Tissue reaction and material biodegradation of a calcium sulfate/apatite biphasic bone substitute in rat muscle.

Background/objective: A biphasic ceramic bone substitute consisting of calcium sulfate and hydroxyapatite has been reported to give good clinical outcome regarding bone regeneration and may serve as a carrier for antibiotics in the treatment of bone infections. Often, the overlying muscle is in direct contact with the synthetic graft. The dissolving bone substitute induces inflammation, which may be harmful to the surrounding soft and muscle tissue. The aim of the present study was to evaluate the surrounding soft tissue reaction and the biodegradation of the biphasic bone substitute.
Methods: Rods (3 mm × 6 mm) were cast and implanted in the rat abdominal rectus muscle. The rods were either soaked or not soaked in autologous bone marrow before insertion to induce bone formation. Thirty-two rats underwent bilateral operation. After 6 weeks and 12 weeks, the bone substitute material and the surrounding muscle were harvested. The right rod was evaluated by histology to study tissue reaction and the left rod was analysed with micro-computed tomography and scanning electron microscopy to study bone substitute degradation.
Results: The muscle tissue around the material was similar at 6 weeks and 12 weeks, with or without prior treatment with bone marrow. The remaining material showed close contact with the muscle, and blood vessels penetrated the material in both groups. Wide bundles of collagen were embedded around the apatite particles, more at the 12-week time point. No bone formation was found, either at 6 weeks or 12 weeks, and scanning electron microscopy showed that the calcium sulfate phase was resorbed after 6 weeks with the calcium phosphate phase remaining intact. Micro-computed tomography showed significantly more hydroxyapatite at 6 weeks than after 12 weeks.
Conclusion: Calcium sulfate hydroxyapatite bone substitute can be used as a carrier for antibiotics or other drugs, without adverse reaction due to the fast resorption of the calcium sulfate. No bone formation was seen despite treating the bone substitute with autologous bone marrow.