Experimental investigations of forces and torque in conventional and ultrasonically-assisted drilling of cortical bone
Introduction
Bone cutting is one of the oldest surgical procedures in the history of medicine. Nowadays, knee and hip implant surgeries are performed around the world and considered to be amongst the most commonly performed operations in clinical practice. A total of 300,000 knee arthroplasties are performed each year in the United States alone with the number increasing every year [1]. Different methods of bone cutting include scraping, grooving, sawing, drilling, boring, grafting, shearing etc. Amongst these methods, drilling is a surgical operation most discussed in literature. A considerable manual force on a drill is required by a surgeon to produce a hole for fixation purposes. A major concern in bone drilling is the penetration force, which may induce unnecessary damage possibly resulting in trauma. The process can also be inefficient because of the flutes clogging. Studies found that cortical bone overheating, which can cause thermal necrosis, is strongly linked to the level of the drilling force [2], [3]. A force transmitted to a bone in drilling is not always appropriate for generation of the required cut. Hence, there is an increasing demand to minimize the cutting force in order to avoid injuries to nerves in the treated area. Another concern is that the torque in drilling can result in the drill breaking.
Recent technological improvements, aimed at achieving minimal invasion in bone drilling, are mainly focused on the design of drills [4], [5]. Another approach is to use a robot-assisted surgery system, where a sensitive force feed-back system controls the tool action. A concept of mechatronic drill for automatical detection of breakthroughs at bone/soft tissue interfaces in order to avoid unnecessary damage, was proposed in [6], [7], [8]. Improved drill designs, reducing the thrust force and allowing efficient removal of bone chips, were reported in [9], [10]. It was noted that drilling a predrilled hole significantly reduced thrust forces due to elimination of the chisel edge thrust at the tip of the drill. Those results demonstrated that a point angle of 118° and a helix angle of 36° with parabolic flutes, decreased the thrust load by 45 percent compared with other existing surgical drill bits. It was also shown that a twist drill with the same point angle and a 28° helix angle, required a much lower torque per unit area of hole and energy per unit volume of bone drilled at a given feed rate, compared to a drill with a 60° point angle [11].
A bone–tool interaction in conventional and ultrasonically-assisted cutting modes has been of interest to researchers for the last few decades. Presently, a mechanical rotary drill is the main type of drilling equipment used in clinical practice. Various drilling techniques have been introduced to improve the cutting process in order to minimize invasiveness of the operation. One such modern drilling techniques utilizes high-frequency (ultrasonic) vibration of the drill along its longitudinal axis and is called ultrasonically-assisted drilling (UAD). Another technique is laser drilling but this has been shown to induce severe tissue burning [12], [13]. Ultrasonic vibration has been already successfully applied on a wide scale in cutting high-strength aerospace alloys [14], composites [15] and soft materials [16]. In medical applications an ultrasonic tool can reduce cutting forces and provide a surgeon with better control in cutting the bone tissue [17].
In previous research into bone drilling, the focus was largely on either the selection of drilling parameters or design of surgical drills. Also, research conducted was mainly focused on forces in conventional drilling (CD) [10], [18], [19]. Studies pertinent to the improvement of the process itself are limited. The ultrasonic drill cuts the bone in a different way compared to a conventional one, and the process should be studied in detail. Despite the benefits of UAD in materials other than bone, no attention has been paid to its application in orthopaedics. The goal of this paper was to realize the benefits of UAD compared to CD. An experimental programme was performed to measure and compare, quantitatively, the drilling thrust force and torque for both types of drilling. The difference in forces for CD and UAD is explained by the process of chip formation using high-speed filming.
Section snippets
Specimen preparation
Fresh dead cortical bones were cut from bovine femur. The bovine bone was of interest since it replicated the properties of human bone according to [20]. The bones were obtained from a local butcher and were stored frozen at −10 °C before the experiment. The epiphysis was then cut off with a hacksaw thus leaving the bone's diaphysis to be tested. The bone pieces were approximately 80 mm in height with an average thickness of the cortical wall of 8–9 mm. The shape of the bone was not suitable for
Mechanism of ultrasonically-assisted cutting
In UAC the displacement x of vibrating tool is given bywhere a, f and ω are, respectively, the amplitude, frequency and angular velocity of the tool. Thus, the tool vibration speed is . In UAC, the tool-work piece contact is intermittent, i.e. the tool remains in a contact with a bone only for a certain part of vibration cycle. The vibrational cutting condition is satisfied if the tool speed is more than the work piece cutting velocity, resulting in a
Results and discussion
Typical force-time graphs obtained for both drilling techniques are shown in Fig. 4. After the initial engagement, the force gradually increased with time and attained a plateau when the drill lip was fully engaged with the bone. Small oscillations recorded at peak values are due to high sensitivity of the measurement system and vibrations in the drilling equipment. The force suddenly vanished when the drill penetrated the entire thickness of the cortical bone (approx. 9 mm).
Conclusion
The obtained experimental results for drilling cortical bone revealed that the penetration force and torque dropped significantly when ultrasonic vibration was superimposed along the drill's longitudinal axis without cooling. The drilling force was nearly halved in the case of vibration for the range of drilling speeds used. The lower drilling torque in ultrasonically-assisted drilling reduced the chance of drill skidding and breakage. In addition, the risk of damage caused by the drilling
Conflicts of interest
None.
Acknowledgement
The authors wish to thank Dr Alan Meadows (Loughborough University, UK) for his support with the experiments.
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