Long-Term in Vivo Performance of Low-Temperature 3D-Printed Bioceramics in an Equine Model

Bone has great self-healing capacity, but above a certain critical size, bone defects will not heal spontaneously, requiring intervention to achieve full healing. Among the synthetic calcium phosphate (CaP) bone replacement materials, brushite (CaHPO₄·2H₂O)-based materials are of particular interest because of their degree of solubility and the related high potential to promote bone regeneration after dissolution. They can be produced tailor-made using modern three-dimensional (3D) printing technology. Although this type of implant has been widely tested in vitro, there are only limited in vivo data and less so in a relevant large animal model. In this study, material properties of a 3D-printed brushite-based scaffold are characterized, after which the material is tested by in vivo orthotopic implantation in the equine tuber coxae for 6 months. The implantation procedure was easy to perform and was well tolerated by the animals, which showed no detectable signs of discomfort. In vitro tests showed that compressive strength along the vertical axis of densely printed material was around 13 MPa, which was reduced to approximately 8 MPa in the cylindrical porous implant. In vivo, approximately 40% of the visible volume of the implants was degraded after 6 months and replaced by bone, showing the capacity to stimulate new bone formation. Histologically, ample bone ingrowth was observed. In contrast, empty defects were filled with fibrous tissue only, confirming the material's osteoconductive capacity. It is concluded that this study provides proof that the 3D-printed brushite implants were able to promote new bone growth after 6 months' implantation in a large animal model and that the new equine tuber coxae bone model that was used is a promising tool for bone regeneration studies.