SEQUENTIAL DRILLING AND DRILL ANGULATION REDUCE THE ACCURACY OF

DRILL HOLE START LOCATION IN A SYNTHETIC BONE MODEL

Edith Bishop BVSc(Hons) MANZCVS GradDipEd MRCVS – corresponding author

Hospital for Small Animals

University of Edinburgh

Easter Bush

Roslin, Midlothian

EH25 9RG

United Kingdom

ebishop@exseed.ed.ac.uk

+44 7487 227 606

Jon Hall MA VetMB CertSAS DipECVS MRCVS

Hospital for Small Animals

University of Edinburgh

Easter Bush

Roslin, Midlothian

EH25 9RG

United Kingdom

Ian Handel - BVSc MSc PhD CStat MRCVS

The Royal (Dick) School of Veterinary Studies

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University of Edinburgh

Easter Bush

Roslin, Midlothian

EH25 9RG

United Kingdom

Dylan Clements BSc BVSc PhD DSAS (Orth) DipECVS FRCVS

Hospital for Small Animals

University of Edinburgh

Easter Bush

Roslin, Midlothian

EH25 9RG

United Kingdom

John Ryan MVB CertSAS DipECVS MRCVS

Hospital for Small Animals

University of Edinburgh

Easter Bush

Roslin, Midlothian

EH25 9RG

United Kingdom

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Abstract:

The accuracy of drill hole location is critical for implant placement in orthopaedic surgery.

Increasing drill bit size sequentially has been suggested as a method for improving the

accuracy of drill hole start location. The aim of this study was to determine whether

sequential drilling or drill angulation would alter accuracy of drill hole start location. Three

specialist veterinary surgeons drilled holes in synthetic bone models either directly, or with

sequentially increasing drill bit sizes. Drilling was performed at 0o, 10o and 20o to

perpendicular to the bone models. Three synthetic bone models were used, to mimic canine

cancellous and cortical bone. Sequential drilling resulted in greater inaccuracy in drill hole

location when assessing all drilling angles together. There was no influence of surgeon or

synthetic bone density on drilling accuracy. The combination of drill angulation and

sequential drilling increased inaccuracy in drill hole start location. We conclude that

sequential drilling decreased accuracy of drill hole location in the synthetic bone model

when drilling was angled. Inaccuracy associated with the drill hole start location should be

taken into account when performing surgery, although the magnitude of inaccuracy is low

when compared to other sources of error such as angulation.

Introduction:

Bone drilling is often required to place surgical implants, and the accuracy of both the drill

angle and location of the drill hole is vital to optimise implant positioning. Human error has

been identified as a source of inaccuracy during bone drilling procedures (1, 2). Handedness

of the surgeon has been shown to effect angulation of drilling, with deviation of angulation

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towards the right for right handed surgeons, and towards the left for left handed surgeons

(2). More acute angulation of the drill bit relative to the bone surface has been shown to

result in greater inaccuracy in the desired angulation of the bone tunnel (3). In order to

increase drill accuracy, techniques such as intraoperative imaging, stereotactic drilling and

production of custom designed drill guides have been used (1, 2, 4, 5).

Sequential drilling refers to the progressive enlargement of a drill hole through the

consecutive use of incrementally larger drill bits until the desired diameter is reached.

Sequential drilling is often recommended for implant placement in equine and human

orthopaedic surgery as it reduces heat generation in the adjacent bone (6-9). Although heat

generation during orthopaedic implant placement in canine bone is a concern (10), the

clinical benefit of sequential drilling in canine orthopaedic surgery not been tested.

Sequential drilling is also recommended by some surgeons to increase the accuracy of

drilling (11, 12), particularly in anatomical structures where there are narrow margins for

error, such as the canine humeral condyle or the canine or feline sacrum (13, 14). To the

authors’ knowledge there has not been previous published research investigating whether

sequential drilling of a drill hole or angulation of the drill relative to the surface of the bone

affects the accuracy of the drill hole origin.

The aim of this study was to determine whether sequential graduated increase in drill bit

size, at different drill angles, would affect the positional accuracy of a drilled hole. Our

hypotheses were that using sequentially increasing drill bit size would result in greater

accuracy in final location of a drill hole compared with drilling a hole of the final target

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diameter with a single drill bit, and that drilling accuracy would be reduced at more acute

angles relative to the surface of the synthetic bone substitute.

