S. Saha Associate Professor and Coordinator of

Bioengineering, Mem. ASME

S.Pal Research Associate,

Mem. ASME; on leave from the Department of Mechanical

Engineering, Jadavpur University,

Calcutta, India

J. A. Albright Professor and Head.

Biomechanics Laboratory, Louisiana State University Medical Center,

Department of Orthopaedic Surgery, Shreveport, La. 71130

Surgical Drilling: Design and Performance of an Improved Drill1

The majority of twist drills used in orthopaedics are very similar to chisel pointed metal drilling bits. Modifications usually observed are reduction of the point angle to 90 deg and sometimes grinding of the entire cutting lip at 0 deg rake angle, which appeared to have been made arbitrarily without any advantage. We have attempted to design a surgical drill bit with the objective of minimization of the drilling thrust and temperature and effective removal of bone chips. Our results showed that the presence of the chisel edge was mainly responsible for increasing the thrust force and the temperature developed. The effects of a constant feed rate and thrust on the peak temperature were also examined. The combined effect of the helix and the point angles on the rake angle which in turn determines the cutting efficiency was analyzed for various types of surgical bits. Based on our results and previously published data from the literature an optimized drill bit was designed with a split point, a point angle of 118 deg, a parabolic flute, and a helix angle of 36 deg and its performance was compared with other existing surgical drill bits. For drilling in compact bone, the new design decreased the thrust load by 45 percent and thefpeak temperature rise by 41 percent. Simlar improvements were also recorded for drilling bone cement. The time of drilling a bone cortex was also significantly reduced and "walking " on the curved bone surface was eliminated and dimensional tolerance on hole sizes was improved. The new design is likely to reduce the time of surgery and also minimize the tissue damage.

Introduction

Human skeletons from early civilizations, displayed in museums, sometimes show surgically produced holes in their skulls, obviously performed with crude tools. In modern times bone drilling re-entered orthopaedic surgery and, as early as 1886, Hansman drilled bone to fix a fracture with metal plates [1]. In 1912, Sherman improved this technique, but no systematic study on the development of a more suitable drilling tool was conducted until Bechtol [2] recommended changes in the design of tool bits, based on his surgical ex­perience. In 1964, Sneath [3] identified the requirements of surgical drill bits and suggested standards, but unfortunately, no experimental data was presented. Subsequently, interest in the design of bone drills increased and experiments by Jacob, et al. [4] and Wiggins and Malkin [5] showed that drills designed according to Bechtol's recommendation required greater cutting force and higher energy than some other types of drills. Jacob, et al. [4] compared the performance of seven different drill designs ranging in speed from 100 to 2360 rpm and with feed rates of 0.254, 0.508, and 1.27 mm/min. Nevertheless, his design recommendations did not strictly

This new drill bit for surgical use is presently available from BIOMET, Box 587, Airport Industrial Park, Warsaw, IN 46580.

Contributed by the Bioengineering Division and presented in part at the Winter Annual Meeting, Washington, D.C., November 15-20, 1981, of THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS; at the 34th ACEMB Meeting,

Houston, Texas, September 21-23, 1981; and at the 28th Annual Meeting of the Orthopaedic Research Society, New Orleans, Louisiana, Jan. 1982. Manuscript received by the Bioengineering Division, July 20, 1981; revised manuscript received April 16,1982.

relate to his experimental results. He, for example, recom­mended a 90-deg point angle drill bit, despite his findings that a 110-deg point angle drill bit yielded less thrust and torque. He offered no reasons for his judgment overriding his results.

The process of drilling bone produces heat which, if ex­cessive, produces tissue necrosis [6]. This is often evident by the presence of a ring sequestra in the radiographs of bones with drilled holes. One of the reasons for this high tem­perature rise is that surgical drill bits are adapted from existing metal drills with minor modifications, e.g., reduced point angle and web-thinning. However, Sneath [3] has shown that many of these modifications did not improve the per­formance of surgical drills. Also, it has not been definitely established what the optimum cutting speed should be, to maintain temperatures below the threshold level of thermal damage.

In the present study the performance of surgical drills was analyzed for various geometrical parameters of the drill bit, concentrating primarily on the temperatures generated and on the thrust loads developed, since these two factors are of special significance in surgical drilling. The contributions of the rake angle, point angle, helix angle, flute geometry, chisel edge, etc., were considered. Based on these considerations, an optimized drill bit was designed and its performance with respect to the temperature generated and the thrust load was compared with other existing surgical drill bits.

Functional Requirements

Drilling is the process of producing holes in solid material;

Journal of Biomechanical Engineering AUGUST 1982, Vol. 104/245

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surgical drills create holes in bone for fixation of various implants. Drills perform multiple functions such as direct penetration into cortical bone, removal of bone chips from the cutting zone, advancement in the direction of movement, and production of a uniform size hole. The geometry required for these multiple functions is incorporated in the design of a twist drill.

Commercially available twist drills designed for use on metal can also be used to drill bone, but the performance is not optimum because the mechanical behavior of bone tissue differs widely from that of metal. The ideal design requirement for drilling bone differs significantly from the industrial requirements for drilling metal [7]. For instance, it is not always possible to locate a flat surface when drilling a hole in bone nor is it always practicable to use a jig or a guide. Therefore, a surgical drill should be self-centering and it should not "walk" when initiating a hole in the cortex of a tubular long bone since it is not practicable to indent a pilot center as is done in metal drilling. At times the drill must be quite long compared to its diameter, but it should not deflect under the operating thrust load, which perferably should also be kept low to minimize the danger of breakage and the possibility of tissue damage.

