Differential blood contamination levels and powder-liquid ratios can affect the compressive strength of calcium phosphate cement (CPC): a study using a transpedicular vertebroplasty model

Calcium phosphate cement (CPC) is a potentially useful alternative to polymethylmethacrylate (PMMA) for transpedicular injection into osteoporotic vertebral fractures. Unlike PMMA, CPC is both biocompatible and osteoconductive without producing heat from polymerization, but it has lower compressive strength compared to PMMA. This in vitro model experiment analyzed how different CPC powder-liquid ratios (P/L ratios) and injection methods may minimize blood contamination in the CPC and, thereby its reduction in compressive strength. (1) CPC of different P/L ratios of 4.0, 3.5, and 3.2 was equally mixed with different amounts of freshly obtained human venous blood, producing cylindrically shaped CPC samples. (2) Using a transpedicular vertebroplasty model containing blood in the bottom, CPC pastes of different P/L ratios were injected with the nozzle of an injection gun affixed either to the bottom (Bottom method) or to the top of the container (Top method). All cylindrical CPC samples thus obtained were immersed in simulated body fluid and then underwent compressive strength tests at 3 h-7 days post-immersion. In CPC equally mixed with blood, lower P/L ratios and a larger amount of blood contamination reduced compressive strength more significantly. Of the two methods of CPC injection, the 'Bottom method' produced significantly greater compressive strength values than the 'Top method'. When performing CPC-assisted vertebroplasty, a greater load bearing-support can be obtained by injecting CPC paste of a high P/L ratio of 4.0 into the deepest part of the space inside the vertebral body to minimize blood contamination.