Materials and methods:

Synthetic bone model testing:

Three different densities of polyurethane synthetic bone blocks (Sawbones, Vashon Island,

WA) were used to mimic canine cancellous and cortical bone respectively; 20 pounds per

cubic foot (PCF) cellular foam block (130 mm x 180 mm x 40 mm), 40 PCF rigid sheet and 50

PCF rigid sheet (130mm x 180mm x 3mm). Canine femoral diaphyseal cortical bone has

been shown to have a density similar to that of the 50 PCF foam (12), so 40 PCF foam sheets

were used in addition to replicate other areas of the canine skeleton likely having a lower

density than the femoral diaphysis, such as humeral diaphysis and condyle (11). The 20 PCF

cellular foam block has a similar density to canine femoral cancellous bone (12). For the

polyurethane sheets, the material was cut into 3cm x 3cm squares with an oscillating sagittal

saw and mounted into a vice with a central marked target for the surgeon to begin the drill

hole. The sheet was confirmed horizontal using a spirit level. Six points were marked on the

surface of each square, consisting of four points each 1.5cm apart, and one point directly in

the centre of the other four (Point C), with an additional marker between two peripheral

points in order to determine direction for drill angulation (Figure 1). Points were marked

using a template created from radiographic film with perforations to allow consistent

marking of all polyurethane sheets. A Canon 750D (Canon: Canon Europa N.V. The

Netherlands) with a 70-200 f/4L USM lens set in automatic focus was mounted directly

above the square, checked with a plumb line, and photographs were taken before and after

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drilling to allow measurement of accuracy of the drill hole and stored as a .jpeg image of

6000 x 3368 pixels.

The 20 PCF polyurethane foam was not cut into individual squares due to the difficulty of

creating cuts at 90 degrees to the surface of the block. Instead 3cm x 3cm squares were

drawn on the surface of the block, and points were marked within each square using the

same template as used for the polyurethane sheets.

Three diplomates of the European College of Veterinary Surgeons drilled into the blocks,

aiming for Point C the centrally marked point, using either a 4.5mm drill bit, or drilled

sequentially using graduated drill sizes starting with 2mm, then 2.5mm, 3.5mm and finally

4.5mm drill bits (310.190, 310.250, 310.350, 310.480: Synthes GmbH, Oberdorf, Switzerland

). As no guidelines exist for sequential drilling the drill sizes chosen for sequential drilling

were based on the surgeons’ observations of how they had seen the technique applied

clinically. Drill holes were angled at 0o (perpendicular to the synthetic bone surface), 10o and

20o from perpendicular, with guidance provided by a second investigator using angled pins

(bent Kirschner wires with angles checked using a goniometer) mounted directly in line with

the drill. Photographs were taken of each drilling trial (with the camera positioned at 90o to

the surgeon), so that accuracy of drill angulation could be checked. A subset of photographs

of each surgeon (three trials for each angle for each surgeon) were analysed to check drill

angulation accuracy using the Angle tool in ImageJ (ImageJ 1.51; National Institutes of

Health, Bethesda, Maryland). All measurements were within 1o either side of the desired

drill angulation. 10o from perpendicular was chosen as this is close to the recommended drill

bit angulation for the safe placement of the thread hole in the canine sacrum (15), and 20o

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from perpendicular was chosen to identify the effects of increased angulation, for example

when placing implants into canine vertebrae(16).

Universal drill guides (2mm, 2.5mm, 3.5mm (Universal Drill Guide 2.0, Universal Drill Guide

3.5: Synthes GmbH, Oberdorf, Switzerland ), and 4.5mm tissue protection sleeve (Protection

Sleeve 4.5: Synthes GmbH, Oberdorf, Switzerland )) were used while drilling to mimic a

clinical setting. Each method (direct drilling (DD) or sequential drilling (SD)) was repeated

three times at each of the specified drill angles (0o, 10o and 20o) by all surgeons in each of

the three different synthetic bone materials. Time taken to perform each drilling procedure

was not recorded as this was not considered relevant to the purpose of our study

Distance of the drill hole from the four peripheral marked points was measured on images

taken before and after drilling to determine the accuracy of drill hole start location, and the

direction of any deviation from the intended target was recorded. Adobe Photoshop