The heat generated during drilling of bone is partially dissipated by the presence of blood and tissue fluids, and part of the heat is carried away by the chips formed. However, bone is a poor conductor of heat and the temperature rise could be significantly high [6-8]. Moritz and Henrique [9] established that (a) a temperature of 70 °C will damage epithelial cells immediately, (b) a temperature of 55 ° C will do so after 30 s, and (c) a temperature of 45"C requires more than 5 hr to produce the harmful effect. Therefore, the geometry of the drill bit should be such that heat generation due to the noncutting action of the cutting edge or friction from the flutes, is avoided or minimized.

As a twist drill rotates and penetrates bone, chips are formed which follow the two spiral paths along the flutes to the surface. The flutes of twist drill bits often tend to clog when the depth of the hole being drilled becomes appreciable compared to its diameter. Once clogged, the friction increases excessively and overheating or even charring of the organic matrix of bone may result [10]. Wiggins and Malkin [5], using a general purpose twist drill, have shown that the cutting torque and the specific cutting energy increase with increasing hole depth while drilling bone in the axial direction. The torque varies linearly and the energy exponentially. The torque may increase further with clogging. Therefore, the design of the drill flute should be such that it avoids clogging and minimizes friction. For rapid completion of surgical drilling, flutes eliminate the need for "woodpeckering," i.e., periodic withdrawal of the drill for chip removal. Ideally, the rate of penetration or (feed rate) should be high, while the thrust or end load remains within tolerable limits (2 to 3 kgf) [3], Wiggins and Malkin [5] showed that the "specific cutting

CHISEL EDGE LAND WIDTH i /

\ ^ ^ ^ \ V DRILL k / % * \y DIAMETER L _ ^ . ; I ! S ^A^ L_

POINT ANGLE/ /

LIP CLEARANCE C" I S E L E ° ° E A " g l ^

HELIX ANGLE f -f K \ _ ~ — _ /

I L ^ ^ - ~ / -*• i=—f MARGIN*" I ) / / > ^--FLUTES-/ | / X ^ / L^ g

Cutting Lip /WEB-7~STV?-CHISEL EDGE - * ^

Fig. 1 Three views of a conventional 2-lip twist drill showing its im­portant functional geometrical parameters

246/Vol. 104, AUGUST 1982

energy," i.e., the energy expended per unit volume of material removed in the form of chips, decreases with in­creasing feed rate at a given speed of drilling. Therefore, a drill bit which allowed faster penetration (a greater feed rate) at a given thrust load would have definite advantages for surgical drilling. A lower thrust load also decreases the likelihood of buckling of the drill. This can be important for surgical drills since the l/d ratio may be quite high, e.g., 40-50, where / is the length and d is the diameter of the drill bit [11].

Mechanics of Drilling and Design Analysis A twist drill containing two lips, as shown in Figs. 1 and 2,

can be simulated as a combination of two single-edged turning tools twisted to meet at the chisel edge. The mechanism of chip formation in drilling is quite complex as the cutting conditions vary along the entire cutting edge from axis to periphery. The chisel edge at the center does not cut in the ordinary sense (shear). It displaces the material ahead of it resembling the action of a small indenting chisel while the outer edge of the cutting lip (Fig. 2) produces a smooth chip. Thus the chisel edge, where cutting velocity is low and the rake angle highly negative, greatly increases the thrust during drilling. The cutting velocity is given by ird/N where d-, is the diameter at any intermediate position and N is the rpm.

Figure 2 shows the average force system acting on one of the cutting lips during drilling. The resultant cutting force R is resolved into three orthogonal directions, X-Y-Z. Pz is the principal cutting force which coincides with the cutting velocity direction, and the drilling torque is mainly due to this force. Px is the thrust force at the cutting edge acting in the feed direction, and Py is a radial force which balances a similar force acting on the other lip. The presence of the chisel edge, which does not have any cutting action, except some sort of extrusion effect, gives rise to a considerable amount of thrust force.

In the analysis of drilling thrust force, Px is the component due to the cutting action of the edge. However the total thrust force consists of the cutting thrust and the thrust due to the extrusion effect at the chisel edge plus the frictional force at the edge. Experimentally (details shown later) we have shown that, a pilot hole with a diameter equal to the width of the chisel edge, decreases the total thrust force by 50 percent or more when drilling bone or bone cement. This is a very im­portant consideration for achieving improved performance in the new design of a surgical bit.

Fig. 2 The resultant cutting force R on one of the lips is resolved into three orthogonal components: Px (thrust force), Py (radial force) and Pz

(principal cutting force). Section AA shows the clearance and the rake angle at any position of the cutting edge, the bone chip and the plane of shearing of the chip. The positive, zero and negative rake angles (7) are shown in a section of tool at the right.

