Effect of drill speed on the strain distribution during drilling of bovine and human bones

Drilling is an operation commonly required in orthopaedic surgery for insertion of screws and internal fixation of bone fractures. Induced damage is one of the undesired effects of drilling mainly due to the use of inadequate drilling parameters. During the recent years, scientists have been trying to describe the relationship between drilling parameters and bone injury. However, no studies have examined the level of strain generated in the bone during the drilling process. This paper focuses on the analysis of different drill speeds during drilling of fresh bovine femora and human cadaveric tibiae. The main contribution of this work is to determine how differences in applied drill speeds affect the strain of cortical tissue near the drilling site and the drill bit temperature. Strains were measured in ex-vivo material during the osteotomy preparation with three drill speeds (520, 900 and 1370 r.p.m.). Additionally, a thermographic camera was used to measure the drill bit temperature. As the drilling operations are blind in nature with unknown depth, the osteotomies were performed using a drill press machine without control of the feed rate or depth. Drill bit geometry was kept constant with 4 mm of diameter, point angle 1200 and helix angle 300. The tests were conducted at room temperature without applying cooling at the drilling zone. Bone strains near to the drilling sites were recorded with high accuracy using linear strain gages mounted around the diaphyseal cortex. It was noted that the bone strain and drill bit temperature increased with an increasing drill speed. Human and bovine bone samples presented significantly different levels of strain and temperature. Both strain and temperature were higher when drilling bovine femora than when drilling human cadaveric tibiae. DOI: https://doi.org/10.24243/JMEB/2.5.170


Introduction
The induced damage from the interface between bone tissue and cutting tool in surgical drilling procedures has been investigated for years. Prediction and control of bone drilling forces and heat generation have been the main concerns when evaluating the drilling performance [1], [2]. It is currently known that the drilling parameters such as drill speed, feed rate, drill geometry, the use of irrigation, predrilling, cortical thickness and bone density have significant influence on bone injury [1]. However, bone drilling in orthopaedic surgery is performed manually without detecting drilling force or depth and the drilling quality mainly depends on surgeon's manual skills. Drill speed is one of the parameters that can be controlled and generally recommended by the manufacturer. Studies have shown that high speeds result in increased trauma [3]- [5].Nevertheless, the literature review on the drill speed during drilling suggests no consistent trend [1]. Few studies have examined the effect of drill speed in human bone and no studies have inquired into this aspect for the strain magnitude during drilling. The challenges on bone drilling should not be limited to avoid thermal damage or excessive drilling forces but also to predict the strains affecting the bone tissue.
The present study is focused on an experimental methodology to analyse the effect of different drill speeds on the strain generation when drilling bovine femora and human cadaveric tibiae. Simultaneously, the temperature distribution on the drill bit surface was measured with the thermography equipment. This research complement our previous contributions in bone drilling [4]-[10].

Bone samples preparation
Ex-vivo study was performed on fresh middle diaphysis segments of bovine femur and non-embalmed sections of human cadaveric tibiae as the test models.
The human cadaveric tibiae were obtained with the permission of the author's institutional research ethics board. The tibiae samples were processed in the Body Donor's Service and Dissection Room of the University of Barcelona. Human tibiae were stripped of soft tissues and then visually inspected to exclude bone pathology. Donor medical histories, when available, were accessed to verify the absence of bone pathology, see Fig.1 Bovine femora were selected since it's considered the closest animal bone to replicate the characteristics of the human osteonal remodelling process [12]. No animals were sacrificed for the current ex-vivo study, the bones were acquired in the local slaughterhouse. Samples represented in Fig.1(b)were formed from femur diaphysis with average cortical thickness of 6mm. In order to preserve their thermo-mechanical characteristics, all samples were maintained in saline solution and stored in plastic bags at -4 °C according to the guidelines specified in [13]. Prior to the experimental tests, the samples were completely thawed at room temperature for 24 hours.

