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Is the instrumented-pointer method of calibratinganatomical landmarks in 3D motion analysis reliable?DOI:10.1016/j.jbiomech.2017.01.019

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Citation for published version (APA):Tawy, G., & Rowe, P. (2017). Is the instrumented-pointer method of calibrating anatomical landmarks in 3D motionanalysis reliable? Journal of biomechanics, 53, 205-209. https://doi.org/10.1016/j.jbiomech.2017.01.019

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Tawy, Gwenllian Fflur and Rowe, Philip (2017) Is the instrumented-

pointer method of calibrating anatomical landmarks in 3D motion

analysis reliable? Journal of Biomechanics, 53. pp. 205-209. ISSN 0021-

9290 , http://dx.doi.org/10.1016/j.jbiomech.2017.01.019

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1

This manuscript is being submitted as a short communication.

Manuscript Title

Is the instrumented-pointer method of calibrating anatomical landmarks in 3D motion

analysis reliable?

Author names and affiliations:

Gwenllian Fflur Tawya

Philip Rowea

aUniversity of Strathclyde

Affiliation address:

The Department of Biomedical Engineering,

University of Strathclyde,

Wolfson Centre,

106 Rottenrow,

Glasgow,

G4 0NW

United Kingdom

Author email addresses:

gwenllian.tawy@strath.ac.uk

philip.rowe@strath.ac.uk

Corresponding author:

Name: Gwenllian Fflur Tawy

Email addresses: gwenllian.tawy@strath.ac.uk or gwenllian.tawy@gmail.com

Telephone number: +44 7791184029

2

Work Address: The Department of Biomedical Engineering, University of Strathclyde,

Wolfson Centre, 106 Rottenrow, Glasgow, G4 0NW, UK

Keywords:

Motion analysis, Anatomical landmark calibration, subject calibration, Protocol

reliability, instrumented-pointer.

Word Count (Introduction through Acknowledgements):

1,992

1

3

IS THE INSTRUMENTED-POINTER METHOD OF CALIBRATING ANATOMICAL 2

LANDMARKS IN 3D MOTION ANALYSIS RELIABLE? 3

ABSTRACT 4

Instrumented-pointers are often used to calibrate anatomical landmarks in biomechanical analyses. 5

However, little is known about the effect of altering the orientation of the pointer during calibration 6

on the co-ordinates recorded. Incorrect positioning of a landmark influences the axes created, and 7

thus the kinematic data recorded. This study aimed to investigate the reliability of the pointer 8

method for anatomical calibration. Two points were drawn onto a fixed box to resemble knee joint 9

epicondyles, then a custom-made pointer was used to define the positions of these landmarks in 10

three-dimensions. Twenty different pointer-orientations were chosen, and the position of the 11

pointer in each of these orientations was recorded 8 times. Euclidean distances between single 12

points were calculated for both landmarks and compared statistically (g = 0.05). Average 13

Euclidean distances between all reconstructed points were 3.2±1.4mm (range: 0.3-7.1mm) for one 14

landmark and 3.3±1.5mm (range: 0.3-7.9mm) for the other. The x- and y-co-ordinates recorded 15

differed statistically when the pointer was moved about the X and Y axes (anterior/posterior and 16

superior/inferior to landmark) (p < 0.05). No statistical differences were found between co-17

ordinates recorded when the pointer was moved around the Z axes (p > 0.05). ICC values for all 18

co-ordinates were excellent, highlighting the reliability of the method (ICC > 0.90). These results 19

support this method of anatomical calibration; however, we recommend that pointers be 20

consistently held in a neutral oriented position (where the handle is not anterior, posterior, superior 21

or inferior to the landmark) during calibration, to reduce the likelihood of calibration errors. 22

23

4

1. INTRODUCTION 24

The use of skin-surface reflective markers to represent bony anatomical landmarks has been 25

described as inaccurate, unreliable, and time consuming (Alexander & Andriacchi, 2001; Baker., 26

2006; Benedetti et al., 1998; Benoit et al., 2006; Sholukha et al., 2013). 27

One notable source of error is ‘soft tissue artefact’ (STA) (Baker, 2006; Leardini et al., 2005; 28

