Temperature measurement at the cutting edge

Objectives: Hippocrates recognised the importance of avoiding excessive heat generation when drilling bone, writing in his theory and practice of medicine, he suggested that cooling should be applied to the trepanning tool when discs of bone were removed from the skull. Attempts to evaluate events that result in thermal injury are restricted by lack of any satisfactory means of measuring the temperature rise at the drilling surface. We report a method whereby a thermocouple housed inside the cutting tip of the drill enables study of heat production during drilling. Methods: A rig attached to a Lloyd tensometer* enabled a drill, spinning at 2,000 rpm, centred in line with the load cell to be subjected to a constant force of 1.7Kg ( average load applied by an experienced operator). A diamond coated tapered drill# (diameter 3.5mm, length 13mm) had a K type thermocouple, 0.5 mm diameter connected to an portable data logger, placed at the drill tip via the drills internal irrigation channel. External irrigation, normal saline at room temperature, was employed for cooling purposes. Drilling was carried out in acrylic and bovine bone. Results: Drilling in acrylic blocks produced a rapid and consistent increase in temperature, mean time to reach 47ºC = 2.19 seconds (SD±0.47). Drilling in bovine bone produced a smaller increase, which was more variable, mean time to reach 47ºC= 13.6 seconds (SD± 4.98). The minimum period of continuous drilling in dense cortical bone before the critical temperature of 47°C reached was 8.4 seconds. Conclusion: This study demonstrates the potential for using the internal irrigation channel as a means of accessing the drill tip for thermal measurements. Intermittent drilling protocols applying force for less than 8 seconds should give a wide margin of safety. *Model LRX, Lloyd Instruments Ltd. UK. #Nobel Biocare Replace®.UK. @Omega Engineering, Manchester, UK.