An in-vitro study of temperature rise during rotary ultrasonic bone drilling of human bone
Introduction
Orthopaedic injuries are common especially in sports injuries, industrial accidents, age factor, and various bone diseases, etc. These injuries are broadly divided into minor and major injuries. A minor injury is treated by non-invasive techniques such as by plasters to immobilize the affected site. In case of major injury, the fracture is usually fixed by direct approach either by internal or external fixation. In an internal fixation, which is commonly used in orthopaedic surgery for treating major injury, the fractured parts of the bones are joined together and fixed with the help of metal plates. In both internal and external fixation bone drilling is required to fix the fractured parts by inserting screws. During drilling, the bone remains in contact with a high-speed rotating tool and the mechanical rotational energy is converted into heat energy [1]. The friction between the rotating tool and the bone generates heat at the drilling site. The temperature rise at the drilling site is also due to poor thermal conductivity of bone, which usually lies between 0.2–2.27 W/mK [2]. High temperature in bone may lead to reduced synthesis of proteins [3,4] and permanent death of bone cells (osteonecrosis) [5,6]. Thermal osteonecrosis may play a significant role in the failure of osteosynthesis process.
A large number of studies [7], [8], [9], [10] have been performed in last decades on animal bones to establish the role of various conventional drilling (using a medical twist drill bit) parameters on temperature rise, and to determine the threshold value above which osteonecrosis occurs. It has been reported that the exposure of bone to a temperature of 47 °C for 60 s [11] and 50 °C for 30 s [12] results in the permanent death of the osteocytes (bone cell). Augustin et al. [8] performed experimental study using conventional tool on cortical bone and found that internal irrigation (cooling) resulted in minimum temperature rise which was below the necrosis threshold temperature. Bachus et al. [10] in the experimental study by conventional drilling process found that the temperature rise during bone drilling is inversely related to cutting force. In a comparative study on human, dog and rabbit femora drilling, Eriksson et al. [13] observed that the heat generation in human and dog bones was above the threshold value of thermal osteonecrosis.
Various attempts using different irrigation techniques [8,12,[14], [15], [16], drill bit designs [1,5,8,[17], [18], [19] and new drilling technique [18], [19], [20], [21] were made to minimize the temperature rise during the drilling process. An irrigation (internal/external) was the most efficient method to reduce heat generation during drilling [12], but it can cause infection at the treated site. Many prior studies [21], [22], [23], [24] showed that imparting ultrasonic vibration to the rotating tool during drilling resulted in lesser temperature rise as compared to the conventional drilling. Alam et al. [21] performed a comparative study between the conventional drilling and imparting ultrasonic vibration to the conventional drill bit and reported that the later produced lesser temperature rise at the same drilling conditions. Similar findings were observed by Shakouri et al. [20] in their work. Gupta et al. [18,24] recently introduced a novel rotary ultrasonic bone drilling (RUBD) technique using a diamond abrasive coated tools. Authors found that temperature generated in drilling by RUBD technique was below the threshold temperature of necrosis (47 °C) and comparatively lesser than conventional drilling tool. The experiments were performed on porcine femur by a computerized numeric controlled (CNC) machine. In an another histopathological study by the author [19] it was reported that RUBD produced no necrosis to the bone cell.
The RUBD technique which showed potential over conventional drilling was performed on heavy and bulky CNC machine, which has a limitation to be used in operation theatres (OT). Moreover, the effect of drilling parameters on temperature rise in RUBD technique has not been tested on human bones till now. It has been reported [25] that human bone has higher shearing strength than porcine bone. Various prior studies [9,[25], [26], [27] also found that the biological and mechanical behaviour of human bone is different from animal bone. Also, the effect of varying the abrasives particle size which may influence the temperature rise during drilling in RUBD has not been reported yet.
The present work focuses on the experimental investigations on human tibia (hard bone) using a recently developed operation theatre (OT) compatible ultrasonic bone drilling machine. A comparative study was performed to study the effect of RUBD and Conventional surgical bone drilling (CSBD) techniques on temperature rise with the new developed machine. Response surface methodology (RSM) was used to plan the set of experiments to study the effect of various drilling parameters including abrasive particle size on temperature rise in RUBD process. Regression equation was used to develop a statistical model at a confidence interval of 95 percent to predict the temperature rise at different drilling parameters in RUBD process.
Section snippets
Work specimen
The human cadaver bone specimen for the experimental investigation of temperature rise was obtained from All India Institute of Medical Sciences (AIIMS), New Delhi. The bone samples from the tibial shaft were obtained at autopsy and immediately preserved in 10 percent buffered formalin solution (Formaldehyde – Sigma Aldrich) to retain the morphological characteristics of the bone [28]. The bone marrow and all the soft tissues were removed from the bone samples and experiments were conducted on
Temperature at the surface of the drilled hole ‘T’
It is difficult to measure the temperature at the inner surface of the hole during drilling with the help of K type thermocouples. For measuring temperature from Th1, Th2, Th3 and Th4, which were inserted at 0.5 mm, 1.0 mm, 1.5 mm, and 2.0 mm, respectively, from the wall of the hole, following equations were used to derive temperature from the four positions.where T1, T2, T3 and T4 represent temperature recorded by thermocouple Th1, Th2, T
Experimentation techniques
A set of 7 experiments as shown in Table 2 were performed to compare the temperature genarted during drilling by the two techniques. Three experimental trials were performed for each combination of parameters and the average of the temperature rise at the bone and tool interface ‘ΔTavg’, was calculated (Ref. Supplementary Table 1 and Table 2). Table 2 displays the average value of ΔT for each experiment set in both RUBD and CSBD technique. In the second phase, Central Composite Design (CCD)
Comparative study
The gross examination as shown in Fig. 5 indicates that at same drilling parameters, CSBD process generated higher temperature, which produced yellowish chips indicating burning of the bone. Whereas, in RUBD process, the chips were whitish in colour indicating no burn.
Fig. 6 illustrates the effects of N, Frand Dd on temperature rise in both RUBD and CSBD processes. It can be clearly elucidated from Table 2 and Fig. 6 that with any combination of drilling parameters, the temperature rise in RUBD
Optimization
The optimization of ΔT was done with the help of Genetic algorithm (GA) tool box with the MATLAB 2014 software. The prime objective of the optimization was to obtain the set of drilling parameters which could minimize the ΔT. Eq. (2), the obtained model for ΔT was used as the objective function. Following equations represent the objective and the constraints used for the optimization.
The experiment was performed to compare the
Conclusions
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RUBD produces lesser temperature rise (40 −50 percent) than that of CSBD at same bone drilling conditions.
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The larger size chips, which were produced in CSBD process, are associated with more temperature rise than that of smaller chips, which were produced in RUBD.
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The statistical model was developed successfully and validated at a confidence interval of 95% to predict the temperature rise in RUBD.
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Rotational speed contributes maximum (51.8%) followed by drilling diameter and abrasive particles
Declaration of Competing Interest
None.
Funding
This work was supported by the Society for Research and Initiatives for Sustainable Technologies - SRISTI (Sanction No.: MOU BET. IITD AND SRISTI), Ahmedabad, Gujarat.
Ethical approval
The work included experimental investigation on human cadaver bone, therefore, the ethical approval was required from the institute ethics committee (IEC) of both IIT, Delhi and AIIMS, New Delhi. Ethical approval was received from both IEC AIIMS Ref No: IEC-273/01.06.2018, RP-32/2018 and from IEC IITD Ref No: Ethics Application P-024.
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