Development Of Beta-Tricalcium Phosphate Based Injectable Synthetic Bone Graft Materials

Abstract

As the global population ages, the number of patients suffering from bone tissue loss due to trauma, degenerative diseases, or surgical intervention increases. To meet the demand for bone graft materials, synthetic substitutes have been developed. In cases of critical-sized or irregularly-shaped bone defects injectable bone substitutes (IBS) provide more utility over pre-fabricated blocks or granules. Non-setting IBSs (putties) act as bone void fillers in non-load-bearing areas. IBSs are employed in a range of medical fields including orthopaedic, cosmetic, reconstructive and dental surgeries. A clinically-successful IBS will be easy-to-handle and apply, have osteo-stimulatory properties, avoid premature degradation, have standardised quality, be sterilisable and have a viable shelf-life. The aim of this research was to develop an ideal IBS composed of β-tricalcium phosphate (β-TCP)-bioactive glass particles in a poly(ethylene glycol) (PEG) – glycerol carrier. Fourier Transform Infrared Spectroscopy (FT-IR) spectroscopy verified that gamma sterilisation (25kGy) did not affect its chemical composition. Scanning electron microscopy (SEM) and micro-computerised tomography (-CT) revealed a homogenous phase distribution of the constituents with the sample preparation technique used. Compositional effects on IBS thermal properties were investigated with thermogravimetric analysis and an approximately 50% loss in weight was recorded in both 1:1 and 2:3 (%w) carrier-to-ceramic compositions. The effects of both varying IBS carrier composition and the gamma-sterilisation condition on the rheological properties of the biomaterial were evaluated. The highest peak injection force (73 N) recorded for any combination of poly(ethylene glycol) (PEG) content and sterility conditions in prepared samples was seen in sterile samples with high PEG content. Oscillatory viscosity measurements of the samples revealed an increase in IBS viscoelasticity (tan δ changed from 0.38 to 0.31) with increased PEG concentration. The washout-resistance and cohesiveness of the IBS was examined according to weight loss in and Ca2+ ion release in dissolution media, respectively. Both PEG and glycerol were found to have competing effects on these two properties. Furthermore, calcium release profiles showed a dynamic ion release/uptake behaviour of the ceramic particles in water. Cytotoxicity testing of the IBSs were performed using the MEM elution method with L929 mouse fibroblasts. Cell viability was quantified with a 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide analysis kit. 24-hour extracts of IBS composites sterilised with gamma irradiation of 10 kGy and 25 kGy resulted in a cell viability value of 22.94 % and 56.53 %, respectively. No relationship between extract incubation times and cell viability could be established. A significant (p<0.0001) increase in cell viability in extract dilutions of 10 kGy – treated samples was observed, increasing from 63.40 % (100 % extract concentration) to 86.17 % (50 % extract concentration). These results concur with literature regarding the limitations of in vitro testing for IBSs and the commonly reported discrepancy between in vitro cytotoxicity and in vivo successful healthy bone formation for similar materials.