Eur Spine J (2006) 15: 742–751
DOI 10.1007/s00586-005-0960-0
Falk Mörl
Reinhard Blickhan
Received: 23 December 2004
Revised: 16 March 2005
Accepted: 22 April 2005
Published online: 27 July 2005
Springer-Verlag 2005
F. Mörl (&)
FSA mbH, ZfB, Dubliner Str. 12,
99091 Erfurt, Thüringen, Germany
E-mail: fmoerl@yahoo.de
Tel.: +49-361-26244166
Fax: +49-361-2624428
R. Blickhan
Institute of sport science, motion science,
Friedrich-Schiller-University Jena,
Jena, Thüringen, Germany
ORIGINAL ARTICLE
Three-dimensional relation of skin markers
to lumbar vertebrae of healthy subjects in
different postures measured by open MRI
Abstract The debate is to which extent external skin markers represent
true underlying vertebral position
and motion. Skin markers and lumbar vertebrae L3 and L4 were
examined by vertically open magnetic resonance imaging (MRI)
within different postures to investigate whether, and to which extent
the position and orientation of skin
markers represent the corresponding
information of assigned underlying
vertebra. Nine healthy volunteers sat
within an open MRI scanner in five
different seating postures: upright,
low flexion, heavy flexion, upright
left turn and upright right turn. Skin
markers were fixed at lumbar levels
L3 and L4. A set of landmarks defines corresponding positions on the
vertebrae. Translation-vectors
quantify the change of co-ordinates
while changing position. Orientation
(Cardan-angles) of each level in
space was calculated from co-ordinates of three skin-markers and the
corresponding vertebral landmarks
respectively. The close relation between the position of the individual
skin marker and its corresponding
Introduction
The position of vertebrae within a global, laboratoryfixed system of co-ordinates is necessary to estimate
spinal loading. The orientation of consecutive vertebrae
determines the curvature of the spine, the deformation
of the intervertebral discs, load on ligaments, and the
landmark on the vertebrae is conserved through all postures (regression coefficients: 0.720<b<0.972,
0.916<r<0.993, p<0.0001). Despite the high sensitivity to measurement errors there is a strong
relation between the translationvectors of external markers and the
corresponding translation of the
landmark on the vertebrae
(0.68<r<0.99, p<0.0001). Within
the resolution of the MR-imaging
the orientation of the vertebrae can
be predicted from the external markers (regression coefficients:
0.936<b<1.189, 0.769<r<0.959,
p<0.0001). Positions and motions
of skin markers can be taken as a
first estimate to calculate the position and spatial orientation of underlying vertebrae. The use of skin
markers may help to investigate
spinal positions in daily life situations where internal imaging is not
possible.
Keywords Open MRI Æ Lumbar
spine Æ Skin markers Æ Soft tissue Æ
Validation
behavior of the facet joints. Especially in relevant
daily life conditions such as lifting a weight this information can only be extrapolated so far from external
observations such as monitoring the position of external
skin markers. However, motions of soft tissue covering
the spine may lead to large errors. So far, only a few
studies investigated the relation between position and
743
motion of skin markers and real bone-motion. Nonexistent validation is claimed to be the main problem of
skin-marker-methods [9]. Concerning the human lumbar
spine there are different opinions about the relation of
skin to vertebrae motion (or position): in static radiological investigations with radiopaque markers mounted
over spinal prominences, it was shown that the skin
marker positions represent the spinous centroids and
curvature quite well [2]. Further, three-dimensional displacement (shift) of skin markers related to positions on
the vertebrae from standing to sitting posture measured
by bi-planar radiography is small, especially markers
mounted over spinous processes represent vertebral
position very well [8]. Comparing functional radiographs of lumbar spine with data of skin markers measured from lumbar spinous processes shows high
correlations for estimation of lordosis and mobility [4].
In contrast, previous investigations on low-back-painpatients in similar situations revealed only a weak correlation between the surface curvature obtained from a
single row of skin markers and the rotation of the vertebrae monitored radiographically (r=0.58) [19]. Notice
that most cited investigations are two-dimensional and
look only on single postures; three-dimensional data are
rare [8]. Information about the relation of spatial orientation of vertebrae to spatial orientation of a set of
skin markers is not available.