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Table 1 Thermal properties of bone and surgical drill bit [12]

Heat Thermal Thermal Density, Sp. heat, capacity, conductivity, diffusivity, Kg/m3 J/kg°C J/m3°C J/m s°C m2/s

Material

Stainless steel

Fresh cortical bone (human)

7.8X103

2.1xl0J

0.46 X 10'

\.26xW

W2nh\

where

Ks

v A W h

specific cutting energy cutting velocity chip cross-sectional area thermal conductivity of work material (bone) thermal capacity (density x specific heat) of work material

C0 and n are constants dependent on the foregoing parameters and they can be obtained experimentally, n = 0.22-0.40 for metal cuting [13]. From equation (1), it may be observed that the temperature T, is directly proportional to the specific cutting energy Ks, which in turn is dependent on the dynamic shear strength of the bone material since cutting is a dynamic shear failure process [13-16]. The specific cutting energy increases with increase of dynamic shear strength for brittle materials [14]. Due to the brittle nature of bone, this may also be true for bone. The thermal capacity and the conductivity of bone material (values shown in Table 1) constitute the other important factors affecting the tem­perature rise. They have an inverse relationship with the temperature, (equation (1)) so that the lower they are the higher the temperature. The thermal conductivity of the tool material has little effect on the temperature. During metal drilling the metal chip carries nearly 85 percent or more of the heat produced [13]. Therefore, unlike metal drilling, the temperature of the bone will be relatively higher during drilling due to its poor thermal capacity and conductivity since chips will carry a smaller percentage of heat. It should be pointed out that, compared to metal, the total heat generated under identical cutting conditions will be much less during bone drilling due to its lower Ks value. This is an important consideration for bone drill design.

Geometrical Parameters of Twist Drills and Their Effect on Cutting Force

Rake Angle. The various cutting angles of a twist drill can be simulated by a single point turning tool, visualizing the drill as a combination of cutting edges twisted to meet at the

3.6x10° 14

2.65x10° 0.38-2.3

3.9x10"'

0.144x10"

An orthogonal section AA (Fig. 2) on the cutting lip shows the rake and clearance angles on the cutting edge of the drill bit, both of which vary along the edge. The bone material undergoes plastic deformation along the shear plane producing chips which flow on the rake face, and most of the mechanical work of the cutting process is converted into heat. The rate of heat generation depends on the various parameters of the cutting process (velocity, feed rate and the shear strength of the material); and this in turn influences the temperature of the tool, bone and chips, depending on their relative thermal properties, which are shown in Table 1.

The general formula correlating the tool and chip interface temperature Tt with five other physical variables was developed [13] using dimensional analysis and is given by equation (1).

C0Ksv2"A"

T,= °J. , ,_ (1)

o 5

0.7

0.6

0.5

0.4

0.3

0.2

•0.1-

0

-0.1

- 0 . 2

- 0 , 3

-0 .4

-0 .5

Fig. 3 Variation of the orthogonal rake angle in radians along the cutting edge at any intermediate diameter dx for a drill of overall diameter d (3.2 mm or 1/8 in.). The right side of the scale is applicable for the NF x 4 drill bit which is the new bit.

Drill type M x 3 M x 5 0 x 1 Q x 2

N F x 4

Helix angle, 6 13.5 deg 30° 23° 24° 36°

1/2 point angle, p 56.5 deg 58° 45° 55° 59°

chisel edge. Unlike a single point cutting tool, the rake angle along the two symmetrical cutting edges of a twist drill is not constant and the back rake angle at the outer edge is equal to the helix angle. The orthogonal rake angle of the cutting edge is measured in a plane perpendicular to the cutting edge and is the angle between the cutting edge velocity vector and tangent to the rake face. The orthogonal rake angle (Fig. 2) at any intermediate position d, is given by [14]

(rf,/fiOtan0-tan[sin"~' (d0/tf,)sinp]cosp tan yoi = : (2)

where

dj = d =

To/ d„

sin p

any intermediate diameter drill diameter helix angle half of point angle orthogonal rake angle at any intermediate position chisel edge length

From equation (2) we have calculated the variation of yoi

along the cutting edge of 3.18-mm (1/8-in.) drill bits of various types, especially those tested by Jacob [4]. Figure 3 reveals the variation of orthogonal rake angle measured in radians along the edge which is shown as a fraction of the nominal diameter of the drill bit along the abscissa. It is clear from Fig. 3 that most drill bits have a high negative rake (Fig. 2) angle for a considerable portion of the cutting edge, e.g., up to 45 to 55 percent. This was especially true for the bit M X 3 which is used for drilling metals.

An optimum rake angle facilitates cutting, decreases deformation of material cut by the tool, improves chip flow and reduces specific cutting energy [15, 16]. It was established

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by Merchant [15] while machining metal and by Jacob, et al. [16] while machining bone that increasing the positive rake angle decreases the principal cutting force. This finding is true for bone regardless of osteon direction [16], and is contrary to the recommendation of Bechtol [2] who suggested a 90-deg point angle and a 0-deg rake angle throughout the cutting edge. It should be pointed out that Bechtol's recommendation did not pertain to cutting force; it was meant to prevent chipping the periostal surface of the bone when the drill penetrates the second cortex, but unfortunately most of the commercial surgical drill bits are designed based on his recommendations.

Therefore from these facts it is apparent that incorporation of a higher positive rake angle along the cutting lip would increase the effectiveness of the bit. This is an important observation which we have identified and incorporated in our design.

