In-vitro analysis of forces in conventional and ultrasonically assisted drilling of bone

Abstract

Background:

Drilling of bone is widely performed in orthopaedics for repair and reconstruction of bone. Current paper is focused on the efforts to minimize force generation during the drilling process. Ultrasonically Assisted Drilling (UAD) is a possible option to replace Conventional Drilling (CD) in bone surgical procedures.

Objective:

The purpose of this study was to investigate and analyze the effect of drilling parameters and ultrasonic parameters on the level of drilling thrust force in the presence of water irrigation.

Methods:

Drilling tests were performed on young bovine femoral bone using different parameters such as spindle speeds, feed rates, coolant flow rates, frequency and amplitudes of vibrations.

Results:

The drilling force was significantly dropped with increase in drill rotation speed in both types of drilling. Increase in feed rate was more influential in raising the drilling force in CD compared to UAD. The force was significantly dropped when ultrasonic vibrations up to 10 kHz were imposed on the drill. The drill force was found to be unaffected by the range of amplitudes and the amount of water supplied to the drilling region in UAD.

Conclusions:

Low frequency vibrations with irrigation can be successfully used for safe and efficient drilling in bone.

Keywords

Orthopaedic bone drilling ultrasonic drilling drilling force irrigation

1. Introduction

Bone drilling is an essential surgical procedure in orthopedics to repair broken bones and secure medical prostheses. One of the desired outcomes of the process is damage free and precisely positioned holes. A mechanical twist drill is widely used for drilling holes in orthopaedic surgical practices. Drilling thrust force and temperature level in bone are important factors affecting the outcome of the process [1,2]. Large and uncontrolled drilling forces and torques are the main reasons for drill-bit breakage during drilling procedures [3–5], which requires additional procedures to remove the broken drill from the bone tissue [6,7]. Mechanical damage in the bone [2,8], heat generation [9], osteonecrosis [10,11] and crack formation near the drilling region [12] are the main consequences of large drilling forces in bone drilling. Therefore, it is critical to understand and explore improved drilling techniques to minimize drilling forces for safe and efficient cutting of bone.

Over the years, extensive research on bone drilling has been conducted to find optimal drilling parameters and conditions to minimize the invasiveness of the procedure. In addition to an experimental work, the process has been extensively researched to find optimal parameters for lower drilling force, torque and temperature using experimental analysis, haptic simulations, finite element analysis and analytical modeling [13–17]. Some studies have reported the benefits of real-time breakthrough detection systems in bone drilling [18,19]. Size and cost associated with the new drilling systems still limit their use in clinical practice. Recently, ultrasonically assisted drilling (UAD) technique has been tested in bone and was found to minimize the drilling thrust force, torque and temperature in bone [16,20]. In UAD, micro vibrations are superimposed on the drill bit along its longitudinal direction (feed direction). The technique was also found beneficial in reducing the cutting force when vibrations were superimposed on the cutter in plane cutting of bone [21]. In previous studies, the use of ultrasonic vibration in bone drilling was without using physiological solutions. The physiological solution was used in studies related to measure temperature only in conventional drilling (CD) [9,22,23].

Currently ultrasonic cutting tools are widely used in oral and maxillofacial surgeries for minimal invasive cutting of delicate bones and tissues [24,25]. Bony defects in the oral cavity can be a challenging task since the fragile bone is sensitive to osteotomes with bulky cutting tips [24]. Cutting tools with ultrasonic assistance allow the surgeons to cut hard tissues while sparing delicate tissues such schneiderian membrane and nerve tissue [25]. This research study is a step forward to investigate and analyze the effect of physiological solutions in minimizing the force in conventional drilling (CD) and UAD of bone. Series of experiments have been conducted to investigate the effect of drilling parameters (drill speed, feed rate) and ultrasonic parameters (frequency and amplitude) on the level of thrust force in bone drilling under physiological drilling conditions (supply of clean water). The influence of the rate of coolant flow on the drilling force is also investigated in both types of drilling.

2. Materials and methods

2.1. Specimen preparation

The drilling was performed on femoral bone of a calf. Fresh femurs were obtained immediately after the slaughter. The age of the animal was around two years with weighing 90–100 kg. The middle portions of the femurs were cut using hacksaw. The average cortical thickness was 8 mm. Specimens were kept moist in saline solution and stored at −10°C to preserve thermo-physical properties. The periosteum was removed from the top surface of the bone to prevent clogging of the drill flutes. Specimen used in drilling experiment is shown in Fig. 1.