Radiological or magnetic resonance imaging (MRI)
methods are precise, but have the disadvantage that
daily-life activities cannot be measured as the mobility
of the patient must be restricted. At present, due to the
limited resolution in time and narrow working space in
three dimensions, only measurements of selected postures are possible [3, 6, 10, 11, 13–15, 20]. Motion
studies of the lumbar spine during flexion were examined two-dimensionally for sagittal plane using fluoroscopy at five images per second [5, 12]. Because of
the dose of radiation especially on the gonads, comparable investigations on healthy subjects are ethically
critical.
Another way to obtain in vivo data of segmental
spinous motion are measurements with bone-pins [1, 7,
18]. These investigations are highly invasive—local
anesthesia, and the bone-pins sticking through skin and
muscles may influence the physiological movement.
Skin markers might be used to monitor spine movements while seated or while changing posture. In this
study, we investigate the validity to infer positions and
movements of vertebrae from a set of surface markers in
five different seated postures. This was possible by
applying vertically open magnetic resonance imaging
(MRI) with its less restrictive environment in comparison to conventional MRI. Regressions of positions,
translations of co-ordinates and Cardan-angles as well
as the relative shift between markers and vertebrae have
been deduced.
Materials and methods
Nine healthy male subjects [age: 25.3 years (SD: 2);
weight: 77.1 kg (SD: 4.4); height: 180.8 cm (SD: 5.9)] sat
in five different postures within the open MRI scanner.
The constitution of the subjects can be characterized as
sportsman-like, with a low percentage of fat under their
skin. Subjects with a history of low back pain and who
did not fulfill the requirements for MRI were excluded.
All subjects were informed (written consent) following
the requirements of the local ethical commission and
were volunteers.
Six special MRI-markers, which could be identified
within the scanned volume, were attached on the backs
of the subjects. The markers were ring-shaped with an
outer diameter of 10 mm, an inner diameter of 3 mm,
and a height of 3 mm. The markers were applied to
mark lumbar vertebrae L3 and L4. The spinous processes of L3 and L4 were palpated in a flexed posture.
Markers were affixed: (i) to the spinous processes and (ii)
to the prominent elevation of the muscle bellies of m.
erector spinae about 3 cm left and right and approximately 3 cm cranial related to the marker on the spinous
process (Fig. 1).
The seat within the coils of the scanner was placed
like described in a previous investigation so the volunteers sat with their lumbar spines within the measurement volume [20]. The subjects had to assume five
different postures and were instructed not to move
during the scan. Images with movement artifacts were
rejected. The five postures were:
– Normal seating: hip- and knee-angle approximately
90, the trunk erect and leaning on the seats back in
the thoracic area (from about T6 to T10);
– Flexed seating: knee-angle also approximately 90,
elbows resting on the thighs with the upper arms
oriented vertically;
– Heavy flexed seating: similar to the previous position,
except the trunk now bend down to the thighs, hands
propped on the feet, so the trunk was in highest
flexion as possible;
– Rotated seating to the left: with the shoulder turned
approximate 90 with respect to the pelvis, hip- and
knee-angle approximately 90, left arm behind the
seat’s back;
– Rotated seating to the right: same as above, except
now the right arm was placed behind the seat’s back
so the trunk was in torsion to the right.
The open magnetic resonance imaging scanner Signa
SP (General Electric Medical Systems, Milwaukee, WI)
was used to measure the subjects’ back. The scanner
consists of two vertical superconductive magnets each
with a magnetic field force of 2 T, so the Signa SP
scanner provides a field strength of 0.5 T between the
744
Fig. 2 Schematically illustration of a lumbar spinous process (a)
and a lumbar transverse process (b) with the five digitized points
Fig. 1 Sagittal slice while flexed seating. On lumbar levels L3 and
L4 the skin markers above spinous processes are to see
magnets where the subjects sat. The width between the
coils is 58 cm.
Measurements were possible within a cubic volume of
approximately 50 cm side length. Concerning the
respective posture and using the sagittal 3-D scanning
sequence Brain 3D (FGRE 3D, T1 weighted, actually
used for brain research) the field of view was set to a
cuboid volume of 60 slices with 256·256 pixels (see
Table 1 for voxels and cuboid).
