Volume effect on biological properties of a calcium phosphate hydraulic cement: experimental study in sheep.

Injectable calcium phosphate hydraulic cements (CPHC) are a new family of bone substitutes within the class of bone reconstruction biomaterials. In this work, CPHC were tested in two consistencies (preset blocks or liquid paste) in an experimental model of cancellous bone defect in sheep. The defects were eight times larger than those investigated previously in rabbits. Three delays (12, 24, and 52 weeks) were used. Before death, a double label of oxytetracycline and alizarine was made intravenously. The distribution of implants was randomized, histomorphometric evaluation was performed and compared with micrographic observation, and optical microscopy of stained sections was performed either under visible, ultraviolet, or polarized light. The results were compared with spontaneous healing of empty defects and with a control group of normal cancellous bone from sheeps of the same age. No significant difference has been observed between premolded and injected implants. In the sheep model, the degradation and new bone formation rates are three times slower, compared with those observed previously in rabbits. New bone formation increased from 5.9% (12 weeks) up to 11.0% (24 weeks) in the empty defect group. In the cement groups, 28.3% new bone was obtained at 12 weeks, which seemed then to level off (27.8% new bone at 24 weeks). Cement residues appear as radio-opaque cylinders on microradiographs. In all cases, a radiolucent layer was observed at the cement/bone interface at 24 weeks. Stained sections showed the formation of a fibroconnective capsule around the residual cement, which presumably slows down new bone formation. Nevertheless, quantitative bone remodeling was accelerated in the cement group; mineral apposition as well as adjusted apposition rates were higher, and the formation period as well as the mineralization of osteoid tissue were faster compared with empty cavities and controls. These results point to higher osteoblast activity and better exchange with surrounding tissues in the defects filled with cement.