Experimental measure of temperature variations in bone due to certain physical parameters during the bone drilling process

Abstract

Background. During bone drilling, the cyclic use of the same drill bit causes progressive wear of the cutting edges. The resulting high frictional forces can lead to a large temperature variation around the drill point. Above 55°C, irreversible damage occurs to the bone tissue. Objective. To follow the evolution of the wear of a bit by measuring the temperature around the focus of the drilling, during repeated drilling by the same bit. Methods. An experimental study was conducted on a test bench, measuring temperatures during bone drilling as a function of spindle speed, feed rate and drill bit duty cycle. Bovine tibia cortices were used due to mechanical properties similar to those of human bone. Six thermocouples were placed in a spiral around the focus of the drilling with an increment of 0.5mm, so as to determine the heat flow during the drilling. The cutting parameters were: spindle speed 100, 200, 300 rpm, Feed rate 30, 60 mm/min; Number of holes 1, 2, 3. The same 3.2 mm diameter wick was used 3 times and the temperatures were measured at each test. Results. The risk of having temperatures above the critical threshold of 55°C was greater at the feed rate of 30 mm/min than at the feed rate of 60 mm/min. Temperature increased with both spindle speed and number of pierces regardless of other cutting conditions. The best cutting conditions were found for a spindle speed of 200 rpm and a feed rate of 60 mm/min and at a spindle speed of 100 rpm for a feed rate of 30 mm/min. When the spindle speed exceeded 200 rpm, the temperature seemed very high. The temperature gradients were calculated under drilling conditions where the maximum temperatures measured at the point closest to the drilling focus were below 55°C. The equations were used to calculate projected temperatures at the bone-wick interface (x=0). Conclusion. It was observed in the majority of cases that the temperatures were above the threshold of 55°C. It became clear that the thermal conductivity of bone is low. The heat remains concentrated around the piercing point.