Mechanical and in vivo performance of hydroxyapatite implants with controlled architectures

Internal architecture has a direct impact on the mechanical and biological behaviors of porous hydroxyapatite (HA) implant. However, traditional processing methods provide minimal control in this regard. To address the issue, we developed a new processing method combining image-based design and solid free-form fabrication. We have previously published the processing method showing fabricated HA implants and their chemical properties. This study characterized the mechanical and the in vivo performance of designed HA implants. Thirteen HA implants with orthogonal channels at 40% porosity were tested on an Instron machine. The compressive strength and compressive modulus measured were 30+/-8 MPa and 1.4+/-0.4 GPa, comparable to coralline porous HA. Twenty-four cylindrical HA implants with two architecture designs, orthogonal and radial channels, were implanted in the mandibles of four Yucatan minipigs for 5 and 9 weeks. Normal bone regeneration occurred in both groups. At 9 weeks, bone penetrated 1.4mm into both scaffold designs. The percent bone ingrowth in the penetration zone was higher in the orthogonal channel design but not statistically different due to the low number of samples. However, the overall shape of the regenerated bone tissue was significantly different. In the orthogonal design, bone and HA formed an interpenetrating matrix, while in the radial design, the regenerated bone formed an intact piece at the center of the implant. These preliminary results showed that controlling the overall geometry of the regenerated bone tissue is possible through the internal architectural design of the scaffolds.