Temperature changes during cortical bone drilling with a newly designed step drill and an internally cooled drill

Abstract

Purpose: Bone drilling causes an increase in bone temperature, and a temperature above 47°C is critical because it causes thermal bone necrosis. Thermal osteonecrosis is common with the drill diameter of ≥4.5 mm without cooling. The aim of this study was to determine the increase of bone temperature during drilling using newly contructed two-step and internally cooled drills.

Methods: An experiment was set up according to a central composite design. An internally cooled drill (3.4 mm and 4.5 mm) and a two-step drill (2.5/3.4 and 3.4/4.5 mm) were used in combination with feed rates of (0.02, 0.04, 0.10, 0.16 and 0.18 mm/rev) and cutting speeds (1.18, 10.68, 33.61, 56.55 and 66.05 m/min) with and without cooling with water of 24°C. Bone temperatures were measured with thermocouples. Drilling was performed on pig diaphyses with a three-axis mini milling machine.

Results: Bone temperatures in all combinations of parameters with internal cooling were below the critical 47°C (p=0.05). The highest temperatures were detected using a 4.5-mm drill (40.5°C). A statistically significant effect other than cooling was found with the drill diameter and feed. A drill diameter of 3.4 mm with internal cooling developed a maximum temperature of 38.5°C and without cooling 46.3°C. For the same conditions a drill with diameter of 4.5 mm reached temperatures of 40.5°C and 55.7°C, respectively. The effect of feed rate is inversely proportional to the increase in bone temperature. With the feed rate 0.16 mm/rev, temperature was below critical even using the 4.5-mm drill (46.4°C, p=0.05). Using the 3.4-mm drill all temperatures were below critical (46.2°C, p=0.05). The two-step drill compared to a standard drill with the same diameter did not show statistical differences in maximum bone temperatures for all combinations of parameters (p=0.05).

Conclusions: A two-step drill does not have any advantages over a standard twist drill of the same diameter. An internally cooled drill causes a significantly smaller increase of bone temperature during drilling with water of 24°C. An internally cooled drill is currently the 'ideal' drill for traumatology/orthopaedics because it produces the smallest increase in bone drilling temperature. If internal cooling is used the regulation of other drilling parameters is of no importance.

Figures

Fig. 1

Complete process of bone drilling and bone temperature measurement. Porcine femoral diaphyses of the hind legs of 75 mm length were used. The maximum bone temperatures during drilling were measured with the thermocouple at a distance between drilling site and thermocouple site of 0.5 mm. All measurements were made on a three-axis mini milling machine, Flexmatic FA 530 S, enclosed in a thermally isolated chamber where the air and bone temperatures were maintained at 37°C. The cooling fluid of water at 24°C went through the tool. Cortical thickness was measured with a depth gauge for screws (Synthes, Switzerland)

Fig. 2

Specific software developed and programmed in LabView providing equal and standardised conditions for every drilling (see text for details)

Fig. 3

Two-step drill with diameter of 4.5 mm and channels through the drill with openings on the tip where cooling fluid exits

Fig. 4

According to the regression line and prediction interval a bone temperature of 47°C will last (mean value) at that temperature for 11 seconds (39 seconds with 95% of upper confidence interval). A bone temperature of 50°C will be above 47°C for a mean of 21 seconds (50 seconds with 95% of upper confidence interval)

Fig. 5

Cutting lips before and after 180 drillings of 2.5/3.4-mm carbide drill with channels for internal irrigation showing no wear on both smaller diameter drill tip and transitional cutting lips to larger diameter part of the two-step drill