Dose-Dependent Effects of Allantoin on Bone Regeneration in Mandibular Distraction Osteogenesis

Objective: To evaluate the dose-dependent effects of locally applied allantoin on bone regeneration during mandibular distraction osteogenesis (DO) in rabbits, focusing on the microarchitecture, collagen deposition, and expression of key osteogenic and chemotactic markers. Methods: Twelve New Zealand rabbits underwent unilateral mandibular DO. Animals were randomly assigned to 4 groups receiving local injections into the distraction gap of saline (group A) or allantoin at 0.5, 1.0, or 2.0 mg/kg (groups B–D) on the surgery day and on days 6 and 15. In vivo CT was used to verify distraction and regenerate continuity. After sacrifice, mandibles were analyzed by micro-CT, histology (HE and Masson), and immunohistochemistry for RUNX2, BMP2, COL1, and CXCR4. Appropriate parametric or nonparametric tests were used ( P <0.05). Results: All animals completed the protocol without major complications. Micro-CT showed significantly higher BMD, BV/TV, Tb.Th, and Tb.N in allantoin groups versus controls, with peak values in group C (1.0 mg/kg); Tb.Sp did not differ significantly. Histomorphometry revealed maximal bone and collagen area fractions in group C, intermediate values in groups B and D, and the lowest values in group A. Immunohistochemistry demonstrated a similar gradient: RUNX2, BMP2, COL1, and CXCR4 expression levels were highest in group C, moderately increased in groups B and D, and minimal in controls. Conclusion: Local allantoin administration enhances regenerative quantity and quality in rabbit mandibular DO in a dose-dependent manner, with 1.0 mg/kg providing the most favorable improvement in trabecular microarchitecture, matrix deposition, and osteogenic/CXCR4 signaling. Higher dosing (2.0 mg/kg) confers no additional benefit, suggesting a bell-shaped dose-response and underscoring the need for dose optimization before clinical translation.