No abstract

No abstract

No abstract

Addition of ductile polymers to calcium-deficient hydroxyapatite (CDHA)-forming bone cements based on α-tricalcium phosphate (α-TCP) is a promising approach to improve the mechanical performance of α-TCP cements and extend their application to load-bearing defects, which is else impeded by the brittleness of the hardened cement. One suitable polymer is poly-(2-hydroxyethylmethacrylate) (p-HEMA), which forms during cement setting by radical polymerisation of the monomer. In this study the hydration kinetics and the mechanical performance of α-TCP cements modified with addition of different HEMA concentrations (0-50 wt% in the cement liquid) was investigated by quantitative in situ XRD and four-point bending tests. Morphology of CDHA crystals was monitored by scanning electron microscopy. The hydration of α-TCP to CDHA was increasingly impeded and the visible crystal size of CDHA increasingly reduced with increasing HEMA concentration. Modification of the cements by adding 50 wt% HEMA to the cement liquid changed the brittle performance of the hardened cement to a pseudoplastic behaviour, reduced the flexural modulus and increased the work of fracture, while lower HEMA concentrations had no significant effect on these parameters. In such a composite, the extent of CDHA formation was considerably reduced (34.0 ± 1.8 wt% CDHA with 50 % HEMA compared to 54.1 ± 2.4 wt% CDHA in the reference formed after 48 h), while the general reaction kinetics were not changed. In conclusion, while the extent of CDHA formation was decreased, the mechanical properties were noticeably improved by addition of HEMA. Hence, α-TCP/HEMA composites might be suitable for application in some load-bearing defects and have adequate properties for mechanical treatment after implantation, like insertion of screws.

No abstract

No abstract

The structural information on the influence of ionic additions in biphasic (hydroxyapatite (HAP) and b-tricalciumphosphate (b-TCP)) mixtures ranging from single ionic substitutions to combined ionic substitutions of most of the essential ions embedded in biological apatite was analyzed through the Rietveld refinement technique. The results have proved that the determined quantitative phase composition of HAP and b-TCP in biphasic mixtures was dependent on the initial calcium (Ca) deficiency of the precursor powders precipitated from the different molar concentrations used in the synthesis. The substitution of cations (Na 1 , Mg 21 , and K 1 ) improved the stabilization of the b-TCP structure whereas anions (F À and Cl À ) were found incorporated at the OH À site of the HAP phase. Rietveld analysis of X-ray powder diffraction data from the present study proved to be a powerful technique to describe the position and occupancy of certain ions like Mg 21 and Cl À in the biphasic mixtures. However, it has also shown limitations in tracking back other ions like Na 1 , K 1 , and F À , which require the use of other complementary characterization methods. Feature D. J. Green-contributing editor Financially supported by the Portuguese Foundation for Science and Technology from the project POCTI/CTM/60207/2004.

No abstract