Comparison of Two Moldable Calcium Phosphate-Based Bone Graft Materials in a Noninstrumented Canine Interspinous Implantation Model

There is a continuing search for novel synthetic materials as an alternative to autologous bone grafting. Different technologies are explored to promote bone formation, which include the addition of BioGlass particles in calcium phosphate (CaP)-based materials and the use of surface modification in the form of submicron surface topographies. In this work, we aimed at comparing the bone formation in a noninstrumented canine interspinous model of moldable formulations of a submicron-surface structured tricalcium phosphate/alkylene oxide copolymer (CaP/AOC) or a tricalcium phosphate/BioGlass/collagen (CaP/BG/C) bone graft material. Intramuscular implantation was carried out as well to evaluate soft tissue responses. Eight mature male mongrel dogs underwent single-level, noninstrumented interspinous implantation, where the bone graft materials were implanted at either side of the spinous processes (L3-L4), with separation by the interspinous ligament ensuring comparison of both materials in each animal (n=8 per material). The materials were also implanted in paraspinal muscle pouches. Animals were euthanized 12 weeks after surgery and the lumbar spines excised and intramuscular implants retrieved. Undecalcified sections were prepared for histological evaluation and histomorphometry was performed to quantify bone formation and material resorption. After 12 weeks, all submicron structured CaP/AOC implants showed abundant bone formation in the (L3-L4) interspinous space (20.8%+/- 6.8%), whereas bone was not found in the CaP/BG/C implants (0%+/- 0%). Intramuscularly, the CaP/AOC material triggered significant bone formation (12.0%+/- 7.8%), whereas CaP/BG/C did not form any bone. In both the spinal and muscular sites, resorption of the CaP/AOC material was evident by a decrease in Feret diameter of the CaP granules as well as in their histological surface compared with the starting material, whereas CaP/BG/C material had a milder resorption. This study shows that a submicron-surface structured CaP/AOC bone graft material has superior bone-forming properties in both an interspinous implantation model and intramuscularly, as compared with a CaP/BG/C bone graft material.