(Adobe. Adobe Photoshop CC. 19.1.0 ed: Adobe 2018) was used to overlay and align images

so that the location of Point Cthe marker point that surgeons aimed for could be compared

to the location of the drill hole. ImageJ (ImageJ 1.51; National Institutes of Health, Bethesda,

Maryland) image processing software was used to obtain pixel coordinates of Point Cthe

marker point and the centre of the subsequent drill hole for each drilling trial. The point

measurement tool was used to identify the pixel co-ordinates of Point Cthe marker point.

The oval function tool was used to outline the edge of the drill hole, and the centre of the

drill hole calculated as the centroid pixel co-ordinates of the oval. The perimeter of the drill

hole was outlined on the top surface of the synthetic bone material only. The use of the oval

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function tool and centroid point measure ensure that the central point of the drill hole was

measured, even where it became elliptical with drill angulation. Distance in millimetres was

determined by calibration based on pixel number compared to known distance between

marker points. Due to a degree of assumed inaccuracy in positioning of marker points, ten

measurements were randomly selected from photographs in each study session. There was

no significant difference between these measurements determined by Student’s t-test (see

Appendix 1, Supplementary Material), and so an average of the scaling for each session was

obtained from these ten measurements, and used to apply calibration to distance

measurements for the corresponding session.

A Shapiro-Wilk test was performed to determine whether the distance of the centre of drill

hole from Point C the drill aiming marker was normally distributed. Linear regression

analysis was performed to determine which factors (surgeon, procedure, synthetic bone

material, and drill bit angulation) influenced distance of the centre of the drill hole from the

marker point.

All variables were compared to direct drilling at 0 degrees in the 20 PCF foam by surgeon 1.

Direction of deviation of the centre of the drill hole from the marker point was divided into

four quadrants:

1. away from and to the right of the surgeon;

2. away from and to the left of the surgeon;

3. towards and to the left of the surgeon; and

4. towards and to the right of the surgeon.

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A chi-squared test was used to determine whether deviation differed significantly towards

versus away from the surgeon and to the left vs the right of the surgeon. A Kruskal-Wallis

test was used to determine whether surgeon, procedure, drill angle or density of synthetic

bone material affected the direction of deviation from marker point. A Spearman correlation

coefficient was used to determine whether direction of deviation and distance of deviation

were correlated. Significance was set at a P value of 0.05 for all tests. R programming

software (17) was used to perform all statistical analysis.

Results:

A total of 158 data points were used for statistical analysis; four data points were missing

due to technical errors in camera operation.

Distance of centre of drill hole from marker point:

The distance of the centre of the drill hole from the marker point was not normally

distributed. The median distance of the centre of the drill hole from the marker point was

0.55mm. The median distance of the centre of the drill hole from Point Cthe marker point

using sequential drilling was 0.6mm (range = 0.07 – 1.83), compared to 0.51 mm (0.02 –

1.54) mm for direct drilling. Results of linear regression analysis are shown in Table 1.

Sequential drilling was statistically significantly less accurate than direct drilling when

assessing all drill angles together (Figure 2). Increasing drill bit angulation from 0o to 10o and

from 0o to 20o statistically significantly decreased accuracy of drilling (Figure 3). There was

no significant influence of surgeon or synthetic bone density on distance of the centre of the

drill hole from the marker poinPoint Ct (Figure 4). Median and range values for distance of

centre of drill hole from marker point are shown in Table 2.

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Estimate Standard Error t value p value

Surgeon 1 (reference)

Surgeon 2 0.00257 0.06524 0.039 0.9686

Surgeon 3 0.08524 0.06492 1.313 0.1912

Direct drilling (reference)

Sequential drilling 0.11236 0.05322 2.111 0.0364*

20 PCF (reference)

40 PCF -0.07182 0.06434 -1.116 0.2661

50 PCF -0.03230 0.06528 -0.495 0.6214

Drill angle

0o vertical

(reference)

Drill angle 10o 0.13128 0.06463 2.031 0.0440*

Drill angle 20o 0.40665 0.06589 6.171 6.02x10e-9***

Table 1: Linear regression model for factors influencing distance of centre of drill hole from central

marker point (Point C) marker point. Adjusted R-squared: 0.2248. Significance codes: ‘***’ = 0.001,

‘*’ = 0.05.