It is interesting to note that an N F x 4 bit (nomenclature used, following Jacob, et al. [4]) which is the design suggested by the present authors, has a considerably lower negative rake angle over a smaller percent of its cutting lip, while nearly 60 percent of its edge has a positive rake angle (Fig. 3) which increased the cutting efficiency of this drill bit.

Point Angle. Half the point angle, p, is considered equivalent to the principal cutting edge angle of a single point turning tool.

True feed per cutting edge for a drill bit is given by

a, = — sin p where s0 = feed, mm/rev

The chip-tool interface temperature [18], T, is related to true feed, a| as

r,ocVo|

T/oeVsin p if s„ is kept constant.

Therefore, a larger point angle will lead to a higher tem­perature at the chip-tool interface. It has also been shown by Boston and Gilbert [17] and Oxford [18] that the thrust force increases and the torque decreases with higher point angles in the range of 90 to 140 deg while machining metal. Therefore, it is logical to expect that an optimum point angle exists for bone and stainless steel work-tool pair. Jacob, et al. [4] evaluated drill bits of various point angles combined with

Table 2 Measured thrust and torque for drilling bone with various drill geometry (after Jacob, et al. [4])

'ype

0 T M O Y

Point angle,

deg

110 88

113 90 86

Helix angle,

deg

24 27

13.5 23

17.2

Thrust F.,N

14.33 22.0 23.57 24.0 27.13

Torque M ; , x l 0 ~ 2 N m

1.02 3.50 4.86 5.55 4.75

different helix angle. Some of their results on drilling torque and thrust are shown in Table 2. We have calculated theoretically the effect of those angles on the orthongonal rake angle along the cutting edge of the drill bit. Figure 3 shows the variation of the rake angle for various point angle and helix angle combinations. It may be observed from Fig. 3 that larger helix and point angle imparts a positive rake angle for greater portion of the cutting lip. Jacob, et al. [4] showed experimentally that a 110-deg point angle drill bit (Q type, see Table 2) produced the least torque and thrust amongst the seven different types (all results are not shown in Table 2) of drill bits used to drill fresh bovine bone.

Now we may look into the equations of Table 3 developed by Wiggins and Malkin [5] by statistical regression analysis of their experimental data on fresh bone drilling. They showed that a twist drill of 118-deg point angle and 28-deg helix angle required much less torque per unit area of hole and energy as well, per unit volume of the bone material drilled at a given feed rate when compared to a surgical drill bit of 60-deg point angle and 20-deg helix angle. It may be seen that with a typical feed rate of /=0.128mm/rev, a surgical bit developed a 0 .718xl0~ 2 Nmm torque and required 0.3504 J/mm3 of energy, the corresponding values for a twist drill of 118-deg point angle are 0.654x 10"2Nmm and 0.3286 J/mm3 . This once again substantiates the fact that the 118-deg point angle will peform better than a 60-deg point angle. Farnworth and Burton [19] also studied the effect of different point angles on the rate of penetration, and the torque and thrust developed during drilling of bone. They examined three forms of drill point geometry namely radial-relief, four facet and spiral point. Their result indicates ([19, p. 232]) that a point angle in the range of 110-130 deg is very effective for optimum drill performance. In conclusion they have recommended a 120-140-deg point angle. Their recommendations were based on a spiral point and they did not measure the temperature developed. We have incorporated a split point by grinding the chisel edge. As a higher point angle leads to the generation of a higher temperature, we decided to use a compromised value of 118-deg point angle. We are suggesting a range of 110-118 deg for the point angle based on the foregoing experimental observations.

Lip Clearance Angle. Figure 1 shows the lip clearance angle which is provided to avoid rubbing of the drill clearance surface with the newly machined surface (clearane angle is shown in Fig. 1 and Fig. 2 on Sec. A-A). For steel this angle varies from 6-9 deg and for hard plastics this is made 12-15 deg [14]. Hardness of fresh cortical bone is of the order of RH

80 to 97 (Rockwell-H) which is equivalent to RB 20-29 [20]. For soft metals it varies in the range of RB 60-100. Therefore, for bone it is quite logical to select an angle of 15 deg at periphery to 18 deg toward the center for l /8-l /4in- (6.35-mm-) dia drills. In existing surgical bits it varies between 12 and 15 deg (Table 4).

Helix Angle. In equation (2), we have seen that the rake angle and helix angle are related and that a larger helix angle

Table 3 The torque (I) required per unit area (A) of the drilled hole and the specific cutting energy (U) during bone drilling at a constant thrust load (after Wiggins and Malkin [5])

Type

Surgical

General

Geometry Point Helix

angle, deg angle, deg

60

118

20

28

Rake angle, deg

0

- 3 0 to

+ 30 (approx)

Equation

7 / / ! = 1 .6x l0" 2 / 0 - 3 9

(7 = 0.1 x/~ 0- 6 1

r M = 1 . 4 x l 0 ~ 2 / ' 3 7

i/=.09x/-°-6 3

Remark

77A = torque/area of hole, Nmm; /= feed, mm/rev

U= Specific cutting energy, J/mm3or energy required for unit volume of material drilled out

248/ Vol. 104, AUGUST 1982 Transactions of the ASME

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the flute profile is shown in Fig. 4. This design was found tobe more effective in ejecting and smoothly removing the bonechips from the cutting zone. This was especially true when thelength of the hole was more than five to six times its diameter.