Fig. 1.

Fresh bone specimen for drilling.

2.2. Equipment and procedure for drilling

Schematic of ultrasonic drilling system is shown in Fig. 2. Electrical power with low frequency is converted into electrical power with high frequency by a high power sine wave generator. The high frequency electrical signal is transmitted to traducer which converts it into high frequency low amplitude vibration. Essentially, the main function of a transducer is the conversion of electrical energy to a vibratory motion. The amplitude of vibration produced by transducer is very low and requires amplification for machining purposes. The horn or concentrator is attached between the transducer and the tool which acts as a wave-guide for amplification and concentration of the vibration to the tool from the transducer. The force data from a dynamometer attached to the work piece is magnified, processes and transfer to a display system.

Fig. 2.

Components of ultrasonic drilling system.

All measurements were made on vertical drilling machine with the options of regulating the spindle speed and feed rate up to a maximum value of 1000 rpm and 10 m/min respectively. A standard steel twist drill of 4 mm diameter, 110° point angle, 20° helix angle and 120° chisel edge angle were used in drilling experiments. The ultrasonic transducer was gripped in its chuck of the drilling machine. The transducer was based on the principle of piezoelectric effect. The rigid structure and high power of the drilling machine enabled precise identification of drilling speed and feed rate. The force was measure and monitored with a two-component dynamometer (Kistler type 9271A). The Picoscope series 2000 oscilloscope with a maximum frequency of 10 MHz was used to acquire the data for force in a digital format. A Schematic of experimental set up for drilling process is shown in Fig. 3.

Fig. 3.

Schematic illustration of the experimental system.

Force measurements were conducted at room temperature of 25°C. Drilling tests were performed perpendicular to the major anatomical direction of the bone (along longitudinal axis of the femur bone). In each test, the drill was penetrated through the whole cortical thickness. Filter water with temperature of approximately 20°C was used as a coolant medium to mimic condition in real bone drilling procedure in orthopaedics. The volume flow rate of water was controlled by a control valve. The drill was replaced with a new drill after forty holes to eliminate the effect of wear on drilling force data. Each set of test was repeated three times to represent repeatability in the results. Drilling and ultrasonic parameters used in experiments are shown in Table 1.

Table 1

Parameters used in drilling experiments

Parameters	CD	UAD
Spindle speed (rpm)	1000–3000	1000–3000
Drill’s diameter (mm)	4	4
Feed rate (mm/min)	30–70	30–70
Vibration amplitude (µm)	N/A	5–20
Vibration frequency (kHz)	N/A	5–25
Water flow rate (ml/min)	20–180	20–180

All the data points in the subsequent plots represent the maximum value of the force which was noted when the cutting edges of the drill bit were fully engaged with the bone during cutting. The maximum value of the drilling force remained at a maximum constant value till the drill exit the cortex of the cortical bone. Each experiment was repeated three times for a particular set of drilling parameters and ultrasonic parameters to observe repeatability in the measurements. The effect of spindle speed, feed rate, ultrasonic frequency and amplitude on the level of thrust force in drilling is discussed below.

3. Results

3.1. Effect of drilling parameters on drilling force

The level of force was measured by varying the drill speed from 1000 rpm to 3000 rpm at a constant feed rate of 50 mm/min. The force experienced by the drill with and without ultrasonic vibrations is shown in Fig. 4. The drilling speed was found to have an inverse relationship with the force in both types of drilling techniques. The force was dropped from a mean value of 62 N to 36 N when the drilling speed was increased from 1000 rpm to 3000 rpm in CD. Similarly, the force was dropped from 41 N to 20 N in UAD for the same increase in drilling speed. A decrease of 34% and 44%, respectively, was noted for 1000 rpm and 3000 rpm when ultrasonic vibrations were imposed on the drill. The drop in force with increase in drilling speed was due to the alteration in the friction between the drill and the bone.

Fig. 4.

Variation of drilling force with drill rotation speed in CD and UAD (feed rate – 50 mm/min, frequency – 15 kHz, amplitude – 10 µm, water flow rate – 100 ml/min).

The effect of feed rate on drilling force in both types of drilling techniques was also studied. The drilling speed was kept constant at 2000 rpm in CD and UAD. The frequency and amplitude were kept constant at 15 kHz and 10 micrometers respectively to see the effect of feed rate on drilling force alone in UAD. The variation of drilling force with feed rate in both types of drilling techniques is shown in Fig. 5. The trend with which the force rose with increase in feed rate in CD was different than that measured in UAD. A significant increase in drilling force was found with increase in feed rate in CD. The force was increased by 80% when the feed rate was changed from 30 mm/min to 70 mm/min. However, the effect of feed rate on force in UAD was not significant compared to CD. The force was increased from 23 N to 29 N in UAD for the same increase in feed rate as in CD.

