Production and characterization of reinforced hydroxyapatite for bone replacement.

Abstract

Hydroxyapatite(HA) is a highly biocompatible calcium phosphate material which in porous form, promotes rapid bone ingrowth and revascularisation. As such it has potential for use as a synthetic bone graft substitute. However, due to poor mechanical strength, its use has been limited to non-major load bearing applications. In response, secondary phase additions such as calcium/phosphate-based glasses have been used to reinforce HA. However, the improved mechanical properties obtained by secondary-phase reinforcement are often associated with decomposition of the HA to tricalcium phosphate(TCP), which may be undesirable due to the increased solubility and controversial biocompatibility of the latter. The aim of this thesis was to produce a calcium/phosphate-based additive for reinforcing HA and to investigate the mechanical and chemical stability of this composite in a physiological environment. Furthermore, the possibility of transferring this technology to porous structures was investigated. Prior to investigating the effects of second phase addition on strength and phase stability, the HA used in this study was characterized as having a biaxial flexural strength (BFS) of 65+11MPa and being chemically stable up to sintering temperatures of 1350°C. Two calcium/phosphate-based additives were produced with Ca/P ratios of 0.5 (CAP I) and 0.8 (CAP2); CAP1 was found to be amorphous, whilst CAP2 was predominantly crystalline in nature where the crystalline phase was primarily Ca2P2O7. The maximal BFS value found for HA doped with 2.5 wt% CAP I (CAPIHA) was 27MPa with up to 73%TCP, whilst the CAP2-doped HA (with 2.5 wt% CAP2) achieved a maximal BFS of 102±21MPa with up to 13%TCP. Thus CAP2HA was judged to be a successful composite suitable for more comprehensive investigation. Studies were carried out to decipher the ideal wt% of CAP2 to promote mechanical reinforcement with a minimal presence of TCP, using 1,2.5 , 3.25 ,4 and 5 wt % CAP2. The results indicated that 2.5 wt% CAP2HA was optimal in terms of both the mechanical and chemical criteria. For investigating mechanical and chemical stability, the HA and CAP2HA samples were soaked in 50% strength Ringer's solution for periods of 1-30 days. HA retained 60% of its original strength, whilst CAP2HA retained 78% of its original strength. In view of the success in using CAP2 as a reinforcing additive, a preliminary investigation was carried out using HA and CAP2HA. This involved development of a technique for producing porous HA, using a reticulated foam template, with a highly interconnected structure and mean porosities and strengths of 66% and 2MPa. Porous samples of CAP2HA were also successfully produced however, only mean porosities and strengths of 75% and 0.6MPa were achieved. This was attributed to processing complications arising from the solubility of Ca2P2O7in water leading to inhibited sintering, which may be overcome by the use of a different binder system.