It is difficult to predict the number of retractions that mightbe required to drill a long hole. However, we have observedexperimentally that the parabolic fluted drills are able to drilla three times deeper hole than conventional surgical drillsbefore retraction and require only one-third, or less, of thetotal number of withdrawals required by standard surgicaldrills. Therefore, we expect that this design will avoid periodicwithdrawal of the drill for chip removal and shorten the timeof surgery.

Materials and Methods

Comparison of Drilling Torque, Thrust and Time. Tocompare the performance of existing drill designs with thenew drill bits a lathe cum drilling machine (Maximat V-7) wasused. A two-component dynamometer (KIAG, Type 9271A)simultaneously monitored the torque and the thrust duringdrilling of bovine bones and bone cements. A small mountingvise was fixed to the dynamometer to hold the samples. Thetorque and the thrust signals were amplified in a matched pairof charge amplifiers (KIAG, 5001) and then fed to a stripchart recorder (Cole Parmer Instrument Co., Recorder 8384­42). The drilling time was estimated from the thrust forcerecord and the calibrated chart speed. The drilling experimentat a constant applied thrust force was performed by hanging adead weight on a lever fixed to the feeding handle of thedrilling head of the lathe cum drilling machine. Figure 5shows the details of the experimental setup.

Samples of bone cement (Surgical grade, Simplex P,Howmedica, Inc.), 3mm thick, were prepared by castingPMMA in flat teflon molds. Rectangular compact bonespecimens (20mm x 25mm x 8-12mm) were machined, usinga band saw, from the middiaphysis of fresh bovine longbones. The samples were frozen at - 20 0 C and kept immersedin saline solution when not used. Unless mentioned otherwise,standard commercially available surgical drill (Table 4) bitswere compared with the experimental bits.

To avoid the effect of the anisotropic nature of bone [16],all experimental drillings were performed normal to the longaxis of the bone, drilling in the radial direction. Standard drillbits of 3/16 in. and 1/8 in. diameter and experimental bits of11/64 in. and 1/8 in. diameter were tested at 940 rpm. Thisrpm was chosen, based on our finding of optimum cuttingspeed [10J and recommendation of Jacob, et al. [4].

Fig. 5 The experimental setup used for drilling experiment. The lathechuck (1), the two component dynamometer (2) for measuring torqueand thrust, the ~rill~ng he.ad (3), dead weight (4) for applying constantthrust, and longItudinal slide (5) for constant feed motion are shown.

6-10

12-1512-1512-15

15-1815-1815-18

Liprelief, deg

15

242527

343536

Helixangle, deg

118

118118118

909090

Pointangle, deg

Diameter,in.(mm)

1/4 (6.35)

1/8(3.18)11/64 (4.37)1/4 (6.35)

1/8 (3.18)3/16 (4.76)1/4 (6.35)

I

Type

Metal cutting (M)

Experimental (N)

Surgical (S)

Fig. 4 The important design parameters of surgical drill (left) and theexperimental drill bit (right) are shown both in line diagram (or·thographic) and photograph. The additional lip and the absence ofchisel edge is visible in the new design, (top right). Lip clearance angleis shown at the bottom (right).

Table 4 Geometrical parameters of the drills used in thisstudy

will generate a larger rake angle, which is a desirablecharacteristic at the edge. Torque and thrust during drillingalso decreases with an increase in the helix angle [14, 17, 18]and a larger helix angle assists in clearing bone chips whichare usually short, flaky, and broken in nature. If chips are notremoved properly from the cutting zone, they are more likelyto clog in the flutes. This in turn, may increase the tem­perature beyond the threshold of tissue necrosis. Usually, thehelix angle of drill bits varies from 13 to 35 deg, depending onthe diameter. Larger angles are used for greater diameters.Based on these facts, and considering the effect of helix angleon the rake angle of the cutting edge (Fig. 3), we recommendthat the helix angle should be in the range of 24-36 deg forsurgical drills. As a matter of experimental evidence, theperformance of the Q-type drills used by Jacob [4J can benoted. It was observed to be the best overall drill bit amongthe bits he used (Table 2). Farnworth and Burton [19] alsorecommended a helix angle of 27 deg based on their ex­perimental results.

Flute. Helical flutes with U-grooves are very common intwist drills used for general purpose metal machining. Fordrilling wood, one may recall the use of auger bits which arevery efficient in removing wood chips. As bone has somestructural similarities with wood, we felt that the flute designsshould be similar. All commercially available surgical drillbits have a helical flute with a U-groove. Many authors [4, 7,10] have reported the serious problem of flute clogging withbone chips eventually leading to high temperature generation.We have also demonstrated this before [10]. To solve thisproblem we have tried a parabolic flute design. The details of

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Probe 0 ^ t — / !.

Up. \

y

\

y / F e e d Recorder

Digital Thermometer

(BAT-12)

Bone Sample

Chuck

Fig. 6 Experimental setup for measurement of temperature during drilling of bone in a lathe at a constant feed rate. The drill is held in the rotating lathe chuck while the bone sample in a vise fixed ot the longitudinal slide of the lathe which was engaged to lead screw for providing the feed motion.

