Prevention of Thermal Tissue Injury Induced by the Application of Polymethylmethacrylate to the Calvarium

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Abstract
The exothermic reaction produced during the polymerization of polymethylmethacrylate yields temperatures in excess of 180 degrees F (81.4 degrees C). At these elevated temperatures, significant bone and dural necrosis occurs. In an attempt to prevent thermal injury during craniofacial surgery, surgeons irrigate the polymerizing implant with cold saline, assuming that this will decrease both the absolute temperature elevation and the duration of the exothermic reaction, making the use of methylmethacrylate safe. Unfortunately, no experimental evidence exists to support this claim. To test the safety of methylmethacrylate during craniofacial surgery, we constructed an in vitro model that allowed us to measure directly the heat transferred from the methylmethacrylate to the underlying dura or bone during polymerization. In this model, the methylmethacrylate implants were 6 cm in diameter, but they varied in thickness. We hypothesized that methylmethacrylate could be safely applied up to a certain thickness, after which the temperature rise on the undersurface of the implant would no longer be controlled by any amount of cold saline irrigation, and thermal necrosis would occur. We found that without irrigation all implants produced temperatures in excess of 180 degrees F (81.4 degrees C) internally. This peak temperature lasted for approximately 45 seconds and then declined to a baseline temperature of 98 degrees F (36.3 degrees C) over 5 to 6 minutes. Upon irrigation of the implants with cold saline (4 degrees C), the level and duration of temperature elevation conducted to the underlying surface varied significantly on the basis of implant thickness. Implants smaller than 5 mm thick produced temperatures on the bone surface of 108 degrees F (41.8 degrees C) for 11 seconds. This temperature control was partially lost at 7 mm when the underlying surface reached a temperature of 134 degrees F (56.1 degrees C) for 15 seconds. Methylmethacrylate implants with a thickness greater than 7 mm produced temperatures of 145 degrees F (62.2 degrees C) for 22 seconds, indicating that the cold was insufficient to protect against thermal injury. Therefore, we conclude that polymethylmethacrylate can be applied safely to the skull if the implant is 6 mm thick or smaller. With polymethylmethacrylate thicker than this, the heat produced during polymerization cannot be controlled, and thermal necrosis to the underlying tissues is inevitable.

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