Sterilization Mode Effect on the Hydration Rate of Calcium Sulfate Hemihydrate
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STERILIZATION MODE EFFECT ON THE HYDRATION RATE OF CALCIUM SULFATE HEMIHYDRATE
T. R. Devine USG Corporation, 700 N. Highway 45, Libertyville, Illinois 60048
ABSTRACT Plaster bandages have been found to support bacteria. The sterilization of plaster (calcium sulfate hemihydrate) for use as an implant biomaterial is, therefore, a primary concern. Heat sterilization can significantly alter material properties and the kinetics of plaster hydration. A study has been done of the effects of sterilization by gamma radiation and by ethylene oxide on the hydration rate of plaster. Three hemihydrate bases with differing water requirements and hydration rates were sterilized by each method. Adiabatic temperature rise was used to follow hydration. Five outcomes were measured and analyzed. These outcomes include the induction period, maximum rate of temperature rise (MRR), time to MRR, total temperature rise, and time to maximum temperature. Neither sterilization mode was found to significantly alter the hydration process of any of the plaster bases. From a material property standpoint, either gamma radiation or ethylene oxide treatment is preferred to heat sterization of calcium sulfate hemihydrate. INTRODUCTION A prime concern of medical product companies which have evaluated the use of a medical grade of calcium sulfate hemihydrate for bioimplant applications has been the degree to which the performance of the material would be effected or degraded by sterilization. The most common methods of sterilizing materials are heating, exposure to ionizing radiation, and exposure to chemical agents. The beat treatment of calcium sulfate hemihydrate can alter the water demand, particle size distribution, hydration kinetics, and strength of the hydrated cast. Sufficient heat could cause a chemical change involving the conversion to soluble anhydrite with the loss of combined water, and subsequently to insoluble anhydrite involving a rearrangement of the crystal symmetry. For these reasons, our recommendation has always been to use gamma radiation as the preferred method of sterilization. The bulk of the scientific literature on the subject (vid infra) suggests minimal effects due to sterilization by radiation. This work, however, has sometimes been performed by researchers unfamiliar with the details of the material properties involved. Also, only one report of ethylene oxide sterilization has appeared.1 Prior work has examined the time to hardening or ultimate strength development as the measure of utility of sterilized CSH articles. Because the hydration of CSH to calcium sulfate dihydrate is exothermic, the adiabatic reaction can followed by monitoring the temperature rise profile. Significant kinetic information is contained in this profile and material property changes can be correlated to changes in it. Therefore, it was decided to investigate the effect of sterilization on adiabatic temperature rise of CSH. EXPERIMENTAL DETAIL Materials Three calcium sulfate hemihydrate (CSH) bases were prepared with various normal consistencie
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