Fig. 5.

Variation of drilling force with feed rate in CD and UAD (drill speed – 2000 rpm, frequency – 15 kHz, amplitude – 10 µm, water flow rate – 100 ml/min).

3.2. Effect of water flow rate on drilling force

In this section, the variation in drilling force with flow rate of water supplied to the drilling region in both types of drilling is discussed. The selection of the range of flow rate tested in this study was based on the experience of orthopeadic surgeons and values reported in the literature [26]. The force was observed to decrease significantly up to the flow rate of 100 ml/min in both drilling techniques. The flow rate above 100 ml did not influence the force in both types of drilling. The drop in force was due to the decrease in the friction between the drill bit and the bone up to 100 ml/min, which was believed to remain unchanged, with further increase in the flow rate. The effect of water flow rate on the drilling force in both types of drilling is shown in Figs 6 and 7.

Fig. 6.

Effect of volume flow rate of water on drilling force in CD (feed rate – 50 mm/min).

Fig. 7.

Effect of volume flow rate of water on drilling force in UAD (feed rate – 50 mm/min, frequency – 15 kHz, amplitude – 10 µm).

3.3. Effect of ultrasonic parameters on drilling force

The feed rate and water flow rate were kept at constant level to investigate the influence of ultrasonic frequency and amplitude on the level of drilling force in UAD using two values of drilling speeds (1000 rpm and 3000 rpm). The effect of ultrasonic frequency on the force is shown in Fig. 8. The force was dropped significantly when frequency up to 10 kHz was applied on the drill using two values of drilling speeds. Further increase in frequency was found ineffective in decreasing force in the presence of water irrigation. The lower force in UAD compared to CD was the intermittent contact between the drill and the bone in UAD which reduced the average friction between the drill and the bone surfaces. One of the possible reasons for no drop in the force using frequency higher than 10 kHz was the squeezing of the fluid medium from the drilling region that promoted flow the in the contact regions of the dill and the bone, thereby, reducing the effect of frictional conditions. The effect of vibration amplitude on the drilling force is shown in Fig. 9. The force was slightly increased when amplitude was increased from 5 µm to 15 µm using two different drilling speeds. The increase in force with increase in amplitude may be attributed to the increase in contact length and bone in a single cycle of vibration.

Fig. 8.

Effect of ultrasonic frequency on drilling force in UAD (feed rate – 50 mm/min, amplitude – 10 µm, water flow rate – 100 ml/min).

Fig. 9.

Effect of amplitude on drilling force in UAD (feed rate – 50 mm/min, frequency – 15 kHz, water flow rate – 100 ml/min).

4. Discussions

The level of thrust force in drilling operation depends on factors such as the strength of the workpiece material, feed rate, rotational speed, cutting fluids, drill diameter, drill geometry and the friction between the drill and the workpiece material. The level of force is significantly reduced with increase in the number of revolutions in CD and UAD. UAD produced lower force for similar drilling parameters due to the improved chip formation mechanism in UAD compared to CD. The decrease of the drilling force was due to the decrease in mean friction between the drills and the bone and quick chip formation and removal mechanism at higher speeds [16]. The decrease in drilling force with increase in drilling speed was also observed in other studies performed on bovine cortical bone [27]. In a recent study performed on porcine bone [28], the drilling force was declined markedly under dry and physiological conditions in CD. Contrary to our findings, that study found large drilling force under physiological condition compared to dry conditions. The drop in the force was attributed to the decreases in thickness of the bone tissue extracted per revolution which decreased the rate of deformation within the material.

The drilling thrust force was increased with the speed of penetration (feed rate) of the drill in CD only. The increase in drilling force using higher feed rates was due to the increase in material removal rate per revolution of the drill. This condition led to the rise in the average friction between the drill and the bone, thereby, increase pressure on the drill. The results presented in this study overlapped with a recent study [29]. The drilling force was also found to be strongly influenced by varying feed rate while simulating dry drilling using finite element analysis [29,30]. In the current study, the force was less affected by varying feed rate in UAD. The obvious reason was the large difference between the speed of the tip of the drill in UAD due to imposed vibrations and the speed of drill penetration (feed rate) [16].

