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The University of Manchester Research
Is the instrumented-pointer method of calibratinganatomical landmarks in 3D motion analysis reliable?DOI:10.1016/j.jbiomech.2017.01.019
Document VersionAccepted author manuscript
Link to publication record in Manchester Research Explorer
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
Published in:Journal of biomechanics
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Download date:11. May. 2020
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:
Corresponding author:
Name: Gwenllian Fflur Tawy
Email addresses: [email protected] or [email protected]
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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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