Bone formation and implant degradation of coralline porous ceramics placed in bone and ectopic sites

Purpose: This study was undertaken to determine the resorption rate of porous ceramic implants. The hypothesis was that implants placed in soft tissues would degrade more rapidly than implants placed in bone. Materials and Methods: To test this hypothesis, implants were manufactured by applying a thin coating of hydroxylapatite onto an interconnected, porous calcium carbonate substrate. Control implants were made entirely of hydroxylapatite with identical microstructure. Two adult dogs received a total of 56 implants placed in the femur, skeletal muscle, and subcutaneous tissues. After killing the animals at 4 months, the specimens were removed, embedded in plastic, sectioned, and either stained for light microscopic examination or subjected to quantitative image analysis using a scanning electron microscope. Results: Contrary to the hypothesis, the rate of degradation was faster for implants placed in bone than in soft tissue. Within the 4 months, degradation was 24% to 63% in bone, depending on the composition. However, it was not statistically significant in either intramuscular or subcutaneous tissue. A surprising observation was that bone ingrowth occurred in 67% of the implants placed in soft tissues. On average, it was 4.3% in intramuscular sites and 6.6% in subcutaneous sites. This bone was histologically normal in 71% of the implants containing bone. Conclusion: This study demonstrates that porous ceramic implants composed of hydroxylapatite on calcium carbonate will degrade more rapidly in bone defects that in soft tissue sites. In addition, implants with interconnected porosity and surfaces of hydroxylapatite will become ingrown with bone even after placement in soft tissues. The exact mechanisms for both of these phenomena are not understood.