Cellular ultrastructure of the ruptured anterior cruciate ligament. A transmission electron microscopic and immunohistochemical study in 55 cases.

To evaluate the cellular ultrastructure following injury, we examined the anterior cruciate ligaments in 55 patients with complete tears in different phases after the injury and compared them to a control group of 39 cadaver knees. Samples were analyzed by electron microscopy, immunofluorescence, and ultramorphometry. After an invasion of inflammatory cells into the stumps of the ruptured ligaments, a marked proliferation of fibroblasts was found at the end of Phase 1 (2-3 days after the ligament injury), that was even more pronounced at the beginning of Phase II (4-17 days). These cells were initially highly metabolically active and secreted Type III collagen precursors. In Phase III (4-45 days), fibroblast degeneration occurred with increasing frequency. Furthermore, some fibroblasts showed signs of cell death. Our findings suggest that the structural alterations of the intraligamentous fibroblasts diminish their function and, consecutively, disorganization of the developing repair tissue occurs. This mechanism might contribute to the poor healing potential of the ruptured anterior cruciate ligament.