Introduction: Thermal injury to nerves and vasculature is a feared complication during endonasal sphenoid drilling. A threshold of 43°C delineates risk of thermal injury to nerves, though no quantitative data guide drilling in this regard. We have developed a low-fidelity physical surgical simulator of the endonasal approach to the anterior skull base. Thermal properties were evaluated, as a part of an ongoing effort to develop this training tool.

Methods: A patient CT was used to produce a 3-D image for modeling. The simulator consists of three components: a replaceable plaster insert for drilling, a rigid skull frame, and silicon-based face mask. The insert was designed specifically to re-create the laminar structure of bone. Thermal properties of the bone-mimicking material were refined through machine-controlled drilling tests comparing plaster and bone. Thermocouples were embedded along the optic canals of 10 cadaver heads and 10 simulator inserts. Test drilling was performed under dry and irrigated conditions. Using 13% temperature increase above baseline as upper threshold, the percentage of time above threshold was utilized for comparison, in addition to absolute temperature range during drilling.

Results: In machine-controlled drilling tests, average measured tool-bone interface temperatures were 134°C and 122°C for plaster and bone, respectively. This difference was not significant. In hand-held drilling tests, the maximum temperature spikes during irrigated drilling were 22°C and 13°C in simulator and cadaver, respectively. The percentage of time over threshold temperature under irrigated drilling was more than 25% for cadaver and simulator tests. Without irrigation, temperatures exceeded maximum thresholds for >70% of the time in both simulator and cadaver, and rose as high as 60°C.

Conclusions: Simulator drilling produced temperature responses similar to those of cadavers. These results demonstrate the potential for thermal injury during sphenoid drilling, and provide a platform by which it can be measured, studied, practiced, and ultimately, mitigated.

Patient Care: Few options for training and practice in the endoscopic endonasal approach to the anterior skull base are available. We have created a low-fidelity simulator for sphenoid drilling. In addition to anatomic accuracy and the ability to use the same tools as in real cases, our simulator offers the ability to test for heat spread through bone to the optic and carotid canals. This innovation could potentially revolutionize the way that the endoscopic endonasal approach is taught and practiced. Validation tests as a surgical simulator is ongoing.

Learning Objectives: By the conclusion of this session, participants should be able to: 1) Describe our the importance of understanding the risk of thermal injury to nerves and arteries during bone drilling. 2) Discuss, in small groups, the potential implications upon heat dissipation mechanisms used during surgery. 3) Identify the potential applications for surgical training with the development of this simple, inexpensive surgical simulator.

References: Xu D and Pollock M. Experimental nerve thermal injury. Brain 1994; 117(2): 375-384.