Measuring a single posture of one subject took
3:30 min with this setup. Including pre-adjustment,
subjects had to maintain a given posture for 5 min.
Using the software MRIcro V1.35 (http://www.mricro.com), the 60 slices per measurement were reconstructed to 3-D-volumes. After this, markers, spinous
and transverse processes were manually digitized using
the Matlab-Toolbox SPM99 (Statistical Parametric
Mapping, http://www.fil.ion.ucl.ac.uk). The laboratoryfixed reference point was in the center of the scanner and
given by the hardware. Interpolation and averaging can
increase the accuracy for visual digitization up to a
factor of 10. The skin markers were digitized with one
point in the center of the marker. To digitize spinous
and transverse processes five landmarks on each process
were used (Fig. 2): spinous processes were viewed in
sagittal plane. One point was set on the tip of the process, two points on the region with highest curvature of
the processes, and two points on the location where the
curvature ended. The transverse processes were viewed
in transverse plane. One point was the tip of the transverse process, one point was set halfway between the
vertebral body and transverse process, and another one
halfway between the transverse process and the process
of the facet joint. In order to obtain the position of one
process, the mean co-ordinate of these five points was
estimated. Repeated digitization of the same image by
three instructed investigators resulted in an error of
0.9% corresponding to 0.31 voxel (from 0.09 to 0.78
voxel) for the mean co-ordinate.
The co-ordinates were transformed from voxels to
metric units (mm) and to a standard, global Cartesian
system of co-ordinates (x-axis: right–left, y-axis: anterior–posterior, z-axis: caudal–cranial in relation to the
subject). Further, three-dimensional translation-vectors
from one posture to the other were calculated for each
process and marker.
Table 1 Side lengths (mm) of voxel and resulting cuboid of the considered MRI-volume
Subject
S1, S8, S9
S2, S3, S4
S5, S6, S7
dir: right-left
dir: anterior-posterior
dir: caudal-cranial
voxel
volume
voxel
volume
voxel
volume
2.0000
2.5000
0.9375
120
150
240
1.1719
1.1719
0.9375
300
300
240
1.1719
1.1719
3.0000
300
300
180
The directions (dir) are anatomically orientated with respect to the subjects (S1,...,S9)
745
Per lumbar level the three mean co-ordinates of spinous and transverse processes and the corresponding set
of three skin markers were used to calculate body-fixed
systems of co-ordinates for vertebrae and skin surface
respectively. The rotation of the body-fixed systems with
respect to the lab-fixed co-ordinate system is given by
Cardan-angles. They represent a sequence of rotations in
our case starting with a rotation about the sagittal axis
(flexion), the new frontal axis (lateral bending), and finally about the resulting longitudinal axis (axial rotation). The angles are calculated via co-ordinate
transformation [22]. The errors in the Cardan-angles
were estimated numerically by assuming an error of the
co-ordinates of each marker of one-tenth of the side
length of a voxel (compare position error and Table 1)
and calculating Cardan-angles using the transformation
matrix. It is assumed that the absolute errors induced
from each of the nine co-ordinates are superimposed.
This procedure results in a measurement error of 3.22
for a (flexion), 3.07 for b (lateral bending) and 2.27 for
c (torsion).
While changing the posture the skin and its markers
shift with respect to the corresponding vertebrae, i.e. the
vector pointing from the respective process to the marker given in the vertebrae-fixed co-ordinate system
should change its components. This change is given as a
measure of skin shift. Shifts for the four different postures are represented and statistically tested against zero.
Also shifts during the different postures were compared.
To quantify the relationship between external skin
markers and the landmarks on the vertebrae regressions
were calculated for the positions, the translation-vectors
and the Cardan-angles. The quality of the relation was
judged by the correlation coefficient and the coefficient
of determination. All calculations and statistics were
made with Matlab under using library Spacelib (Matlab
StatisticsToolbox, The MathWorks, Inc.).
Results
Positions, translation-vectors and spatial orientation of
skin markers show a strong linear relation to positions,
translation-vectors and spatial orientation of landmarks
on the vertebrae. For our sample using simple linear
regression equations
Y ¼ bX þ a
the internal positions, translation-vectors or spatial
orientation Y are calculable using the external measured
or calculated data X and the estimated regression coefficient b and intercept a. Positions and motions of
lumbar vertebrae are detectable from markers on skin
surface (Table 2).
