P. Sharrock, M. Fiallo (Castres, France)
We have prepared a low viscosity bone cement containing 7% by weight of Hydroxylapatite powder. The polymerization of this cement was studied as a function of the cement’s temperature at the moment of mixing and as a function of the environmental temperature. The cement was placed in an isolating medium (pelvic duck bone) or a conducting medium (stainless steel) in various quantities. A thermocouple was placed in the hardening cement to follow the temperature rise during the polymerization reaction. Simultaneously, an infrared temperature sensor was used to record the temperature on the outside of the holding medium.
We observed that the temperature peak decreases as the mass of cement decreases. In bone, it goes from 55_C for 8g of cement to 38_C for 2g of cement when the initial temperature is near 20_C. The maximum temperature is reached at 10 minutes for 8g of cement and at 14 minutes for 2g of cement. There is a significant temperature difference between the inside and outside temperature of the bone ranging from 2_C to 8_C.
For 2g of cement placed in stainless steel sandwich, le initial cement temperature was set at 15_C and the metal at 10_C, 15_C or 20_C. The corresponding temperature rises were 19.3_C, 21.0_C and 28.8_C, while the Tmax times were 14, 11 and 13 minutes. When both the metal and the cement were at the same initial temperature (15_C), the polymerization began at 5 minutes and a constant temperature difference near 1.5_C was observed between the cement and the metal.
When the metal was at 10_C, the cement’s temperature decreased to 13_C after 6 minutes then rose slowly to reach at 4_C difference above the metal’s temperature. When the metal was at 20_C, it’s temperature decreased to 17_C at 7 minutes then followed closely the cement’s temperature rise with less than one degree difference. We conclude that the metal acts as a heat sink for the exothermic polymerization and retards the time to reach the maximum temperature except when the metal is initially warmer than the cement.
DSC measurements revealed the heat of reaction to be constant for the different conditions but split into one broad and one sharp exothermal peak. Residual monomer analyzed by GC was less than 1% in all cases. The mechanical shear strengths were statistically equal for all temperatures. This leads us to propose that pain relief during vertebroplasty is not related to monomer release or temperature effects but simply to mechanical stabilization.