PREVENTION OF THERMAL TISSUE-INJURY INDUCED BY THE APPLICATION OF POLYMETHYLMETHACRYLATE TO THE CALVARIUM

The exothermic reaction produced during the polymerization of polymeth ylmethacrylate yields temperatures in excess of 180 degrees F (81.4 de grees C). At these elevated temperatures, significant bone and dural n ecrosis occurs. In an attempt to prevent thermal injury during craniof acial surgery, surgeons irrigate the polymerizing implant with cold sa line, assuming that this will decrease both the absolute temperature e levation and the duration of the exothermic reaction, making the use o f methylmethacrylate safe. Unfortunately, no experimental evidence exi sts to support this claim. To test the safety of methylmethacrylate du ring craniofacial surgery, we constructed an in vitro model that allow ed us to measure directly the heat transferred from the methylmethacry late to the underlying dura or bone during polymerization. In this mod el, the methylmethacrylate implants were 6 cm in diameter, but they va ried in thickness. We hypothesized that methylmethacrylate could be sa fely applied up to a certain thickness, after which the temperature ri se on the undersurface of the implant would no longer be controlled by any amount of told saline irrigation, and thermal necrosis would occu r. We found that without irrigation all implants produced temperatures in excess of 180 degrees F (81.4 degrees C) internally. This peak tem perature lasted for approximately 45 seconds and then declined to a ba seline temperature of 98 degrees F (36.3 degrees C) over 5 to 6 minute s. Upon irrigation of the implants with cold saline (4 degrees C), the level and duration of temperature elevation conducted to the underlyi ng surface varied significantly on the basis of implant thickness. Imp lants smaller than 5 mm thick produced temperatures on the bone surfac e of 108 degrees F (41.8 degrees C) for 11 seconds. This temperature c ontrol was partially lost at 7 mm when the underlying surface reached a temperature of 134 degrees F (56.1 degrees C) for 15 seconds. Methyl methacrylate implants with a thickness greater than 7 mm produced temp eratures of 145 degrees F (62.2 degrees C) for 22 seconds, indicating that the cold was insufficient to protect against thermal injury. Ther efore, we conclude that polymethylmethacrylate can be applied safely t o the skull if the implant is 6 mm thick or smaller. With polymethylme thacrylate thicker than this, the heat produced during polymerization cannot be controlled, and thermal necrosis to the underlying tissues i s inevitable.