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Objective: To evaluate the potential for injectable, permanent bone augmentation by assessing the biocompatibility and bioactivity of subperiosteal hydroxylapatite (Radiesse) deposition in a rat model.

Methods: Fourteen adult Sprague Dawley rats were injected in the parietal skull with hydroxylapatite (n=10) or a carrier gel control (n=4), using a subperiosteal injection technique on the right and a subcutaneous injection technique on the left. At 1, 3, and 6 months, 3 rats (1 negative control, 2 variables) were sacrificed. At 12 months, the remaining 5 rats were sacrificed. After each harvest, the calvaria were examined under both light and polarized microscopy.

Results: The inflammatory response was limited in all specimens. Injectables were still present 12 months after the injection. New bone formation was only seen when the injection was located deep to a disrupted periosteum The odd of new bone formation was 48.949 times higher (95% CI (2.637, 3759.961), p = 0.002) with subperiosteal hydroxylapatite injections compared to all other combinations of injection plane and injectable.

Conclusion: This preliminary report of subperiosteal hydroxylapatite (Radiesse) injection in a rat model has verified the biocompatibility of injectable hydroxylapatite at the bony interface and suggests the potential for new bone formation.