Abstract
Hydroxyapatite is the main mineral constituent of bone and teeth and has a great potential for bone repair due to its biocompatibility, osteoconductivity, non-toxicity and bioactivity. Biologically derived hydroxyapatite is of great interest since it is biocompatible and possess the correct bony architecture and porosity to allow tissue ingrowth. Bovine bone sourced from New Zealand, where the incidence of bovine spongiform encephalopathy (BSE) or other serious animal maladies such as Foot and Mouth Disease is non-existent, is an effective and economical source to extract hydroxyapatite. The main purpose of this thesis was twofold. Firstly, the aim was to develop a xenograft material (bovine hydroxyapatite/BHA) from New Zealand sourced bovine cancellous bone through a variety of defatting and deproteinization procedures, secondly to incorporate fluoride and silicate ions into the BHA to produce two novel bone grafts namely bovine derived fluorapatite (BFA) and silicon substituted bovine hydroxyapatite (SiBHA).
Chemical, physical, structural and thermal properties of the developed bone grafts were characterised. Fourier transform infrared (FTIR) spectroscopy confirmed removal of organic matter from the bone matrix after subcritical water extraction and sintering at 650oC with the presence of carbonate (CO32-), phosphate (PO43-) and hydroxyl (OH-) functional groups. FTIR spectroscopy further confirmed that both fluoride and silicate ions were substituted into the hydroxyapatite lattice. X-ray diffraction studies confirmed the crystalline nature of the three materials. Energy dispersive X-ray analysis showed that the main inorganic phases contained calcium and phosphorus as the major elements with trace amounts of sodium magnesium, potassium, zinc and strontium. Scanning electron microscopy showed the three materials had retained their natural macro porous architecture. Thermo gravimetric analysis showed that the three materials were stable up to 1000oC losing only ~2% of their weight. Meanwhile, the BHA, BFA and SiBHA scaffolds showed excellent chemical and structural stability in simulated body fluid (SBF) after 28 days of incubation. Mechanical testing on the three materials were carried out by compression testing on an Instron materials testing instrument. The fluoride and silicate substituted bovine hydroxyapatite samples (BFA and SiBHA) showed an improvement in Young’s modulus and compressive strength compared to bovine hydroxyapatite (BHA). Finally, the biocompatibility of the three materials were tested using Saos-2 cell culture testing. In-vitro studies showed the three materials were cytocompatible, supporting proliferative growth of Saos-2 osteoblast cells. The three materials exhibited osteo-inductive features which suggested that these scaffolds constitute a good substrate for Saos-2 cell differentiation leading to extra cellular bone matrix formation.
This research proved that the developed bovine hydroxyapatite scaffold was successfully substituted with fluoride and silicate ions. However, further research is necessary to evaluate the clinical feasibility of the xenografts as a bone graft substitute material.