Effect of hyperthermal treatment on the viability of bone-derived cells

Polymethylmethacrylate (PMMA) bone cement has been widely used in orthopedic surgery for fixing prostheses and stabilizing collapsed vertebral fractures. Although it is the most popular biomaterials in orthopedics, heat generation during polymerization may cause thermal injury to the surrounding cells. Bone cells exposed to the thermal injury would secret a number of biological factors, and then locally influence a balance of bone remodeling. However, thermal tolerance of bone cells is not well understood. The aim of this study was therefore to quantify in vitro thermal injury of bone-derived cells and to establish a model to predict accumulation of the cell damage. Osteocyte, osteoblast, and fibroblast cell lines were exposed to steady supraphysiological temperatures ranging 40-75°C, and change of cell mortality depending on heating time and temperature was determined by using a dye, propidium iodide. When the cells were exposed to thermal treatment, all cell lines exhibited approximately exponential increase of cell injury at the initial phase, and then gradual decline of the increasing rate as the cell mortality approached 1. This kinetics of cell injury was described well by a logistic curve with high correlation coefficient. By comparing the slope of the logistic curves and the time to reach 50% of cell mortality, it is found that thermal tolerance of osteocytes was significantly low among three cell lines. This result indicates that thermal injury of osteocytes would be induced by the heating at a temperature that is harmless to other cell types, and both necrotic and apoptotic responses of thermally injured osteocytes might stimulate osteoclastic bone resorption.