Thermal stability of acrylic bone cement powder under shelf storage conditions: An isothermal microcalorimetric study

Heat-conduction isothermal microcalorimetry was used to measure the exothermic heat flow rate (Q) from the powder of a widely used commercially-available acrylic bone cement, Palacos (R), when it interacted with ambient laboratory air, as a function of time, t, in the calorimeter, for up to 200 h. Four variants of the powder were used, these being unsterilized, sterilized using ethylene oxide gas, gamma-irradiated, in ambient air, with a minimum dosage of 2.5 Mrad, and gamma-irradiated, in ambient air, with a minimum dosage of 4.5 Mrad. In each case, the powder variant was tested after being stored on the shelf, under ambient conditions, for 2 days, 3 weeks and 9 months immediately following sterilization. Best-fit correlations between Q and t for each powder variant were determined. Then, this relationship was integrated over the period 14 h <= t <= 200 h to give an estimate of the "effective" heat flow, Q(eff). For powder variants tested 2 days after being sterilized, the difference in their thermal stabilities (Q(eff) ranged from 0.19 +/- 0.01 to 0.62 +/- 0.03 mu J/g, respectively) was significant in the case of some pairs and not for others. However, for powders tested either 3 weeks or 9 months following sterilization, there was no significant difference between the means of Q(eff) (they ranged from 0.18 +/- 0.01 to 0.31 +/- 0.07 mu J/g) for any pair. These results suggest that an acrylic bone cement in which the powder is EtO-sterilized may be mixed with the liquid monomer for use in cemented arthroplasties after any length of time of shelf storage of the powder, under ambient conditions, whereas, for powders that are gamma-irradiated and then stored under the same conditions, at least 3 weeks should elapse before they are used in these procedures.