DRILLING AT CONST. FEED RATE

|RPM=940) D=6.35mm(X")

8 0 -

7 0 -

6 0 -

5 0 -

4 0 -

30 -

2 0 -

1 0 -

BONE CEMENT

T

i

M M N

BOVINE BONE

T 1

M

T

''4

M f,

| [ No Pilot holelM. Drill)

| j ^ j P.H.{M. Drill) 1.6mm dia

| I No P.H.(N. Drill)

Fig. 7 Comparison of thrust forces developed by the new drill bit (N) and by a conventional metal cutting drill (M) while drilling bone and bone cement. Presence of a pilot hole (P.H.) or the use of the new drill bit both reduced the thrust force significantly (p < .001).

Effect of Chisel Edge. In the section on mechanics of drilling, we have already described that the chisel edge of a drill bit (Figs. 1 and 2) produces an increased thrust load and a higher temperature. It also hampers centering of the drill bit on an irregularly curved surface of a bone. To evaluate the effect of a chisel edge on drilling thrust, an experiment was performed on samples of bone and bone cement by pre-drilling a minute hole (pilot hole) of diameter equal to the length of the chisel edge of a metal drilling bit (diameter 6.35 mm, Table 4) before the experimental hole was drilled. Our objective was to evaluate the reduction in the thrust due to the absence of a chisel edge action as ensured by the presence a pilot hole. The thrust load was measured (as described in the foregoing subsection, "Comparison of Drilling Torque, Thrust and Time,") subsequently while drilling at a constant feed rate of 0.128mm/rev over the pilot hole and also on the solid specimens of the fresh bovine bone and bone cement. The solid bone and bone cement samples were also drilled with a new bit of the same diameter under identical cutting parameters and conditions.

Measurement of Temperature. The temperature gen­erated during drilling is dependent on the cutting parameters, e.g., the feed rate, drill speed, thrust force, type of work materials and the cutting conditions, i.e., wet or dry cutting. The temperature developed can be significantly reduced by

u

UJ <£ 3 r-

< tx UJ CL

> UJ h-

7 0 -

6 0 -

5 0 -

40 -

3 0 -

2 0 -

1 0 -

Drill Dia. = 3 mm Feed = 0.128 mm / rev

R P M

Fig. 8 Variation of temperature during drilling of fresh bovine bone with respect of cutting speed at constant feed rate. The temperature was measured 1mm away from the drilled hole.

suitable modifications of the existing design under identical cutting conditions and parameters. The experimental set-up for temperature measurement is shown in Fig. 6. For these tests, a constant feed rate was maintained; the drill bit was held by the lathe chuck and the bone specimens were fixed to the tool post (Fig. 5). The temperature was measured with copper-constantan microprobes (model BT-1, Bailey In­struments) placed one mm away from the pheriphery of the drilled holes in the bone. The probe was held in position by inserting it (sliding-fit) into a 0.2-mm-dia hole predrilled in the bone for this purpose. The time response of the probe was 0.15 s. The microprobe was connected to a strip chart recorder (Cole Parmer Inst.) for continuous recording of the bone temperature during drilling. The microprobe along with the recorder was calibrated by comparing it with a mercury thermometer in water. The bone samples were kept moist by soaking them in saline water. Just before drilling, the samples were removed from the water but no external irrigation was used to avoid any error due to uncontrolled rate of irrigation at the drilling zone. After completion of every drilling, the samples and drill bit were again soaked in water. Therefore the measured temperature values will be comparable to the upper limit of temperatures reached during actual surgical procedures with no, or minimum, irrigation. The temperature was also measured during drilling bone with an applied constant thrust load. In this case the drilling head (Maximat V-7) was used to provide rotation to the drill and the sample was held in a vise. For temperature measurement the same arrangement and instruments as described earlier was used as shown in Figs. 5 and 6.

As bone is anisotropic in nature, the cutting force and possibly the temperature will vary with the orientation of osteons during orthogonal cutting [16]. In all of our ex­periments, drilling was performed only in the radial direction (perpendicular to the long axis of bone). This direction was chosen, as clinically bone screws or pins are mostly inserted in this orientation.

Results

Effect of Chisel Edge and Point Modification. The thrust force measured (described in the foregoing subsection, "Effect of Chisel Edge") during drilling of bone cement showed that a small pilot hole (1.5 mm) equal to the diameter of the chosel edge, reduced the thrust load of a 6.35-mm (1/4-in-)dia drill bit by 45 percent when drilling at a constant feed rate (Fig. 7). Similarly, the same pilot hole experiment on beef bone reduced the thrust load by 40 to 50 percent (Fig. 7). A similar reduction in thrust while machining metals with twist drills has been shown by other authors [21, 22]. This study formed the basis of the chisel edge modification.

An analysis of the drill geometry indicates that the straight

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TEMPERATURE DURING DRILLING BOVINE BONE. RPM = 940 CONST. THRUST LOAD = 93.4N.

