Guiding bone formation using semi-onlay calcium phosphate implants in an ovine calvarial model

The restoration of cranio-maxillofacial deformities often requires complex reconstructive surgery in a challenging anatomical region, with abnormal soft tissue structures and bony deficits. In this proof-of-concept, the possibility of vertical bone augmentation was explored by suspending hemispherically shaped titanium-reinforced porous calcium phosphate (CaP) implants (n = 12) over the frontal bone in a sheep model (n = 6). The animals were euthanized after week 13 and the specimens were subject to micro-computed tomography (μCT) and comprehensive histological analysis. Histology showed that the space between implant and the recipient bone was filled with a higher percentage of newly formed bone (NFB) versus soft tissue with a median of 53% and 47%, respectively. Similar results were obtained from the μ-CT analysis, with a median of 56% NFB and 44% soft tissue filling the void. Noteworthy, significantly higher bone-implant contact was found for the CaP (78%, range 14%-94%) versus the Titanium (29%, range 0%-75%) portion of the implant exposed to the surrounding bone. The histological analysis indicates that the CaP replacement by bone is driven by macrophages over time, emphasized by material-filled macrophages found in close vicinity to the CaP with only a small number of single osteoclasts found actively remodeling the NFB. This study shows that CaP based implants can be assembled with the help of additive manufacturing to guide vertical bone formation without decortification or administration of growth factors. Furthermore, it highlights the potential disadvantage of a seamless fit between the implant and the recipient's bone.