Acrylic bone-cement. Influence of mixer design and unmixed powder.

The effects of hand mixing with two different mechanical mixing systems (fixed versus rotating central axis) on unmixed powder content, macroporosity, density, and bending strength of acrylic bone-cement are compared. The effects of voids and unmixed powder on cement bending strength are also evaluated. In acrylic cement, both unmixed powder monomer and voids 1 mm and larger can be easily visualized and analyzed on radiographs of 3-mm-thick samples. Image analysis allowed demonstration 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 rotating-axis system produced cement of higher density compared with hand mixing only (P = .004). There was a significant correlation between the number of voids measured per square centimeter and cement bending strength (P < .0001), as well as an independent and significant correlation between unmixed powder content and cement bending strength (P < .0001). Mechanical mixing using a fixed central axis produced significantly weaker cement compared with both hand mixing (P < .015) and the rotating-central-axis system (P < .0001). A 15% drop in strength between the two mechanical mixing systems was observed. It is therefore concluded that the use of different rotating systems in mechanical mixers can influence void and unmixed powder content and, consequently, the mechanical properties of acrylic cement, and that unmixed powder is an independent factor affecting the bending strength of the cement.