Peters et al., 2010). STA is caused by the movement of a marker in relation to its underlying bony 29

position (Cappozzo et al., 1996; Leardini et al., 2005). As the markers are often attached directly 30

to skin, movement of the limb naturally causes the soft tissue (especially skin and fat) surrounding 31

the bone to move (Baker, 2006; Cappozzo et al., 1996). Consequently, the marker attached to the 32

skin may move to a position where it no longer truly represent the position of the bony anatomical 33

landmark. This error can be amplified if the marker is placed on clothing; especially if the clothing 34

is loose-fitting (Baker., 2006; Benedetti et al., 1998). 35

Placement errors translate to errors in kinematic and kinetic data as they affect the anatomical axes 36

calculated from marker positions (Alexander & Andriacchi, 2001; Benoit et al., 2006; Della Croce 37

et al., 2005). 38

An alternative method of calibration uses a pointer attached to a cluster of asymmetrical markers; 39

an instrumented-pointer (Benedetti et al., 1998; Cappozzo et al., 1995). This method involves 40

creating a local co-ordinate system from the markers on the pointer. This technique, known as 41

C.A.S.T (calibrated anatomical systems technique), was introduced by Cappozzo and colleagues 42

in 1995 (Cappozzo et al., 1995). The C.A.S.T method has been successful in orthopaedic surgery 43

to calculate the mechanical axis of the femur (Belvedere et al., 2011; Smith et al., 2014). It is also 44

5

commonly used in biomechanical research (Besier et al., 2003; Cappozzo et al., 1995; Fantozzi et 45

al., 2003; Hagemeister et al., 2005; Lin et al., 2015, Remelius et al., 2014). 46

Implementing a C.A.S.T is believed to have advantages over individual reflective markers stuck 47

onto skin or tight clothing, such as reduced soft tissue artefact (depending on the type of cluster 48

used and activity carried out) (Besier et al., 2003). Preparation of an individual is also quicker and 49

simpler (Benedetti et al., 1998). 50

Despite these benefits, it is currently unknown whether the way in which the pointer is held against 51

a landmark (its orientation) during calibration affects the co-ordinates recorded. Thus, this 52

investigation aimed to determine whether changing the orientation of the pointer significantly 53

influences the 3D-position of two virtual landmarks used to create an axis. This investigation could 54

therefore be used to identify pointer orientations which should be avoided during anatomical 55

landmark calibration. 56

57

2. Methods 58

2.1. Pointer Development 59

A pointer with 4 fixed retro-reflective markers was created then labelled as a cluster in Vicon 60

Tracker software (ver.2.2, Vicon Motion Systems, Oxford). A local co-ordinate system was 61

created within the pointer using this software. A temporary marker (without its base of support) 62

was used to determine the position of the pointer tip relative to the fixed markers on the pointer. 63

This information was used to calculate the position of a virtual point (representing the tip of the 64

pointer) into the local co - ordinate system of the wand. Marker width was taken into consideration 65

in these calculations. 66

6

A custom-written function in D-Flow saved the three-dimensional positions of the pointer-tip 67

(Motekforce Link, Amsterdam). 68

2.2. Recording Pointer Co-Ordinates 69

To replicate the positions of anatomical landmarks (e.g. lateral and medial knee epicondyles), two 70

red dots were drawn onto two sides of a sturdy box. The dots were placed half-way across the 71

width of the box, and a couple of centimetres below the top of the box. 72

The box was placed onto a stool in the field of view of 8 Vicon Bonita B10 cameras (Vicon Motion 73

Systems, Oxford). Elasticated straps attached the box to the stool to prevent movement. 74

Ten different types of pointer orientations were investigated. Each was analysed with the pointer 75

parallel to the ground as well as perpendicular to the ground. Thus, twenty combinations were 76

recorded for each landmark (Fig. 1). Fig. 2 shows examples of the orientations analysed. 77

x-, y- and z-co-ordinates of a landmark were recorded 8 times per orientation, completely 78

removing the pointer from the box between recordings. Three-dimensional graphs of the mean 79

vectors produced between the two points per orientation type were generated with Matlab® (ver. 80

R2014a: Mathworks Natick, MA). The x-axis was anteroposterior, the y-axis was vertical and 81

the z-axis was mediolateral. 82

To confirm that the box did not move as the pointer was used against it, a marker was glued onto 83

the box and the co-ordinates of the marker were recorded as the pointer was used twenty times 84

(once for each orientation). 85

The cameras were calibrated as recommended by the manufacturers. The image error of each 86

camera was <0.3mm (average camera error = 0.257mm). 87

7

2.3. Analysis of Data 88

Statistical analyses were carried out in Minitab software (ver. 16: Minitab Inc., State College, PA, 89

USA). Intra-class correlation coefficients (ICCs) were determined per Shrout & Fleiss’ schema 90

