Anatomical reassessment of the rat cortical drill-hole injury model

The drill-hole injury model is used to evaluate the efficacy of interventions aimed at enhancing fracture healing and supporting clinical application. However, despite its frequent use, the detailed anatomical understanding required for accurate assessment of bone repair remains limited. This study aimed to conduct morphological and histological analyses of cortical bone healing following drill-hole injury in rat tibiae, providing fundamental anatomical insights to support future use of this model in translational research. A 2.0-mm drill-hole injury was made bilaterally in rat proximal tibiae of rats. The rats were euthanized at predetermined time points, and the tibiae were collected for analysis. The outer surface and longitudinal and transverse sections of the defect were examined. For cross-sectional evaluation, the drill-hole injury was divided into defect and intramedullary regions. By day 5, a transition from inflammation to bone formation was observed. New bone was visible on the intramedullary region inner surface by day 7, expanding toward the defect region and narrowing progressively. Bone remodeling began in the intramedullary region after day 10, while bone formation in the defect region continued. By day 28, surface closure was macroscopically apparent on scanning electron microscopy images, but histology showed incomplete internal architecture and bone quality. Accurate evaluation of the healing process in the drill-hole injury model requires a three-dimensional perspective and the incorporation of morphological and histological analyses. Such anatomical insights may have been overlooked previously and provide critical baseline data for precise bone-healing assessment in future translational research using this model. • Drill-hole healing in rat tibiae was evaluated morphologically and histologically. • Bone formation progresses from the intramedullary region toward the defect region. • Surface closure does not reflect complete internal healing at the tissue level. • The findings refine existing views on healing in the drill-hole injury model.