Density (PCF) Drill Angle (degrees) Direct Drilling (mm) Sequential Drilling (mm)

20 0 0.631 (0.17-1.54) 0.423 (0.25-0.86)

10 0.41 (0.08 – 0.78) 0.79 (0.11 – 1.38)

20 0.701 (0.15 – 1.3) 0.68 (0.33 – 1.83)

40 0 0.378 (0.02 – 1.05) 0.217 (0.07 – 0.54)

10 0.365 (0.09 – 1.13) 0.559 (0.27 – 1.01)

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20 0.698 (0.32 – 1.42) 1.15 (0.39 – 1.47)

50 0 0.365 (0.13 – 0.67) 0.287 (0.17 – 0.55)

10 0.568 (0.13 – 0.89) 0.835 (0.23 – 1.11)

20 0.604 (0.43 – 1.11) 0.813 (0.60 – 1.58)

Table 2: Median (range) values for distance of centre of drill hole from central marker point (Point

C)marker point expressed in millimetres.

Direction of deviation of centre of drill hole from central marker point:

Where a drill hole was not completely centred on the target, the count of deviations

towards each quadrant as a result of drill angle (Table 3) and procedure (DD versus SD,

Table 4) are shown. There was no significant difference in direction of deviation between

surgeons. There was no significant difference in incidence of deviation to the left or right (p

= 0.87).

Away and right Away and left Towards and left Towards and right

0 degrees 12 21 9 10

10 degrees 14 30 5 6

20 degrees 33 12 2 4

Table 3: Incidence of deviation of centre of drill hole in each of four categorical directions from the

central marker point (Point C) when drilling was performed at 0o, 10o and 20o to perpendicular to

the synthetic bone material.

Away and right Away and left Towards and left Towards and right

Direct drill 32 24 10 15

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Sequential drill 27 39 6 5

Table 4: Incidence of deviation of centre of drill hole in each of four categorical directions from the

central marker point (Point C) when drilling was performed directly (DD) or using sequentially

increasing drill bit sizes (SD).

The direction of deviation was significantly more often away from the surgeon (p<0.001);

this became increasingly marked with each sequential drill angle (Figure 5). There was no

significant effect of synthetic bone density, procedure or surgeon on direction of deviation

of the centre of the drill hole from Point Cthe marker (p = 0.10) (Figure 4).

Estimate Standard Error t value p value

Direct drilling (reference)

Sequential drilling -0.14357 0.08984 -1.598 0.1121

Drill angle 10o -0.0589 0.08632 -0.682 0.4960

Drill angle 20o 0.24191 0.08887 2.722 0.0072**

Sequential

drilling*Drill angle

10o

0.40731 0.12514 3.255 0.0014**

Sequential

drilling*Drill angle

210o

0.34969 0.1275 2.743 0.0068**

Table 5: Linear regression model for factors influencing distance of centre of drill hole from central

marker point (Point C) (drill angle and sequential drilling) and their interaction. Adjusted R-

squared: 0.2248. Significance code: ‘**’ = 0.01

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The results of initial linear regression analysis were probed by evaluating the effects of

sequential drilling, drill angle and their interaction, and are shown in Table 5. Increasing drill

bit angulation from 0o to 20o statistically significantly decreased accuracy of drilling.

Sequential drilling significantly decreased drilling accuracy when drilling at an angle (10o or

20o).

Discussion:

Results of this study revealed that the drill hole start location was less accurate with

sequential drilling when compared to direct drilling in a synthetic bone model when

assessing all drill angles together, although the magnitude of the difference was small.

When probed in isolation, inaccuracies in the drill hole start location were associated with

the combination of drill angulation and sequential drilling. Safe corridors based on anatomic

landmarks have been defined for implant placement where there is limited margin for error,

such as the canine humeral condyle (18) and canine and feline sacrum (15, 19-22). Other

methods of overcoming this inaccuracy include use of intraoperative fluoroscopy (23, 24) or

the use of custom drill guides (4, 5). The increased inaccuracy resulting from sequential

drilling when compared to direct drilling is unlikely to be clinically significant unless the

surgeon drills a hole at an angle close to the safety limit.

