An anatomic and histologic study of the origin and terminal points in the anterior and posterior cruciate ligaments in rats

Abstract

Background: It remains unclear why the anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL) are most commonly torn at their proximal and distal attachments, respectively. This animal study was designed to evaluate the difference in tissue structure between the origins and terminal attachments of the ACL and PCL. Methods: Knee samples from 30 rats were assessed anatomically, histologically, and morphologically to determine differences in tissue structure. Results: The ACL and PCL originate in the epiphyseal plates of the distal femur and proximal tibia, respectively, and they terminate as fibrous tissue into the tibial and femoral bones, respectively. The cruciate ligaments are constructed mostly of collagen, but at their origins the collagen is directly connected to the epiphyseal plate and less so to the bone. At the terminal attachments, the collagen is more fibrous and grows directly into bone tissue. The morphology of the cells at the origins show heterogeneity whereas homogeneity is seen at the terminal attachments. The terminal attachment of the cruciate ligament has more fibrous bundles that contains more fibroblasts than the origin structure that has less fibrous tissue and contains more chondrocytes (P<0.01). Conclusions: At the ACL and PCL origins, the collagen fibers are directly connected to the epiphyseal plate and less so to bone. In contrast, at the terminal attachments, the collagen is directly connected to bone and actually becomes bone tissue, thereby providing secure adherence. This might explain why ACL and PCL cruciate ligament tears more often occur closer to their sites of origin.