(1979). The level of significance was set at g = 0.05. 91

Euclidean distances between the recorded point and the mean of all recorded points for that 92

landmark were calculated for both landmarks. 93

94

3. Results 95

Average Euclidean distance between reconstructed points were 3.2±1.4mm (range: 0.3-7.1mm) 96

for the left-hand side of the box and 3.3±1.5mm (range: 0.3-7.9mm) for the right. 97

Greatest mean differences were between the points reconstructed when the pointer was positioned 98

a) posteriorly with the short arm pointing posteriorly, and b) anteriorly with the short arm pointing 99

anteriorly (7.1mm & 7.9mm for left and right landmarks). x- and y-co-ordinates recorded when 100

the pointer was anterior to the landmark were significantly different to those recorded when it was 101

posterior to the landmark (p <0.0001 & p = 0.002, respectively). Co-ordinates recorded along the 102

medio-lateral axis did not differ between these orientations (p = 0.147). The average Euclidean 103

distance between points recorded with the pointer anterior to the landmark and posterior to it was 104

3.4mm. 105

x- and y-co-ordinates created when the pointer was superior to and inferior to the landmark differed 106

statistically to one another (p = 0.032 & p<0.0001, respectively). Again, the z-co-ordinates were 107

8

found to be similar (p = 0.083). The average Euclidean distance between points recorded with the 108

pointer superior to the landmark and inferior to it was 2.3mm. 109

The smallest differences in Euclidean distances between points were observed when the pointer 110

was rotated about the medio-lateral axis (0.3mm for both landmarks). No statistical differences 111

were found: p = 0.055 for x-co-ordinates, p = 0.070 for y-co-ordinates and p = 0.944 for the z-co-112

ordinates. 113

ICC values of all co-ordinates recorded at both landmarks were excellent (all 0.99). 114

A 3D graph of the mean landmark positions recorded during each orientation was plotted to 115

visualise the effect these mean values would have on the creation of an axis (Fig. 3). The magnitude 116

and directions of these vectors changed as the orientation of the pointer changed (Fig. 3). Mean 117

magnitude was greatest when the pointer was superior to the landmarks with the short arm pointing 118

inferiorly (228.8mm). The smallest mean magnitude (214.6mm) was observed when the opposite 119

orientation was assumed (giving a difference of 14.2mm), highlighting the effect of changing the 120

orientation of the pointer during calibration. On average, moving the pointer from a superior to 121

inferior orientation affected the magnitude of the vector by 1.7mm. When anterior and posterior 122

orientations were adopted, the mean difference in magnitude was 0.4mm. 123

To determine the repeatability of a single point in a given orientation, each x-, y- and z-co-ordinate 124

recorded per orientation were statistically compared. ICC values were 1.0000 for all twenty 125

orientations. 126

4. Discussion 127

Locating an anatomical landmark incorrectly during the calibration stage of a gait assessment can 128

directly affect the kinematics calculated (Baker, 2006; Osis et al., 2016; Schwartz et al., 2004). 129

9

Our results showed that the mean co-ordinates recorded per orientation could lead to the 130

production of different axes, suggesting that the vector produced changed when the orientation of 131

the pointer was not maintained. This in turn could directly affect kinematics. 132

Osis et al. (2016) found that changing the position of a retro-reflective marker by 10mm resulted 133

in a 7.59° change in knee and ankle internal-external rotation angles and a 5.17° change in knee 134

abduction-adduction rotation angles when running. 135

The greatest Euclidean distances between reconstructed landmarks in our investigation were 136

7.1mm and 7.9mm; considerably smaller than those reported by Della Croce et al. (1999). 137

According to their study, differences of up to 25.0mm were recorded at some anatomical 138

landmarks (smallest difference of 4.8mm), where differences were calculated as the root mean 139

squared distance from the mean position. This difference is likely to be since the landmark was 140

pre-defined in this study, and no palpation was required. 141

Although our differences were smaller, an error of approximately 8mm (our maximum) could 142

increase the kinematic error by around 5° (Osis et al., 2016). McGinley et al. (2009) stated that 143

clinically acceptable errors were those <5°. This is a cumulative error, consequently minimising 144

the likelihood of pointer related errors arising is paramount for an accurate calibration. 145