Drilling tests and strain measurements
The drilling tests were carried out in a vertical drilling machine with options of regulation of drill speed. The osteotomies were made with a drill bit (Ø4 mm, point angle of 118º and helix angle of 30⁰) and drill speed rates of 520, 900 and 1370 r.p.m. The selection of bone drilling parameters is based on the existing clinical practice based on handheld drilling machines. This way, feed rate and osteotomy depth depend on the sensibility of the operator. To obtain the feed rate, drilling time and osteotomy depth were measured with an appropriate depth gauge and the average for each sample was calculated. All experiments started from room temperature (20-24°C) without any coolant. Between successive experiments, sufficient time was allowed for the bone tissue and the drill bit return to room temperature.
The strain measurements during bone drilling were made through linear strain gauges (1-LY18-6/120, 120Ω±0.35% from HBM). The strain gauges were bonded directly to the bone surface. A small amount of adhesive was applied to the strain gauge, which is immediately placed in the prepared surface of the bone being careful to observe a correct alignment. To promote the uniformity of results, all gauges were mounted in identical locations, considering the same distance of 3.5 mm between the edge of the osteotomy and the middle of the strain gauge (Fig. 2). An acquisition data system (Vishay Micro Measurements P3 Strain Indicator and Recorder) allowed recording the strains during drilling.
Temperature measurements were also considered in the present study. A thermal camera (Therma CAM 365, FLIR Systems) was positioned at a distance of 1.5 m from the drilling area and allowed to obtain thermal images of the drill bit surface, before and immediately after drilling. The overall experimental setup for bone drilling is shown in Fig.  2. Measurements for each combination of machining parameters were randomly repeated six times.

Results and Discussion
Significant differences in the strain and temperature measurements were found in bovine and human bone samples as drill speed varied. Comparisons of the maximum strain values average are presented in Fig. 3. It became evident that changing the drill speed directly impacts the strain generated at the bone surface close to drilling zone. Overall, increasing the drill speed resulted in an increase in maximum bone strain. In the case of human cadaveric tibia, it was observed that when the drill speed was increased from 520 to 900 r.p.m. the strain increased by 19.95% and when the speed was increased from 900 to 1370 r.p.m. the strain increased by 58.34%. In the case of the bovine femora, the increase of bone strain is still more accentuated than in human cadaveric tibiae. Strain increased significantly as the drill speed was changed from 520 to 1370 r.p.m. Comparing the level of strain for different drill speeds, it was observed that when the drill speed increased from 520 to 900 r.p.m. the strain increased by 47.67% and when the drill speed increase from 900 to 1370 r.p.m. the strain increased by 98.80%. The authors of this study found similar trends in the polyurethane foams with mechanical properties similar to the cadaveric human bone, including FE studies [4].
To display the relations of drilling speed and drill bit temperature, the temperature variation was calculated and compared subtracting the recorded temperature (TR) with the initial temperature of the drill bit (T0) for each osteotomy (ΔT). The results were summarized using means values of ΔT. Fig. 4 shows the result of ΔT considering the different drill speeds and bone tissues. It was observed that the drill bit temperature increased with increasing drill speed. By carrying out this analysis, it can be expected that the bone temperature also increased with the increase in drill speed. One explanation for these results could be that the number of cuts and the amount of friction between the drill and the bone will be relatively increased, thus leading to a higher accumulated friction energy and a higher bone temperature rise. These outcomes are consistent with our already published works, using bovine and porcine bones [10]. In addition, other authors suggest the same results [14].

Fig. 4 Drill bit temperature as a function of drill speed and bone tissue, n number of holes, mean of feed rate (SD) in mm/min
Concerning bovine femora versus human cadaveric tibiae, their behaviour show a similar trend both for strains and drill bit temperatures. However, the strain and drill bit temperature generated when drilling bovine femora were higher than that produced when drilling human cadaveric tibiae. This behaviour has been expected, since the bovine femur is much harder and the depth of the cortical thickness is greater than the human bone. The average cortical thickness for the bovine femorawas6 mm while for the human cadaveric tibiae was 3.5 mm.

Conclusions
Analysis of the relationship between drilling conditions and strain or temperature rise are crucial to prevent bone injury during the osteotomy. This study demonstrated the interaction on the effect of different drill speeds in the bone strain and drill bit temperature generation during drilling of bovine femora and human cadaveric tibiae. The parameters for the drilling tests included three drill speeds and feed rates obtained by the manual drilling, as it happens in clinical practice. The use of different drill speeds and the inherent variations of bone material properties bring a strong variation on the strain magnitudes and drill bit temperatures.
In general, it was observed that an increase in the drill speed could raise the bone strain and the drill bit temperature during bone drilling. Both bone strain and drill bit temperature were higher when drilling the bovine femora than when drilling human cadaveric tibiae. Variability in bone properties and mainly in the cortical thickness lead to significantly variations on the strains and temperatures during drilling. Lastly, the results from the present study can provide reference values for the development of new surgical power tools in orthopaedic bone surgeries application.