Table 5 Effect of drill type on the size of the hole, n = 5

Drill Method

oc D < DC UJ

a. 1

8 0 - i

7 0 -

6 0 -

5 0 -

4 0 -

3 0 -

2 0 -

10 -

8-V'N-l"N4"S-fe"N=4-

Fig. 9 Mean temperature (± 1std. dev) measured 1 mm away from the hole during drilling of fresh bovine bone at a constant thrust load, which was chosen based on threshhold level of penetration of higher size (1/4 in.) drill

DRILLING OF BONE CEMENT

RPM=940. APPLIED F,=45N

90

80

7 0 -

6 0 -

5 0 -

40

30

20

10

SURGICAL DRILL

NEW DRILL

•

S-^"(4 .76mm| N-jj"|4.36mm> S - j "(3.18mm) N = i "(3.18mm)

Fig. 10 The thrust Fz and the torque Mz developed, and the time of drilling (t) a 3-mm thick block of bone cement by standard surgical drills (S) of diameters 4.76 mm and 3.18 mm and the new drills (N) of diameters 4.36 mm and 3.18 mm

line chisel edge has a large negative rake angle [13]. In order to improve the cutting efficiency, the width of the chisel edge is often reduced. In some commercial surgical drill bits, the webs (web is shown in Fig. 1) are thinned by grinding which improves the efficiency to some extent. However, the maximum degree of web-thinning possible by this method is limited by the minimum strength of the web necessary to avoid damage to the drill point due to the cutting force. In the new drill, therefore, a split point was incorporated by grin­ding a notch which reduced the chisel edge almost to a tip and this produced two additional cutting edges as shown in Fig. 4. This transformed the extrusion effect of the chisel edge to a cutting action by imparting positive rake angle at the chisel edge zone. This design change also caused breaking of the chips into smaller pieces so that they could be ejected more easily through the flutes. This modification also facilitated accurate location of the drill bit on the curved surface of a bone. Skidding or walking of the common drill point when starting a hole on a curved, bony surface is a clinical problem. Due to the absence of a chisel edge and incorporation of a pointed tip, this new bit can be located and held in position more conveniently when a portable hand drill is used.

Type

New Surgical

Diameter, mm

3.16 3.15

Hand fed (a)

Hole diameter,

3.17 (0.01) <"' 3.21 (0.02)

Machine (b)

(mm)

3.17(0) 3.16(0.01)

(<,'The figures in the parenthesis shows the (± s.d.)

Temperature. The temperature generated during drilling fresh bovine bones was found to increase at the beginning with rotational speed and there was an optimum range of 750-1000 rpm, at a feed rate of 0.128 mm/rev, where the temperature was below the threshhold level of tissue damage. Figure 8 depicts the results of testing with a surgical bit of 3-mm diameter. Similar results were also obtained for a 4.8-mm-dia surgical bit. Jacob, et al. [4] suggested 750 to 1250 rpm as optimum speed range for bone drilling. The new bit of 3.18-mm diameter, when tested similarly at a constant feed rate of 0.128 mm/rev, did not show any significant change in temperature in the speed range of 65 to 1400 rpm. The peak temperature developed while drilling bovine bone with the new bit varied from 30 to 40°C in the above range of speed.

The result of a comparative study on temperature generation under constant thrust load (which probably is more common clinically) is shown in Fig. 9. Under the same cutting conditions, the new 1/8-in-dia bit reduced the tem­perature by 41 percent when compared with a 1/8-in standard surgical drill bit (Fig. 9). The 1/4-in-dia new bit even developed lower temperatures than a smaller surgical drill bit of 3/16-in. diameter.

Thrust, Torque and Time. The penetration rate under the approximately constant applied load of 45N was significantly higher (p<0.001) in case of the new drill bit (Fig. 10). A thrust load of 45N was chosen since this was slightly above the threshhold value for initiating and continued penetration with a surgical drill of 3/16-in. diameter [3, 10]. Figure 10 shows that a surgical bit required 233 percent more time to drill a 3/16-in-dia hole through the bone cement piece of thickness 3 mm. Similarly a 1/8-in-dia surgical bit used 67 percent more time compared to the new design. The drilling torque was slightly more (5-10 percent) for the new design. Clinical trial of the drill bit also showed promising results as far as thrust and temperature developed and ease of handling was con­cerned.

Size and Quality of Holes. Drill holes when measured accurately are found to be generally larger than the drill diameter due to wobbling, an effect also noted by others, [2]. Bechtol, et al. [2] suggested that enlargement of the hole can be minimized by dulling the flute edge. Compared to surgical bits our split point drill wobbled much less.

To quantify the effect of drill design on the size and quality of the drilled holes, 1/8-in- (3.18-mm-) dia surgical and new bits were used to drill two sets of holes in flat machined pieces of moist bovine bone clamped in a vise. A portable hand-fed power drill rotating at 1000 rpm and a drilling head rotating at 940 rpm in a lathe cum drilling machine as described earlier, were used in this study (Table 5). The holes were examined carefully and their dimensions measured under a microscope (Ortholux, Leitz, 32 X) and also on a TV screen after 200 x magnification. The holes were also measured with internal vernier slide caliper (The Central Tool Co. model 208). The edges of the holes generated by surgical drill looked rough and torn while those by the new drill had sharper, cleaner edges. As expected, the hand-fed drilling generated comparatively rougher edges and produced larger dirnen-

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sional variation than the machine drilling for both types of drill, as shown in Table 5.