Long spiral chips were generated during CD with no water supply [16]. Bone chips may trap in the flutes of the drill and cause additional cutting load on the drill. Water supply to the cutting region helped in effective removal of chips and debris. There was a limit to the rate of supply of water for decreasing drilling force in UAD using lower and higher drilling speeds and for lower drilling speed in CD only. The high-speed to-and-fro motion of the drill in UAD with 15 kHz frequency caused the water to squeeze out from the drilling region. The maximum amount of water accommodated in the drill-bone interface was at the flow rate 100 ml/min. A further decrease in drill speed and frequency may accommodate more amount of water supply, which can affect force differently. In addition, more pressurized water irrigation may change the drilling force and require further research. A previous study measured lower temperature in bone during drilling at higher irrigation rate [26]. Further studies are required to optimize the bone drilling process using drilling and ultrasonic parameters other than used in this study.

Despite many benefits of ultrasonically assisted cutting, the technique still requires further research before implementing it in clinical practice. UAD with frequency up to 10 kHz may be used as an alternative to CD in orthopaedic surgical procedures to avoid necrosis of bone tissue [20]. Frequency above 10 kHz was not beneficial in reducing the drilling force further but can significantly rise bone temperature [20]. Drilling with ultrasonic assistance will allow surgeon to penetrate the drill into bone with less effort. Lower drilling force and temperature using UAD will enhance the current state of drilling procedure in orthopaedics.

5. Conclusions

Experiments were conducted to investigate and analyze the level of thrust force experienced by drill bit when penetrating the bone tissue applying the most influential drilling parameters used in surgical procedures in clinics. UAD produced lower force than CD in the presence of water irrigation. Drilling speed had significant influence in reducing force in both types of drilling techniques. The feed rate was only influential in raising the force in CD only. The maximum drop in the drilling force, measured with water supply up to 100 ml/min to the drilling region, was beneficial in reducing force with all combinations of parameters.

The study has concluded that only using frequency at lower level (up to 10 kHz) in UAD was influential in lowering the force. Ultrasonic frequency above 10 kHz causes overheating of the bone and should be avoided. The drilling force was unaffected by the range of amplitudes used in this study. This study suggests combination of lower drilling speed, feed rate and cooling to avoid overstressing of the bone in drilling operation. The use of internal irrigation (cooling through the hole in the drill) in the presence of ultrasonic vibrations is to be investigated and compared with external irrigation as used in this study. Further research is needed to measure the level of force using pressurized irrigation.

Footnotes

Acknowledgements

The authors wish to thank Prof. Vadim Silberschmidt and Dr. Naseer Ahmed for providing support in drilling experiments.

Conflict of interest

The authors have no conflict of interest to report.

References

Pallan

F.G.

, Histological change in bone after insertion of skeletal fixation pins, J. Oral. Surg. Anesth. Hosp. Dent. Serv. 18 (1960), 400–408.

Brett

P.N.

Baker

D.A.

Taylor

and Griffiths

M.V.

, Controlling the penetration of flexible bone tissue using the stapedotomy micro drill, Proc. Inst. Mech. Eng. I 218 (2004), 343–351.

Jantunen

, A summary of methods applied to tool condition monitoring in drilling, Int. J. Machine Tool Manuf. 42 (2002), 997–1010.

Stein

J.M.

and Dornfeld

D.A.

, Burr formation in drilling miniature holes, CIRP Ann. Manufact. Technol. 46 (1997), 63–66.

Pichler

Mazzurana

Clement

Grechenig

Mauschitz

and Grechenig

, Frequency of instrument breakage during orthopaedic procedures and its effects on patients, J. Bone Joint Surg. Am. 90(12) (2008), 2652–2654.

Bassi

J.L.

Pankaj

and Navdeep

, A technique for removal of broken cannulated drill bit: Bassi’s method, J. Orthop. Trauma 22(1) (2008), 56–58.

Price

M.V.

Molloy

Solan

Sutton

and Ricketts

D.M.

, The rate of instrument breakage during orthopaedic procedures, Int. Orthop. 26(3) (2002), 185–187.

Kendoff

Citak

Gardner

M.J.

Stubig

Krettek

and Hufner

, Improved accuracy of navigated drilling using a drill alignment device, Journal of Orthopaedic Research 25(7) (2007), 951–957.

Augustin

Davila

Mihoci

Udiljak

Vedrina

D.S.

and Antabak

, Thermal osteonecrosis and bone drilling parameters revisited, Arch. Orthop. Trauma Surg. 128 (2008), 71–77.

