Hydroxyapatite-zirconia composites and strontium-hydroxyapatite for skeletal applications

peer-reviewedHydroxyapatite (HA) is inherently bioactive and is successfully used in non-load bearing skeletal applications. However, its use in load-bearing applications is limited due to its poor mechanical properties, particularly under tensional and torsional stresses. For this reason HA is employed in non-load bearing clinical applications such as implant coatings or as bone fillers. In the work contained herein, nano-sized zirconia (ZrO2) is employed as a filler phase in an attempt to improve HA’s mechanical properties. HA was synthesised at four temperatures: 6oC, 25oC, 45oC and 65oC. The resultant powders were characterised in the green state using x-ray fluorescence (XRF), dilatometry, x-ray-diffraction (XRD), BET and high-temperature XRD to determine the synthesis temperature that offers the most suitable stoichiometry and resistance to decomposition when sintering. ZrO2 (1, 2, 3, 4 and 5 wt%) was added to the most promising laboratory synthesised HA. Although HA reinforcement by the incorporation of ZrO2 is reported in the literature, high temperatures of over 1400°C are commonly required to sinter these composites to high density; temperatures at which HA is thermally unstable. The literature reports that microwave sintering (MS) can increase both density and mechanical properties in ceramic bodies with shorter processing times and at lower sintering temperatures than conventional sintering regimes. On account of this, microwave sintering was employed in this study to sinter the HA-ZrO2 composites at 700oC, 1000oC and 1200oC; in addition to comparative sintering in a conventional (CS) furnace. The 700oC samples do not undergo any appreciable densification, irrespective of sintering method. Increasing ZrO2 decreases density of both the CS and MS samples from approximately 88% to 85% in the 1200oC samples. Increasing amounts of ZrO2 had no significant effect on the biaxial flexural strength (BFS) of the composites. The BFS of the samples sintered at 1000oC ranged from 30-45MPa, while the samples sintered at 1200oC ranged from 60-85MPa. Statistically the hardness of the materials is not affected by ZrO2 incorporation, although the MS 1200oC samples do show increased maximum hardness values with added ZrO2. The hardness values for the CS and MS samples sintered at 1000oC range from 0.5 to 1.5GPa, while the samples sintered at 1200oC range from 1.7 to 3GPa. The volume fraction porosity was determined as the main controlling factor in both the BFS and hardness of the CS and MS samples over the temperature range 700oC to 1200oC, however at specific sintering temperatures other secondary controlling factors have increased control over BFS and hardness. SEM analysis showed that the ZrO2 particles collected in the grain boundaries, thereby reducing grain boundary diffusion in both the CS and MS samples. HA was subsequently doped with strontium (Sr2+) ions in an attempt to increase bioactivity, as Sr positively influences bone remodelling. Successful synthesis of strontium-hydroxyapatite (Sr-HA) was achieved, as determined by XRD, Fourier transform infrared spectroscopy (FTIR) and x-ray photoelectron spectroscopy (XPS). 5% Sr incorporation resulted in stabilisation of a number of phases with an increase in BFS from 85MPa to 120MPa in the MS samples. A decrease in the maximum BFS resulted in the CS samples, from 120MPa to 47MPa. The MS samples doped with 10% Sr experienced approximately no change in BFS while the CS samples experienced an increase in the maximum BFS from 47MPa to 73MPa. The maximum hardness for the 5% Sr-HA CS and MS samples increases from between 2GPa (MS) and 3GPa (CS) to approximately 4.7GPa for both regimes. This remained unchanged for the 10% Sr-HA samples. With 5% Sr dopant, the HA phase was stabilised in both sintering regimes, however with increased 10% Sr dopant levels, decomposition increased. β-TCP is stabilised in the CS samples with no α-TCP generation. This does not occur in the MS samples due to destabilising effects of the microwave field. The 10% Sr-HA displayed excellent bioactive properties in a successful animal trial