Coherence of co-ordinates
Co-ordinates of skin markers and corresponding
landmarks on the vertebrae during the five investigated postures show a linear relationship (regression
coefficients and correlations x: b=0.9573, r=0.993; y:
b=0.9720, r=0.988; z: b=0.7202, r=0.916; p<0.0001
for all regressions; Fig. 3, Table 2). A different position of the vertebral landmark results in a proportional difference in the position of the skin marker, the
regression coefficients b are near 1, this means that coordinates of markers on the surface are analogical to
those of the assigned landmarks on the vertebrae.
Deviations of the regression coefficients b from 1
especially noticeable in the z-component points to
systematic deviation from a pure parallel displacement
due to rotation of the segments. For slopes close to 1
the intercept a (Table 2) represent the components of
the distances between the external markers and vertebral landmark as extrapolated from all measurements. For example, on average the spinous process
marker was 13.9±2.4 mm behind the mean landmark
of the spinous process. Especially markers on spinous
processes represent the position of the landmark on
the spinous process very well (r>0.96). The coefficient
of determination (r2) for all co-ordinates is above 0.83,
for spinous processes above 0.92. So 83% of the coordinates were manifested with the regression coefficient (more than 92% for markers on the spinous
processes).
Coherence of translation
The change in position inflicts similar translations in
marker and vertebra position. Again, the components
of the vector quantifying this translation are significantly correlated for markers and corresponding processes (r>0.68, p<0.0001; Fig. 4, Table 2). The
highest correlations and smallest confidence intervals
are found for the vectors from spinous processes and
corresponding markers (r>0.91, CIdata<±27.13 mm).
The vectors for the transverse processes and their
markers show in their x- and y-components correlation coefficients above 0.87, those of the z-component
are lower. In general the translation of a vertebra
results in a proportional translation of the assigned
skin-marker and the regression coefficients b (slopes)
are close to 1. As in the co-ordinates the systematic
deviations are largely due to rotation of the trunk
segment while changing posture an effect which is
more pronounced for longer distances between skin
markers and spinous process. In this case the intercept
(a) to a large extent expresses the amount of average
rotation.
746
Table 2 Linear regression-statistics for calculated parameters for skin markers and assigned landmarks on the vertebrae
b±(CIb)b)
a±(CIa)a)
r
r2
Co-ordinates (mm)
x
y
z
xSP
ySP
zSP
0.9573±0.0135
0.9720±0.0176
0.7202±0.0371
0.9915±0.0184
0.9856±0.0187
0.8342±0.0693
)3.7390±1.3703
)33.1170±2.2779
22.6528±5.5578
)1.5848±1.7573
)13.8877±2.3916
12.1568±9.2119
0.9929
0.9883
0.9158
0.9981
0.9980
0.9639
0.9858
0.9768
0.8388
0.9962
0.9961
0.9291
Translations (mm)
xTP3
yTP3
zTP3
xSP3
ySP3
zSP3
xTP4
yTP4
zTP4
xSP4
ySP4
zSP4
1.1346±0.1418
1.3199±0.0834
0.8217±0.2019
1.0349±0.1106
1.1177±0.0614
0.8962±0.1434
1.0288±0.1271
1.1726±0.1083
0.9101±0.2291
0.9902±0.1354
1.0388±0.0956
0.9724±0.1354
)0.5397±2.0250
)9.9917±3.3888
17.9868±5.4632
)0.9456±1.6458
)5.5495±2.3410
7.2980±5.5621
)0.8118±1.6608
)8.8367±2.9216
20.2703±6.2188
)0.5110±1.5417
)4.3108±2.4986
5.9943±5.4016
0.8829
0.9656
0.6912
0.9548
0.9874
0.9063
0.8851
0.9306
0.6824
0.9483
0.9659
0.9266
0.7795
0.9325
0.4777
0.9116
0.9750
0.8214
0.7835
0.8661
0.4656
0.8993
0.9330
0.8587
Cardan-angles ()
a3
c3
a4
c4
0.9509±0.0861
1.1886±0.1769
0.9364±0.1206
1.0447±0.2672
63.1626±2.2677
)0.1019±0.8187
68.7606±2.9133
0.0931±1.1428
0.9593
0.9001
0.9224
0.7688
0.9202
0.8102
0.8509
0.5911
The table shows regression coefficients with 95% confidence
intervals (b±(CIb)b)), intercept with 95% confidence intervals
(a±(CIa)a)), Pearson’s correlation-coefficients (r) and coefficients