Special Features. While drilling bone, the presence of periosteum often obstructs the flow of chips through the drill flutes [4, 10] and, thus, causes clogging of the chips. The split point drill bit we used hardly caused any such problem. Usually the chisel edge catches the periosteum and eventually carries it to the flutes where it obstructs chip flow. Our split-point design imparted a positive rake angle and a cutting action to the chisel edge thus avoiding such problems. In drilling larger holes with ordinary drill bits, it was sometimes observed [2] that the drill might catch on a piece of bone while entering the marrow cavity. If it was forced ahead at that stage, it would create a small fracture fragment which was usually visible to the naked eye. This damaging phenomenon was not observed at all while drilling with the new bit when observed under a magnifying glass of 10 x . Once again we may recall that with the new design, "walking" of the drill bit was eliminated. The use of a dril guide was also not necessary except for a very long drill bit or to protect the soft tissues.

Conclusion

A new drill bit has been designed and, compared to the standard surgical drill bits, the new bit was much more ef­ficient in drilling bone and bone cement. The new bit penetrated at a faster rate with a reduced thrust load. The peak temperature generated was also significantly lower for the new bit and it avoided clogging of the bone chips and walking on the curved bone surface. We hope that the use of this new drill bit will make bone drilling easier, reduce the time of surgery and minimize the possibility of thermal necrosis of bone. The modified design parameters are shown in Table 4. Figure 4 shows the detailed orthographic views and pictorial view (top right corner) of the drill bit. The design features of the new drill bit has been discussed in this paper. Further details required for manufacturing the drill bit is beyond the scope of the present paper.

References

1 Weisman, S., "The Skeletal Structure of Metal Implants," Biomechanics and Human Factor Symposium ASME, 1967, pp. 87-110.

2 Bechtol, C. O., Ferguson, A. B., and Liang, P. G., "Metals and Engineering in Bone and Joint Surgery," Williams and Wilkins Co., Baltimore, 1959.

3 Sneath, R. S., "The Determination of Optimum Twist Drill Shape for Bone," Biomechanics and Related Bioengineering Topics, Proceedings of the Symposium of Glasgow, Pergamon Press, Oxford, Sept. 1964, pp. 41-45.

4 Jacob, C. H., Berry, J. T., Pope, M. H., and Hoaglund, F. T., "A Study of Bone Machining Process—Drilling," Journal of Biomechanics, Vol. 9, 1976, pp.343-349.

5 Wiggins, K. L., and Malkin, S., "Drilling of Bone," Journal of Biomechanics, Vol. 9, 1976, pp. 553-559.

6 Matthews, L. S., and Hirsch, C , "Temperature Measured in Human Cortical Bone when Drilling," Journal of Bone and Joint Surgery, Vol. 54A, 1972, pp. 297-308.

7 Albright, J. A., Johnson, T. R., and Saha, S., "Principals of Internal Fixation in Orthopaedic Mechanics: Procedures and Devices," eds., D. N. Ghista and R. Roaf, Academic Press 1978, pp. 124-229.

8 Moss, R. W., "Histopathologic Reaction of Bone to Surgical Cutting," Oral Surgery, Vol. 17, 1964, pp. 405-414.

9 Moritz, A. R., and Henrique, F. C. Jr., "Studies of Thermal Injury I I ," American Journal of Path., Vol. 23, 1947, p. 695.

10 Pal, S., and Saha, S. "Effect of Cutting Speed on Temperature During Drilling of Bone," Proceeding oftheACEMB, Vol. 23, 1981, p. 289.

11 Product Encyclopedia, Zimmer, USA, Warsaw, Indiana, 46580, 1978. 12 Huiskes, R., "Some Fundamental Aspect of Human Joint

Replacement," ,4C7>1 Ortho. Scand. Supp., No. 185, 1980, pp. 62-63. 13 Kronenberg, M., Machining Science and Applications—Theory and

Practice for Operation and Development of Machining Process, Pergamon Press, First Edition, 1966, p. 53

14 Bhattacharyya, A., and Ham, I., Design of Cutting Tools—Use of Metal Cutting Theory, ASTME Publication, 1969.

15 Merchant, M. E., "Mechanics of Metal Cutting Process," Journal of Applied Physics, Vol. 19, 1945, p. 876.

16 Jacobs, C. H., Pope, M. H„ Berry, J. T., and Hoaglund, F. T., "A Study of the Bone Machining Process—Orthogonal Cutting," Journal of Biomechanics, Vol. 7, 1974, pp. 131-136.

17 Boston, O. W., and Gilbert, W. W., "The Torques and Thrust in Small Drills Operating in Various Metals," Trans. ASME, Vol. 58, 1936.

18 Oxford, C. J., "On the Drilling of Metals I—Basic Mechanics of Drilling Process," Trans. ASME, Vol. 77, 1955.

19 Farnworth, G. H., and Burton, J. A. "Optimization of Drill Geometry For Orthopaedic Surgery," Paper No. 28, Proceeding of the 14th International Machine Tool Design and Research Conference, 1974, pp. 227-233.

20 Evans, F. G., Mechanical Properties of Bone, C. C. Thomas, Springfield, III., 1973.

21 Pal, A., Bhattacharyya, A., and Sen, Gopal, "Investigation of Torque in Drilling Ductile Materials," International Journal of Machine Tool Design and Research, Vol. 4, 1965, pp. 205-221.

22 Bera, S. K., and Bhattacharyya, A., "Evaluation of the Thrust Force at the Chisel Edge of a Twist Drill," Proceedings of the 1st All India MTDR Conference, Jadavpur Univ., Calcutta, Jan. 1967.

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