Immobilization management of acute phase increases healing ligament strength

Conservative treatment of the anterior cruciate ligament (ACL) is important for restoring functional activity and preventing secondary degeneration. However, the molecular mechanisms underlying ligament immobilization and its precise role in the healing process remain poorly understood. In this study, we investigated the effect of immobilization on the strength of the healed ACL during acute management. We performed surgery to heal the ACL of rats and immobilized the knees using Kirschner wires. The group in which only the surgery to promote ACL healing was performed was designated as the controlled anterior tibial instability group, whereas the group that underwent both surgery and immobilization was designated as the immobilization (IMM) group. After 1-2 weeks of immobilization, histological analyses using hematoxylin-eosin staining and immunohistochemical evaluation of collagen types I and III expression were performed. A comprehensive genetic analysis in the acute phase was performed via RNA sequencing. Furthermore, fibroblasts derived from rat ACL were used to recapitulate inflammation with interleukin-1β, and its effect on elongation stress (110%) was investigated using polymerase chain reaction. Joint immobilization for 2 weeks postoperatively increased the mechanical strength of the conservatively connected ligaments. Stretch stimulation of fibroblasts with interleukin-1β also decreased the expression of the extracellular matrix. Furthermore, bioinformatics analyses identified differentially expressed genes associated with the healing process in fixed versus unfixed ligaments. The results demonstrate that acute-phase immobilization, defined as fixation for 2 weeks following injury, enhances ligament strength by promoting extracellular matrix synthesis and organized regeneration, providing novel insights into optimizing conservative ACL therapy.