When the pointer was rotated about the anterior-posterior and vertical axes, the results recorded 146

were statistically different for x- and y-co-ordinates. Difference between recorded z- co-ordinates 147

may not have reached statistical significance due to the rigid property of the box. Thus, changing 148

the position of the pointer along these axes should be avoided during calibration, as the error may 149

be even greater when used on skin. 150

10

We are confident that the differences highlighted in our results were not due to movement of the 151

box as the pointer was used against it, as y- and z-co-ordinates of a marker glued onto the box 152

remained the same to 3 decimal places as the pointer was used. On occasion, the x-co-ordinate of 153

the landmark became reduced by 0.001mm; otherwise the position was consistent. 154

The pointer should therefore be held in a neutral position with relation to the landmark when 155

calibrating (i.e. not above, below, posterior or anterior to the landmark). Rotating the pointer about 156

the medio-lateral axis did not have a significant effect on the co-ordinates recorded. Consequently, 157

the pointer could be held in any orientation in this plane when calibrating. 158

The co-ordinates recorded were highly repeatable and reliable when a particular orientation was 159

used (ICCs = 1.000). This highlights the importance of a consistent calibration technique, 160

suggesting that using a combination of orientations, even about the medio-lateral axis, could be 161

detrimental to the calibration process. 162

A limitation to this study is that there was no baseline co-ordinate against which the recorded co-163

ordinates could be compared, but this replicates the clinical situation where the true value is 164

unknown. Furthermore, only one pointer was used in this study. 165

5. Conclusion 166

Despite the increase in use of instrumented-pointers in biomechanical research and orthopaedics 167

to calibrate the 3D position of bony anatomical landmarks, no study to date had investigated the 168

effect of pointer-orientation on the co-ordinates recorded. 169

Our results showed that the co-ordinates recorded by the pointer differed to a level which could 170

influence kinematic reconstruction. The greatest Euclidean distance between reconstructed 171

landmarks in our investigation was 7.9mm which could have led to a kinematic error of 172

11

approximately 5°. Errors above 5° are clinically unacceptable. We therefore recommend that the 173

pointer should be consistently held in a neutral position to the landmark (i.e. not inferior, superior, 174

anterior or posterior to the landmark) during anatomical calibration to reduce the chances of 175

introducing error through improper pointer orientation. 176

Overall, we are confident that the pointer-calibration method can be reliably used to record the 177

position of an anatomical landmark in three dimensions. However, accurate location of the 178

anatomical landmark by palpation is still necessary, regardless of whether a pointer or static marker 179

is used to record its location on the body. 180

Acknowledgements 181

This study was supported by the University of Strathclyde and Medacta International SA. Neither 182

had direct involvement with this study or manuscript. 183

Conflict of Interest Statement 184

We have no conflict of interest to declare. 185

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274

17

275

276

Parallel to

Ground

Perpendicular

to Ground

Pointer

Pointer-end in

line with

landmark

Pointer-end

superior to

landmark

Pointer-end

inferior to

landmark

Pointer-end

anterior to

landmark

Pointer-end

posterior to

landmark

Short arm of

pointer facing

anteriorly

Short arm of

pointer facing

posteriorly

Figure 1: Twenty combinations of pointer orientations used to investigate the effect of orientation on

the landmark co-ordinates recorded.

18

277

D E

C B A

Figure 2: Examples of 5 pointer orientations investigated in this study. In all cases the pointer

is parallel to the ground. The pointer-end is A: in line with the landmark, B: superior to the

landmark, C: Inferior to the landmark, D: Posterior to the landmark, E: Anterior to the

landmark.

19

278

Figure 3: The mean landmark positions recorded per orientation were plotted as

vectors to show the way in which pointer orientation would affect the creation of an

axis. Pa = Parallel, Pe = Perpendicular, L = in-line with landmark, S = superior to

landmark, I = inferior to landmark, A = Anterior to landmark, P = posterior to

landmark, Ant = short arm of pointer orientated anteriorly, Pos = short arm of pointer

orientated posteriorly.

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.080.09 0.10 0.11 0.12 0.13 0.14 0.15 0.16 0.17 0.18 0.19 0.20

-0.14

-0.13

-0.12

-0.11

0.80

Medio-Lateral (m)

Vectors Generated from Mean Landmark Positions

Anterior-Posterior (m)

Ve

rtic

al (m

)

Pa LA

Pa LP

Pa SA

Pa SP

Pa IA

Pa IP

Pa AA

Pa AP

Pa PA

Pa PP

Pe LA

Pe LP

Pe SA

Pe SP

Pe IA

Pe IP

Pe AA

Pe AP

Pe PA

Pe PP

20

279