Influence Of Ca₂P₂O₇ On Mechanical Strength And Degradability Of β-Ca3(PO4)2 Bioceramics In Vitro And Inflammatory Reaction In The Host

β-tricalcium phosphate (β-Ca₃(PO₄)₂,β-TCP) has been proved to be biocompatible and resorbable in vivo with new bone growth replacing the implantedβ-TCP. This property imparts significant advantage ontoβ-TCP compared to other biomedical materials, which are not resorbable. Therefore,β-TCP bioceramics are widely used as bone defect filling materials in the field of orthopaedic, oral and plastic surgery.In theβ-TCP powder synthesis, a second phase of calcium pyrophosphate (Ca₂P₂O₇, CPP) will appear, if the Ca/P values were not well controlled and lower than 1.50. The presence of CPP may ultimately influence the properties of the fabricatedβ-TCP bioceramics.But there is so far no detailed report on the effect of CPP impurity inβ-TCP bioceramics.In this study, the influence of CPP on sintering ability, mechanical strength , degradability ofβ-TCP bioceramics and inflammatory reaction in the host were investigated.Section 1. Experiments in vitroIn this section, the effect of Ca₂P₂O₇ on the sintering ability, mechanical strength and degradability of the ceramics were investigated.Theβ-TCP powders were synthesized by the precipitation method using Ca(NO₃)₂·4H₂O and(NH4)2HPO4 as reagents. The CPP powders were prepared by solid-state reaction of CaHPO₄·2H₂O at 1100 oC for 2h.To investigate the influence of CPP on sintering ability, mechanical strength and degradability ofβ-TCP bioceramics, pureβ-TCP and 0.5¹0.0 wt.% CPP-dopedβ-TCP powders were sintered.The phase composition of the sintered samples was characterized by XRD, and the relative density (RD) of the sintered samples was measured by the Archimedian method. The fracture surfaces of the sintered samples were observed using SEM. The 3-point bending strength and elastic modulus of the sintered samples were measured at a mechanical testing machine. The degradability of the fabricated samples doped with different amount of CPP additives was also investigated by their weight loss percentage in Tris-HCl buffer solution.The increase of the CPP additives resulted in a remarkable decrease of the RD. An almost complete sintering of the pureβ-TCP samples (RD as high as 99.4%) can be achieved at temperature of 1050 oC for 5 h. With the increase of the additives to 0.5 wt.%, the RD decreased sharply to 96.8%. With a further increase of the additives up to 2.5 wt.%, the RD maintained at about 96.5%. When the additives increased to 10 wt.%, the RD decreased to 89.1%. The results showed that the presence of the CPP could obviously decrease the sintering ability of theβ-TCP powders.The microstructural observations of the fracture surfaces of the pureβ-TCP bioceramic showed highly densified body. On the contrary, the samples dopped with CPP additives showed poor densified bodies with large numbers of 0.5-1.2μm intergranular pores in the matrix.For samples with 0-10 wt.% addition of CPP, the bending strength and elastic module decreased from 130.46 MPa to 57.88 MPa and from 46.30 GPa to 23.98 GPa, respectively. The bending strength of the samples doped by 10 wt.% CPP was only about 44% of the pureβ-TCP bioceramics.Degradability can be seen to increase with increasing the soaking time. It is clear that the degradation of the pureβ-TCP bioceramics only reached 0.05 % at day 1. With increased soaking time, the degradation increased slowly, and reached 1.03 % at day 7 and then increased step by step and reached 1.90 % at day 14. In contrast, the degradation of the samples doped with CPP additives was much faster than that of the pureβ-TCP bioceramics. Moreover, as the CPP additives increased, the dissolution rate increased steadily. At day 1, the weight loss percentage of the samples doped with 10 wt.% CPP additives reached 0.26 %. With the increase of the soaking time, the degradation increased sharply, and reached 5.04 % at day 14. The results show that the degradation rate of the samples doped with 10 wt.% CPP additives is approximately three times higher than the pureβ-TCP bioceramics.The results showed that the Ca2P2O7 could apparently decrease the sintering ability and mechanical properties ofβ-Ca₃(PO₄)₂. Moreover, the relative density and mechanical strength of the sintered samples decreased gradually with the increase of the Ca₂P₂O₇ additive amount. However, the dissolution rate of the samples increased with the increase of the Ca₂P₂O₇ additive amount.Section 2. Experiment in vivoIn this section, we tested these bioceramics scaffolds made ofβ-TCP and CPP in a rat model. The influence of CPP on inflammatory reaction in the host was investigated.Six kinds of bioceramics which ingredients were pureβ-TCP, 0.5 wt.%, 2.5 wt.%, 5 wt.%, 10 wt.% CPP-dopedβ-TCP and pure CPP were sintered. According to ingredients, the bioceramics cylinders were distributed randomly to seventy-two rats.The perivertebral fascia lumbodorsalis in each rats was exposed under sterile conditions, under anesthesia. After bilateral dorsal subcutaneous pockets were formed, two same constituent bioceramics blocks were inserted into the bilateral pockets. At 1, 2 and 4 weeks after surgery, rats were sacrificed and the implanted bioceramics blocks and the neighboring tissue were extracted.Macroscopical appearance was assessed with regard to inflammatory reaction and overgrowth of the neighboring tissue. After the implanted bioceramics blocks and the neighboring tissue were extracted, the newly born membrane layers that surrounded the scaffolds were stripped. The wall thickness of the membrane layers were measured by a spiral micrometer. Histological thin (5μm) sections and thick (30⁴0μm) were prepared. The thin sections were stained with hematoxylin and eosin (H&E), and the thick sections were surface stained with H&E. Thin sections were used for the histological qualitative and evaluation. The occurrence of soft tissue and inflammatory cells was described.All blocks were found to be biocompatible. Ca₂P₂O₇-dopedβ-Ca₃(PO₄)₂ bioceramics evoked inflammatory reaction of adjacent tissues in vivo. Histological sections showed the infiltration of inflammatory cells. Furthermore, with the increasing percentage of CPP, the inflammation and allergy of tissues became more and more severe. Quantitative evaluation of the tissue response revealed that that the Ca2P2O7-dopedβ-Ca₃(PO₄)₂ bioceramics scaffolds resulted in severe post-implantation inflammation, whereas the pureβ-Ca₃(PO₄)₂ bioceramics scaffolds induced only a slight post-implantation inflammation.Conclusions:1. The relative density and mechanical strength of the sintered samples decreased gradually with the increase of the CPP additive amount. The dissolution rate of the samples increased with the increase of the CPP additive amount.2. With the increasing percentage of CPP, the inflammation and allergy of tissues became more and more severe.