of determination (r2) for co-ordinates (x, y, z), co-ordinates of
spinous processes (xSP, ySP, zSP), translations on transverse processes and their markers (x,...,zTP3 and x,...,zTP4) and translations
on spinous processes and their markers (x,...,zSP3 and x,...,zSP4) and
components of Cardan-angles for flexion (a3/4) and torsion (c3/4)
on lumbar levels L3 and L4 on the surface and the vertebrae. All
parameters show high correlations (r>0.68), especially co-ordinates of markers on spinous processes are strongly related to the
landmarks on the spinous processes (r>0.96). Regression coefficients b for all parameters are near 1 (all statistics are highly significant, p<0.001)
Coherence of spatial orientation
Fig. 3 Relation of co-ordinates of lumbar skin markers and coordinates on the vertebral processes during the five observed
postures with regression lines (solid) and confidence bounds (95%,
dashed). All co-ordinates show strong linear relations (see Table 2)
The described marker-configuration was designed to
calculate three-dimensionally spatial orientation of a
group of skin markers and assigned vertebrae using
Cardan-angles (a: flexion, b: lateral bending, c: axial
rotation). The Cardan-angles calculated with a set of
three skin-markers and those calculated with the three
landmarks of the corresponding vertebrae are significantly correlated (p<0.0001; Fig. 5, Table 2). Taking all
data into account the correlations are: level L3 - component a (flexion/extension) r=0.959, component c
(torsion) r=0.8533; level L4 - component a r=0.856,
and c r=0.564. If outliers resulting from digitization
errors are removed correlations improve and are highly
significant also at level L4 (Fig. 2c, d): L3 a:b=0.9509,
r=0.959; L3 c: b=1.1886, r=0.9; L4 a:b=0.9364,
r=0.922; L4 c:b=1.0447, r=0.769; p<0.0001). From
this it can be deduced, that the dorso-ventral orientation
of the vertebrae is proportional to that predicted from
the skin markers. More than 85% of the data for flexion
are predicted by the regression-analysis, for the torsion
747
Fig. 4 Translations of landmarks on the vertebrae in
dependence of translations of
their associated skin marker
(L3: A, B; L4: C, D) and the
respective processes (transverse
(TP): A, C; spinous (SP): B, D).
A strong linear relation is
observed for all components
and all processes and their
assigned markers (see Table 2
for regression parameters)
Fig. 5 Cardan-angles of lumbar
vertebrae in dependence of
those obtained from skin
markers with regression line,
confidence bounds for regression coefficients and confidence
bounds for obtained data (levels—L3: A, B; L4: C, D; components—a dorsi flexion/
extension: A, C; c axial rotation: B, D). The strong linear
relationship results in regression
lines with small confidence
bounds. Because of the spatial
resolution of the used open
MRI, the resulting error in coordinates and the resulting
error of Cardan-angles depending on nine co-ordinates produce wide confidence bounds of
obtained data (see Table 2)
748
more than 59% of the data are solid. Again the regression coefficients are nearly 1 so spatial orientation for
our sample can reliable measured with markers on the
surface.
The measurements cover only a small range of lateral
flexion angles. The minute lateral flexions observed
represent artifacts probably induced by skin shift. For
Cardan-angle-component b (lateral flexion) these high
correlations could not be found, partially negative correlations are calculated (L4: b=)0.435). As the experimental setup prevented postures with clear lateral
flexion most of the lateral rotations are below 7 and
thus 56% on lumbar level L3 and 60% on lumbar level
L4 are within the expected measurement error (see
Materials and methods).
Similarly, relative angles between L3 and L4 are below resolution of the applied MRI-technique and calculation of Cardan-angles, and are not documented.
