Structural and Molecular Differences in the Epiligament of the Acl and McL: Insights Into Their Divergent Healing Capacities

The anterior cruciate ligament (ACL) and medial collateral ligament (MCL) are essential stabilizers of the knee joint. While ACL injuries frequently necessitate surgical reconstruction due to poor intrinsic healing, the MCL demonstrates superior self-repair capacity. The epiligament (EL) theory suggests that this differential healing potential may be attributed to variations in the EL, a vascularized connective tissue layer enveloping the ligaments. The EL is a crucial reservoir of cells, vessels, and signaling molecules essential for ligament repair. This study compares the EL of the proximal and distal parts of the ACL to that of the MCL to determine whether structural and molecular differences in the EL might explain the superior healing potential of the MCL. Samples were obtained from the proximal and distal portions of the ACL and MCL in fresh human cadaveric knee joints. Histological and immunohistochemical analyses assessed cellular density and the expression of key markers involved in ligament healing: CD34, α-smooth muscle actin (α-SMA), and vascular endothelial growth factor (VEGF). Image-based quantification was performed using ImageJ software. The EL of the MCL contained a significantly higher cell density compared to the ACL EL, particularly in the distal segment. The MCL EL also showed more robust α-SMA expression, particularly in its distal portion, suggesting greater myofibroblast involvement in healing. In contrast, α-SMA expression was weaker in the ACL EL, with the proximal part exhibiting slightly higher levels than the distal part. CD34 expression was more pronounced in the distal ACL EL but remained lower overall compared to the MCL EL. VEGF expression was present in the EL of both ligaments but showed no significant differences between the proximal and distal parts. The current study thoroughly compares the EL in both the proximal and distal regions of the ACL and the MCL. It highlights significant differences in the expression of CD34 and α-SMA between the EL of these two ligaments. These findings offer new insights into the mechanisms of ligament healing and may lead to improved strategies for enhancing ACL repair. This study is financed by the European Union-NextGenerationEU, through the National Recovery and Resilience Plan of the Republic of Bulgaria, project BG-RRP-2.004-0004-C01 “Strategic research and innovation program for development of Medical university - Sofia