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
+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
Word Count: 2807
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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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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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