Repair process after fibrocartilaginous enthesis drilling: Histological study in a rabbit model

Background: Disorders of the enthesis are often a consequence of sports injuries. However, there is uncertainty regarding the process of mechanical stress-related injuries at the enthesis and the subsequent repair process of the injured tissues. To elucidate the repair process of the fibrocartilaginous enthesis, we studied the repair of injured fibrocartilaginous enthesis and the morphological characteristics of the repaired tissue. Methods: We drilled 0.5-mm holes in the right tibial insertion of the patellar tendon of Japanese white rabbits, with their own left sides serving as controls. Specimens harvested at 1, 2, 4, 6, 8, and 12 weeks were examined histologically. Morphologically, the ratios of calcified fibrocartilage-bone interface lengths to enthesial lengths were compared between the control and surgical groups. Results: Repair initiation was observed in the deep bone layer at 1 week, with remarkable progress at 2 weeks. Repair at the enthesis and neoosteogenesis in deep bone layers were detected at 4 weeks, and the drill hole disappeared at 6 weeks. The tendon was partially invaded by fibrocartilage-covered chondroid bone at 8 weeks, and regenerated fibrocartilaginous enthesis and increased calcified fibrocartilage-bone interface irregularity was identified at 12 weeks. The ratios of calcified fibrocartilage-bone interface lengths to enthesial lengths were significantly greater in the surgical group than in the control group. Conclusions: Repair progressed from bone to fibrocartilage and ended at fibrous tissue. Cancellous bone disruption triggered repair in all layers. Removal of the subchondral plate enabled infiltration of nutrients via blood vessels, with the underlying bone acting as a scaffold for the regenerating fibrocartilage.