Shift of soft tissue in relation to lumbar vertebrae
For our sample, the shift of soft tissue in relation to
the corresponding landmark on the vertebrae while
changing posture can be neglected. During the four
investigated postures shift of soft tissue is not significantly different from zero for all components of the shift
vector in relation to normal seating (0.06<P<0.99,
median skin shift: 0.86 mm, Fig. 6). Particularly markers
on spinous processes show lower shifts than markers on
Fig. 6 Marker shift during the
four investigated postures: flexion (A), maximum flexion (B),
left torsion (C) and right torsion (D) in relation to normal
seating over left (TPL3/4) and
right (TPR3/4) transverse processes and spinous processes
(SP3/4) on lumbar levels L3 and
L4 for components x, y and z of
the shift-vector. Boxplots show
median, lower and upper quartiles, rest of the distribution and
outliers (+). All data is not
significantly different from zero
muscle bellies, but these differences are also not significant (median skin shift over spinous processes:
0.36 mm). This is due to the fact that the direction the
skin shifts is unpredictable and individual. The maximum
skin shift to be found within the sample was )36.87 mm
for the z-component during torsion to the left about left
transverse process L4. This huge value is clearly detected
as an outlier. Outliers arose from marker positions on the
verge of scanned volume wherein more bias is to expect.
Without this outlier the maximum skin shift during this
posture about left transverse process L4 for z-component
is )9.86 mm. Corresponding to the smaller angles of
rotation the shifts of markers in the torsion experiments
are closer to zero (0.89<P<0.99). The median marker
shift during torsion is greater than the median marker
shift of all postures (median skin shift of torsion experiments: )1.76 mm, median skin shift during torsion
experiments over spinous processes: )2.02 mm). Differences to flexion and heavy flexion experiments are not
significant (Kruskal–Wallis nonparametric one-way
Analysis of Variance (ANOVA)).
Discussion
New data supporting linear relation of skin markers
to lumbar vertebrae
Three-dimensional co-ordinates, translation-vectors and
spatial orientation of markers on the surface of the
749
lumbar back are in close linear relationship to threedimensional co-ordinates, translation-vectors and spatial
orientation of the assigned lumbar vertebrae. In contrast
to static 2-D-studies investigating only a single posture,
[2] our study adds evidence that this interrelationship is
verifiable during different postures. Further the threedimensional relationship supports former investigations
and expands for five different postures [8]. The study
documents, within the resolution of modern imaging
methods, changes in three-dimensional orientation of
vertebrae and corresponding groups of surface markers
inflicted by changes of posture. For the dorso-ventral
flexion (a) the relation was close to 1, i.e. for the investigated segments the flexion of the external markers are
very close to that of the underlying vertebrae. A similar
conclusion is possible for torsion (c). Moreover the shift
of soft tissue above investigated landmarks on lumbar
vertebrae is quite small and seems to have no effect on
posture or motion measurements on the surface. These
results are comparable to previous investigations with
biplanar X-ray [8]. So, an important step to validate
skin-marker-methods in the region of lumbar spine has
been made [9], and for healthy subjects the conclusion
that positions and motions of skin markers are not
sufficiently correlated with positions and motions of the
underlying vertebrae must be rejected [19].
Conclusions for lumbar spine motion analysis
It is obvious that the possibility to reliably monitor
vertebral movements without going through invasive
surgery represents a distinct advantage in investigations
of daily activities [1, 7, 18]. For our sample, internal
vertebral motions can be derived from information of
carefully placed surface skin markers in free situations
such as picking up minor loads. It is possible to measure
three-dimensional shapes, postures and motions of
lumbar vertebrae using skin markers on lumbar back
surface with this special marker-application. Especially
markers on spinous processes represent vertebral positions and motions very well. Setups with skin markers
above spinous processes have been frequently used to
monitor spinous movements and positions [4, 9, 17, 19,
21, 23]. So, studies with similar application of skin
markers achieve good results.
Notes and limitations
There is to note that positions and motions on the
lumbar back surface are not identical to positions and
motions on the vertebrae. But knowing the linear relationship the position and motion of lumbar vertebrae
are predictable from surface measurements. In some
cases (regression coefficient near 1) the change of the
position of the markers is analogue to that of the vertebral processes (spinous processes and their markers).