10.

Eriksson

A.R.

Albrektsson

and Albrektsson

, Heat caused by drilling cortical bone. Temperature measured in vivo in patients and animals, Acta Orthop. Scand. 55(6) (1984), 629–631.

11.

Lee

Rabin

and Ozdoganlar

O.B.

, A new thermal model for bone drilling with applications to orthopaedic surgery, Med. Eng. Phys. 33 (2011), 1234–1244.

12.

Alam

Ahmed

and Silberschmidt

V.V.

, Comparative study of conventional and ultrasonically-assisted bone drilling, Technol. Health Care 22(2) (2014), 253–262.

13.

Lee

Gozen

B.A.

and Ozdoganlar

O.B.

, Modeling and experimentation of drilling forces, J. Biomech. 45(6) (2012), 1076–1083.

14.

Tsai

M.D.

Hsieh

M.S.

and Tsai

C.H.

, Bone drilling haptic interaction for orthopedic surgical simulator, Comput. Biol. Med. 37(12) (2007), 1709–1718.

15.

Alotta

Belmonte

Bosio

and Dario

, Study on mechatronic tool for drilling in the osteosynthesis of long bone: Tool/bone interactions, modelling and experiments, Mechatronics 6(4) (1996), 447–459.

16.

Alam

Mitrofanov

A.V.

and Silberschmidt

V.V.

, Experimental investigations of forces and torque in conventional and ultrasonically-assisted drilling of cortical bone, Med. Eng. Phys. 33(2) (2011), 234–239.

17.

Abouzgia

M.B.

and James

D.F.

, Temperature rise during drilling through bone, Int. J. Oral. Maxillofac. Implants 12(3) (1997), 342–353.

18.

Ong

F.R.

and Bouazza-Marouf

, The detection of drill bit break-through for the enhancement of safety in mechatronic assisted orthopaedic drilling, Mechatronics 9(6) (1999), 565–588.

19.

Hsu

Y.L.

Lee

S.T.

and Lin

H.W.

, A modular mechatronic system for automatic bone drilling, Biomed. Eng. Appl. Basis Commun. 13(4) (2001), 168–184.

20.

Alam

and Silberschmidt

V.V.

, Analysis of temperature in conventional and ultrasonically-assisted drilling of cortical bone with infrared thermography, Technol. Health Care 22 (2014), 243–252.

21.

Alam

Khan

and Silberschmidt

V.V.

, Analysis of forces in conventional and ultrasonically assisted plane cutting of cortical bone, Proc. Inst. Mech. Eng. H 227(6) (2013), 636–642.

22.

Sezek

Aksakal

and Karaca

, Influence of drill parameters on bone temperature and necrosis: A FEM modelling and in vitro experiments, Comput. Mater. Sci. 60 (2012), 13–18.

23.

Alam

Khan

and Silberschmidt

V.V.

, 3D finite element modeling of drilling cortical bone: Temperature analysis, J. Med. Biol. Eng. 34(6) (2014), 618–623.

24.

Stübinger

Landes

Seitz

Zeilhofer

H.F.

and Sader

, Ultrasonic bone cutting in oral surgery: A review of 60 cases, Ultraschall Med. 29(1) (2008), 66–71. doi:10.1055/s-2007-963507.

25.

Schlee

Steigmann

Bratu

and Garg

A.K.

, Piezosurgery: Basics and possibilities, Implant. Dent. 15(4) (2006), 334–340.

26.

Matthews

L.S.

and Hirsch

, Temperatures measured in human cortical bone when drilling, J. Bone Joint Surg. Am. 54(2) (1972), 297–308.

27.

Hillery

M.T.

and Shuaib

, Temperature effects in the drilling of human and bovine bone, J. Mater. Proc. Technol. 92–93 (1999), 302–308.

28.

Linlin

X.U.

Chengyong

Min

Huiyu

H.E.

Yuexian

Hanyuan

Jingnan

and Jiayong

, Drilling force and temperature of bone under dry and physiological drilling conditions, Chin. J. Mech. Eng. 27(6) (2014), 1240–1248.

29.

Lughmani

W.A.

Bouazza-Marouf

and Ashcroft

, Drilling in cortical bone: A finite element model and experimental investigations, J. Mech. Behav. Biomed. Mater. 42 (2015), 32–42.

30.

Lughmani

W.A.

Bouazza-Marouf

and Ashcroft

, Finite element modeling and experimentation of bone drilling forces, Journal of Physics: Conference Series 451 (2013), 012034.