The inaccuracy of drill hole start location increased as the drill was angled from 0o to 10o,

and from 0o to 20o, and typically the displacement of the start point was away from the

surgeon. However, on the basis of these results the impact of drill hole start location on

screw position is negligible when compared to the previously published impact of drill

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angulation. The drill hole for a sacroiliac screw is recommended to be angled at 7o from

perpendicular to the lateral cortex of the sacrum (15); specialist surgeons typically have

margins of error of approximately 7o or more 25% of the time when drilling at 10o from

perpendicular to the drilling surface (3). Given the relatively small range of inaccuracy seen

in our study (relative to the size of the drill bit used) it is likely that surgeon inaccuracy in

drill angulation, rather than drill hole start location, is more clinically relevant, particularly as

the drill angulation is increased.

Increasing drill bit angulation tended to result in drill hole start location which was more

likely to be away from the surgeon. Thus, the drill bit tended to slip in the direction of

angulation, and our observation was that this occurred when the drill bit rotation was

started prior to the tip gripping bone. This was compounded by sequential drilling, which

may be because the larger drill bit was started with the near edge abutting the existing hole

(thus drifting the centre of the drill bit away from the true centre of the drill hole). To

minimise this slippage, surgeons may start the drill bit in a position perpendicular to the

bone to obtain a small purchase point on the bone surface at the accurate drill hole starting

point, and then apply the required angulation. Measurement of drilling inaccuracy between

drill bit changes during the sequential drilling trial would have helped us to determine

whether inaccuracy increased with each subsequent drilling, however this was not done.

Therefore, although this information may have helped us to determine where inaccuracy

may arise during sequential drilling, this cannot be determined without repeating the study.

Two of the surgeons were right handed and one was left handed. There was no difference in

direction of deviation between the surgeons, however a greater number of surgeons would

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be needed to predict whether handedness influences direction of deviation in drilling as has

been previously documented for drill bit angulation (2).

In human and equine orthopaedic surgery sequential drilling has been shown to reduce heat

production in adjacent cortical bone (6-9, 25, 26) with greater heat production occurring

with increasing cortical bone thickness (9). The thickness of cortical bone in the equine

metatarsal II is reported to be up to 14mm (27), and cortical bone in humans is 3mm to

5mm (6). Cortical bone thickness in the canine humerus and femur is on average 2.9mm and

3mm respectively (11, 12). Therefore, thermal bone necrosis may be less of a concern when

drilling in canine bone, but theoretical benefit of sequential drilling to reduce thermal

necrosis in canine bone is untested.

The polyurethane synthetic bone model used in this study has been previously validated as

a biomechanical study model for human and canine cortical and cancellous bone (28-30).

The 20 PCF foam used to mimic cancellous bone in this study was not sawed into individual

squares as with the 40 and 50 PCF foam sheets due to the difficulty of sawing this material,

which may have influenced drilling accuracy. It has been recommended to use two drill bits

when drilling sequentially to avoid thermal necrosis (8), whereas in this study four drill bits

were used. Using fewer drill sizes during sequential drilling may also have influenced

accuracy, as each additional drill bit may contribute to the mechanism of bone tunnel start

location deviation away from the original marker point. Other limitations of this study

include that there were only three surgeons performing the drilling, and that testing

surgeons of differing degrees of orthopaedic surgical experience may have produced

different results.

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In conclusion, results of this study suggest that sequential drilling reduces drilling accuracy

when drilling at an angle of 10o or 20o, contrary to the original hypothesis. We recommend

that the surgeon should use their own discretion as to whether this technique is employed

in situations where drilling accuracy is important. Additionally, more acute angulation of the

drill bit relative to the bone surface will further increase inaccuracy, which should be

appreciated when there are narrow margins for error.

