Mechanical Properties and Porosity Alterations of Hydroxyapatite/Endodontic Cement Biomaterial Lauris Rupeks1, Viktors Filipenkovs1, Visvaldis Vitins1, Ivars Knets1 Riga Technical University Keywords – natural hydroxyapatite, endodontic cement, mechanical properties, porosity. INTRODUCTION Replacement of biological tissue is very important issue in medicine. Different biomaterials on the basis of hydroxyapatite are used for creation of materials for bone substitute [1]. In the present study had been investigated two variations of biomaterial created from natural hydroxyapatite (NHAp) and endodontic cement (EC). The structure of it was investigated by optical microscopy (OM). Mechanical properties for new material modifications were determined as well. Further this material was implanted into rats. New material consists of two components, resorbable part of NHAp and partly resorbable part of EC. Shortly after implantation cement part of biomaterial hardens and makes good mechanical bond with bone tissue. However, disadvantage of it is brittleness and in longer time period due to the exposition of material to mechanical stress this bone/biomaterial bond becomes unstable. In order to avoid it, in long run second fraction of biomaterial - NHAp is added. NHAp is resorbable material and after implantation it is dissolved by body fluids. Due to this process of resorption implanted material becomes porous. Further bone tissue grows into these pores and provide stable long term bond between biomaterial and implant. MATERIALS AND METHODS Deproteinized bone tissue was crushed to the size of particles 10-90 μm. Such particles retain four highest levels of the natural structure of bone tissue [2]. Further, as a binder we used EC. NHAp and EC particles were mixed with water. Next, the resulting mass was molded into cylinders, diameter 5mm and height 6mm, under pressure of 0.5 MPa. Samples were removed from the mold after 48 hours when the hardening process finished. The structure of composite materials based on NHAp and EC (with different fraction percentage) were investigated by OM, no pores were observed in the material before implantation. The mechanical properties of EC and EC-NHAp composite materials were determined. Further these materials were implanted into rats. In total were fifteen implants, first month five pieces, next month again five and in third month last five pieces. After three months laboratory rats were sacrificed and histological analysis made for the implanted materials. RESULTS AND DISCUSSION A comparison between the composites based on EC-NHAp has shown that a change in the matrix-filler ratio changes the mechanical properties. Highest values of ultimate stress σ* and modulus of elasticity E were with NHAp content 40 vol%, - 16.7 MPa and 0.17 GPa, respectively. However, when the content of NHAp was increased till 60 vol% above mentioned parameters changed to 9.7 MPa and 0.11 GPa. It is obvious that greater percentage of filler decreases σ* and E because mechanical properties of NHAp are weaker than EC. Hystological analysis has shown that part of biomaterial where was NHAp is absorbed, and implant material becomes porous, what fosters ingrowth of bone tissue in new cavities. However, this part of biomaterial which consists of EC stays nearly unchanged and provides appropriate structure with adequate mechanical strength for implanted material [3, 4]. Important is to stress that this tendency, formation of pores, becomes more explicit by time and in such way creates scaffold for live bone tissue integration. The EC-NHAp composites have sufficiently good mechanical properties and biocompatibility, consequently materials can be used for covering bone defects in order to create a new hard tissue structure after surgical interventions. REFERENCES [1] X. W. Li, H. Y. Yasuda, Y. Umakoshi. Bioactive ceramic composites sintered from hydroxyapatite and silica at 1200oC: preparation, microstructures and in vitro bone-like layer growth. J. Materials in Medicine. V.17, No 6, pp. 573-581 (2006). [2] X. Xiao, R. Liu, Q. Huang. Preparation and characterization of hydroxyapatite/polycaprolactone-citosan composites. J. Mater. Sci. Mater. Med. V 20, No 12, pp. 2375-2383 (2009). [3] H. Studenovska, M. Slouf, F. Rypacek. Poly(HEMA) hydrogels with controlled pore architecture for tissue regeneration applications. J. Materials in Medecine. V. 19, No 2, pp. 615-621 (2008). [4] M. Doblare, J. M. Garcia. Anisotropic bone remodeling model based on a continuum damage-repair theory. J. of Biomechanics. V. 35, No 1, pp. 1-17 (2002).