Repair of the immature and mature craniofacial skeleton with a carbonated calcium phosphate cement : Assessment of biocompatibility, osteoconductivity, and remodeling capacity

The apatite compounds used most commonly in craniofacial reconstruction are highly crystalline and biologically inert ceramics. Because their capacity to be replaced by native bone is limited, they have found little application in repair of the growing craniofacial skeleton. Carbonated calcium phosphate cements more closely resemble the mineral phase of bone, thereby offering enhanced bioresorption and osteoconductivity, but their fate in the immature and mature craniofacial skeleton has not been investigated. The authors hypothesized that the capacity for cell-mediated remodeling of carbonated calcium phosphate cements is based on (1) their crystallographic and compositional similarity to the mineral phase of bone and (2) the osteogenic capacity of the host. Four noncritical-sized calvarial defects were created in six 3-week-old and six 16-week-old Yorkshire pigs. The defects were repaired with autologous bone, sintered carbonated calcium phosphate cement disks with a higher crystal order, or carbonated calcium phosphate cement (Norian CRS; Synthes Maxillofacial, West Chester, Pa.). The fourth defect was left empty as a control. Specimens were harvested at 30 and 90 days postoperatively. Empty defects healed with dense fibroconnective tissue in all groups. Autologous bone grafts underwent complete remodeling and replacement with woven bone at both time points. Sintered carbonated calcium phosphate disks demonstrated no bony ingrowth or remodeling. In immature animals, carbonated calcium phosphate cement implants were progressively replaced with woven bone through osteoclast-mediated resorption and osteoblast-mediated bone formation. Only minimal remodeling of the carbonated calcium phosphate cement implants was observed in skeletally mature animals. The results of these experiments suggest that the extent of remodeling of carbonated calcium phosphate cement is dependent on both the composition of the implant itself and the osteogenic capacity of the host and that carbonated calcium phosphate cement may be used successfully for inlay applications in the immature craniofacial skeleton.