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References:

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18. Barnes DM, Morris AP, Anderson AA. Defining a safe corridor for transcondylar screw insertion across the canine humeral condyle: a comparison of medial and lateral surgical approaches. Vet Surg. 2014;43(8):1020-31.19. Shales CJ, White L, Langley-Hobbs SJ. Sacroiliac luxation in the cat: defining a safe corridor in the dorsoventral plane for screw insertion in lag fashion. Vet Surg. 2009;38(3):343-8.20. DeCamp CE, Braden TD. The Surgical Anatomy of the Canine Sacrum for Lag Screw Fixation of the Sacroiliac Joint. Veterinary Surgery 1985;14(2):131-4.21. DeCamp CE, Braden TD. Sacroiliac Fracture-Separation in the Dog A Study of 92 Cases. Veterinary Surgery. 1985;14(2):127-30.22. Burger M, Forterre F, Brunnberg L. Surgical anatomy of the feline sacroiliac joint for lag screw fixation of sacroiliac fracture-luxation Veterinary and Comparative Orthopaedics and Traumatology 2004;17(3):146-51.23. Cook JL, Tomlinson JL, Reed AL. Flouroscopically Guided Closed Reduction and Internal Fixation of Fractures of the Lateral Portion of the Humeral Condyle: Prospective Clinical Study of the Technique and Results in Ten Dogs Veterinary Surgery 1999;28:315-21.24. Tonks CA, Tomlinson JL, Cook JL. Evaluation of closed reduction and screw fixation in lag fashion of sacroiliac fracture-luxations. Vet Surg. 2008;37(7):603-7.25. Davidson SRH. Drilling in Bone: Modeling Heat Generation and Temperature Distribution. Journal of Biomechanical Engineering. 2003;125(3):305-14.26. Bulloch SE, Olsen RG, Bulloch B. Comparison of Heat Generation Between Internally Guided (Cannulated) Single Drill and Traditional Sequential Drilling WIth and Without a Drill Guide for Dental Implants. International Journal of Oral and Maxillofacial Implants. 2012;27:1456-60.27. Lescun TB, Frank EA, Zacharias JR, Daggy JK, Moore GE. Effect of sequential hole enlargement on cortical bone temperature during drilling of 6.2mm-diameter transcortical holes in the third metacarpal bones of horse cadavers. American Journal of Veterinary Research 2011;72(12):1687-94.28. Calvert KL, Trumble KP, Webster TJ, Kirkpatrick LA. Characterization of commercial rigid polyurethane foams used as bone analogs for implant testing. J Mater Sci Mater Med. 2010;21(5):1453-61.29. Szivek JA, Thomas M, Benjamin JB. Technical Note: Characterisation of a Synthetic Foam as a Model for Human Cancellous Bone. Jounral of Applied Biomaterials 1993;4(3):269-72.30. International A. Standard Specification for Rigid Polyurethane Foam for Use as a Standard Material for Testing Orthopaedic Devices and Instruments 2016;F1839 - 80(1):1-6.

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Figure 1: 40 PCF polyurethane sheet mounted in vice. Four marker points at periphery were used

to align images before and after drilling in order to assess distance and direction of deviation from

Point C the centrally marked point. Additional marker on right hand side placed to allow

determine direction of deviation relative to position of surgeon.

Figure 2: Distance of deviation of the centre of the drill hole from Point C the marker point based

on procedure used (direct drilling vs sequentially increasing drill bit size). DD = direct drill; SD =

sequential drill. All surgeons and drill angles combined.

Figure 3: Distance of deviation of the centre of the drill hole from Point C the marker point based

on angle of drill bit in relation of synthetic bone surface. 0 = perpendicular to bone surface; 10 =

ten degrees from perpendicular; 20 = twenty degrees from perpendicular. All surgeons, direct drill

(DD) and sequential drill (SD) combined. Significant differences between angle pairs is denoted by

the horizontal bases, with ‘***’ = 0.001, ‘*’ = 0.05.

Figure 4: Distance and direction of deviation of centre of drill hole from Point C original marker

point separated by procedure (SD vs DD) and synthetic bone density. 20 = 20 PCF foam, 40 = 40

PCF sheet, 50 = 50 PCF sheet.

Figure 5: Distance and direction of deviation of centre of drill hole from Point C original marker

point separated by procedure and drill angle. 0 = perpendicular to bone surface; 10 = ten degrees

from perpendicular; 20 = twenty degrees from perpendicular.

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