variation in terms of degree of distinct separation. This is, to our knowledge, the first study that has directly compared gross and microscopic observations of ACL bundle number.
