Post-natal Molecular Adaptations in Anteromedial and Posterolateral Bundles of the Ovine Anterior Cruciate Ligament: One Structure with Two Parts or Two Distinct Ligaments?

Abstract

The human anterior cruciate ligament (ACL) is a composite structure of two anatomically distinct bundles: an anteromedial (AM) and posterolateral (PL) bundles. Tendons are often used as autografts for surgical reconstruction of ACL following severe injury. However, despite successful surgical reconstruction, some people experience re-rupture and later development of osteoarthritis. Understanding the structure and molecular makeup of normal ACL is essential for its optimal replacement. Reportedly the two bundles display different tensions throughout joint motion and may be fundamentally different. This study assessed the similarities and differences in ultrastructure and molecular composition of the AM and PL bundles to test the hypothesis that the two bundles of the ACL develop unique characteristics with maturation. ACLs from nine mature and six immature sheep were compared. The bundles were examined for mRNA and protein levels of collagen types I, III, V, and VI, and two proteoglycans. The fibril diameter composition of the two bundles was examined with transmission electron microscopy. Maturation does alter the molecular and structural composition of the two bundles of ACL. Although the PL band appears to mature slower than the AM band, no significant differences were detected between the bundles in the mature animals. We thus reject our hypothesis that the two ACL bundles are distinct. The two anatomically distinct bundles of the sheep ACL can be considered as two parts of one structure at maturity and material that would result in a structure of similar functionality can be used to replace each ACL bundle in the sheep.

• anteromedial
• posterolateral
• ACL
• maturation
• collagens
• proteoglycans

Acknowledgments

We thank Leslie Jacques for her assistance with the sheep. We are also grateful to Dr. J.B. Rattner and Dr. E. Huebner for all their assistance with the TEM processing and cutting. We gratefully acknowledge the financial support provided by the Canadian Institutes for Health Research (CBF & NGS) and Alberta Innovates Health Solutions (CBF). Kyla Huebner was supported by a doctoral studentship from Alberta Innovates Health Solutions and the Canadian Institutes for Health Research and Etienne O’Brien through a post-doctoral fellowship from osteoarthritis interdisciplinary team grant.

Declaration of interest

The authors report no conflicts of interest. The authors alone are responsible for the content and writing of this article.