The sintering and mechanical behavior of hydroxyapatite with bioglass additions.

There is increasing interest in the potential of composites of hydroxyapatite with phosphate- or silicate-based bioactive glasses, and certain of these glass additions have been found, in previous work, to aid densification and form a mechanically-reinforced, bioactive material; in particular, large improvements in flexural strength and fracture toughness were obtained through the addition of small amounts of phosphate glass. Less is known about the mechanical behavior of HA/bioglass composites, although in vivo studies by other workers have shown encouraging biological results. In this investigation, the sintering behavior, mechanical properties, and microstructure of composites of HA with up to 50 wt % glass, were analyzed. X-ray diffraction showed the phase composition of sintered composites with up to 5 wt % added bioglass to be non-stoichiometric HA with alpha-TCP or beta-TCP. Phase analysis of composites containing higher glass additions was impracticable due to peak broadening and overlap, although reaction products, at the highest glass additions and sintering temperatures, may include wollastonite-2M and beta-Na2Ca4(PO4)2SiO4. Sintered density, and mechanical properties other than fracture toughness, showed no significant improvement over HA.

A quantitative study of the sintering and mechanical properties of hydroxyapatite/phosphate glass composites.

Previous work has shown that small additions of a phosphate glass (CaO-P2O5) can significantly enhance the sinterability and strength of hydroxyapatite. However, there are no quantitative phase analyses available for these materials which would provide indicators of biocompatibility and resorbability. Similarly, there is little information available about the mechanical properties, especially with high glass additions. In this study, the effects of sintering hydroxyapatite with phosphate glass additions of 2.5, 5, 10, 25, and 50 wt.% are quantified. Each composition was sintered over a range of temperatures, and quantitative phase analysis was carried out using XRD. In addition, the microstructures were studied using RLOM and SEM, and mechanical properties (Vickers hardness, KIC, and MOR) measured. These results may be used to indicate which compositions and processing conditions may provide materials suitable for use in hard tissue replacement. Composites containing up to 10 wt.% glass additions formed dense HA/TCP composite materials possessing flexural strength and fracture toughness values up to 200% those of pure HA. The HA/TCP ratio was strongly dependent on the percentage glass addition. Higher glass additions resulted in composites containing beta-TCP together with large amounts of alpha- or beta-calcium pyrophosphate, and having similar mechanical strengths to pure HA.

A synthetic bone implant macroscopically identical to cancellous bone.

Macroporous hydroxyapatite (HA) and beta-tricalcium phosphate (beta-TCP) are widely used as synthetic bone replacement materials due to their high biocompatibility and osteoconductive properties. The level of porosity, pore size distribution, pore morphology, and the degree of pore interconnectivity in such grafts significantly influences the extent of bone ingrowth. It has been hypothesised that an ideal implant macrostructure may be similar in morphological characteristics to the inorganic matrix of the bone it is replacing. However, to date, clinically available synthetic materials differ structurally from cancellous bone. A method is described for the macrostructural replication of cancellous bone. Reproduction involves a multistage process requiring the manipulation of positive and negative forms of the inorganic matrix. By infiltration of a wax negative mould of cancellous bone with a ceramic slip, followed by removal of the wax, and firing, it is possible to produce a positive replica of the original cancellous macrostructure. Optimisation of slip preparation conditions (pH and percentage deflocculant addition) and sintering conditions have allowed successful replication of cancellous bone using several bioceramic compositions including HA, beta-TCP, and HA/beta-TCP.

Development of a new synthetic bone graft.

A process for the replication of bovine cancellous bone in synthetic bioceramic materials for use as artificial bone graft substitutes is described. The process detailed here may be easily implemented to allow production of large numbers of blocks of material, even on a laboratory scale. The graft material has a pore morphology and interconnectivity identical with that of the original cancellous bone used as a starting material. Strength of the material is adequate, and at lower porosity levels it meets the FDA requirements for coralline materials for spinal applications. The synthetic graft is also shown to have excellent fluid-retention characteristics, making it a potential carrier for morphogenic agents such as solutions of bone morphogenic protein.