Bioactive PMMA-Based Bone Cement Modified with Methacryloxypropyltrimethoxysilane and Calcium Salts-Effects of Calcium Salts on Apatite-Forming Ability-

Abstract

Bone cement consisting of polymethylmethacrylate (PMMA) powder and methylmethacrylate (MMA) liquid is clinically used for fixation of implants such as artificial hip joints. However, it does not show bone-bonding ability, i.e. bioactivity. The essential prerequisite for an artificial material to bond to bone is the formation of an apatite layer on its surface when implanted in a bony defect. This apatite layer can be, for example, formed in a simulated body fluid (Kokubo solution) with ion concentrations similar to those of human extracellular fluid. The present authors recently showed the potential of bioactive PMMA-based bone cement through modification with y-methacryloxypropyltrimethoxysilane (MPS) and calcium chlorides. In this study, the effects of the kinds of calcium salts on apatite-forming ability were examined by exposure to Kokubo solution. PMMA powder (molecular weight of 100000 and 14 μm in average particle size) was mixed with 20 mass% of calcium salts consisting of calcium chloride (CaCl 2 ), calcium acetate (Ca(CH 3 COO) 2 ), calcium hydroxide (Ca (OH) 2 ), calcium lactate (Ca(CH 3 CHOHCOO) 2 ), calcium benzonate (Ca(C 6 H 5 COO) 2 ) and calcium methacrylate (Ca(CH 2 = C (CH 3 )COO) 2 ), while 20 mass% of MPS was added in the MMA liquid. The appropriate setting time was obtained by the usage of calcium acetate or calcium lactate, although the modification with MPS and calcium salts leads to an elongation of the setting time of the cement. Apatite formation could be detected after soaking in Kokubo solution for 1 d when the cements were modified with calcium chloride, calcium acetate, or calcium methacrylate. Apatite formation could be observed on the modified cement with calcium hydroxide after 7 d of soaking in Kokubo solution. The higher solubility of calcium salts allows for a higher rate of apatite formation in Kokubo solution. Furthermore, changes in pH after exposure of the cement are related to the apatite-forming ability, because the addition of calcium hydroxide also gives the cement the ability of apatite formation, even if the cement releases a small amount of calcium ions into the surrounding fluid. Smaller dissolution of calcium salts results in less decrease in compressive strength after exposure to a body environment. The release of calcium ions, followed by changes in pH, must be controlled to achieve a successful design of bioactive PMMA bone cement by modification with MPS and calcium salts.