ACRYLIC BONE-CEMENT - INFLUENCE OF MIXER DESIGN AND UNMIXED POWDER

The effects of hand mixing with two different mechanical mixing system s (fixed versus rotating central axis) on unmixed powder content, macr oporosity, density, and bending strength of acrylic bone-cement are co mpared. The effects of voids and unmixed powder on cement bending stre ngth are also,evaluated. In acrylic cement, both unmixed powder monome r and voids 1 mm and larger can be easily visualized and analyzed on r adiographs of 3-mm-thick samples. Image analy sis allowed demonstratio n of a significant increase in unmixed powder content (P < .0001), in cement prepared using a vacuum mixing system with a fixed central axis compared with both the rotating axis system and hand mixing. The rela ting-axis system produced cement of higher density compared with hand mixing only (P = .004). There was a significant correlation between th e number of voids measured per square centimeter and cement bending st rength (P < .0001), as well as an independent and significant correlat ion between unmixed powder content and cement bending strength (P < .0 001). Mechanical mixing using a fixed central axis produced significan tly weaker cement compared with both hand mixing (P < .015) and the ro tating-central-axis system (P < .0001). A 15% drop in strength between the two mechanical mixing systems was observed. It is therefore concl uded that the use of different rotating systems in mechanical mixers c an influence void and unmixed powder content and, consequently, the me chanical properties of acrylic cement, and that unmixed powder is an i ndependent factor affecting the bending strength of the cement.