For the individuals used in our sample for example the x
co-ordinate of the spinous processes can be predicted
based on our equations with an accuracy of ±8.7 mm.
Considering the small sample of nine subjects the data
base is to narrow to allow general predictions for arbitrary populations. Combined with a complex model
using external anthropomorphic data and locations of
centers of rotation even more reliable and general predictions may be possible [23].
Restrictions of the used setup
Actually at least four lumbar levels were scanned within
the described volume but due to change in posture only
lumbar levels L3 and L4 remained in the middle of the
volume. In the verge of the scan volume outside of this
restricted middle area large bias is to be expected
affecting the precision of our measurements. For the
whole lumbar spine the anatomical structures are comparable so similar results for other lumbar levels are to
be expected (see also [2, 4, 8]).
Due to static measurements—subjects had to rest for
up to 5 min to measure a single posture—only five different seating postures were tested. This poor resolution
in time restricts the number of postures feasible to
investigate and the relations of skin markers, and their
assigned landmark on the vertebrae during other postures can only be extrapolated. Especially during highly
dynamic motions like jumping or running the relation of
skin markers and their assigned vertebrae is unpredictable. While standing lumbar paraspinal muscle bellies
contract and expand in the transverse direction. Probably these changes are small over wide areas in change of
posture and do not affect the relation of skin markers to
their bony landmark. The influence of muscle contraction per se was not investigated as the subjects sat relaxed within the scanner to persevere the measuring
time. In our dataset influences of different swellings of
back-muscles within different postures are not detectable. A hint towards an influence of transverse muscleswelling during shortening is given by the results of the
torsion experiments wherein muscles shorten asymmetrically. The medium marker shift for the torsion experiments show larger (but not statistically different) values.
The investigated subjects were young, sportsman-like
males with no history of low back pain. The percentage
of fat under their skin was very low so only the behavior
of skin, muscles, connecting tissue, ligaments etc. as soft
tissue was measured. There is no information about
the influence of thick layers of fat so the shift of soft
tissue in fat subjects is also unpredictable. Further,
low-back-pain-patients have deviant paraspinal muscle-
750
performance inducing deviations in the relation between
skin markers and vertebrae.
Technical limitations
The most important limitation is the spatial resolution
of the used MRI-scanner (Signa SP, General Electrics). Despite enhancing the resolution via interpolation (see Materials and methods) the corresponding
error in co-ordinates propagates in all calculated
parameters (translation-vectors, especially Cardan-angles). For better spatial resolution of the available
instrument the subjects had to sit for more than 5 min
in a fixed posture. The subjects perceived these exposures in a motion free, static attitude as tiring.
Keeping these caveats in mind it is remarkable that
the special arrangement and resulting orientation of
the used set of external markers gives reasonable
prediction for the angles of orientation of the underlying vertebrae. At this time no other three-dimensional device with important better resolution in time
and space is available.
Only spatial orientation of single vertebrae on level
L3 and L4 and their assigned marker-set were calculated. Relative inter-segmental angles are smaller than
the measurement error using this possible spatial
resolution and the algorithms to calculate the Cardanangles. The propagation of the measurement error
due to the volume resolution of the MRI limits the
estimation of the Cardan-angles and the possibility to
compare external and internal relative rotations between
adjacent segments.
Finally, there is no space between the coils within the
scanning volume to allow lateral flexion. Thus, for this
component of rotation (b) no clear relation could be
derived and the calculated differences in spatial orientations are also smaller than the measuring error.
Conclusions
For the investigated group of subjects positions and
motions of lumbar spine skin markers are analogical to
positions and motions of the assigned underlying lumbar
vertebrae. So, skin markers can be used to estimate
lumbar vertebral position and spatial orientation while
seated. Similarly, lumbar motions during closely related
daily life activities might be measurable with the skin
marker method. The here presented coherence may not
be true for elderly, overweight and scoliotic patients
because of their behavior of soft tissue may diverge.
Acknowledgements This work was supported by Forschungsgesellschaft für angewandte Systemsicherheit und Arbeitsmedizin
mbH. Special thanks to the clinic and polyclinic for diagnostic
radiology, university of Leipzig for using the open MR scanner,
especially Dr. H. Busse and the technical employees. All investigated subjects were informed (written consent) following the
requirements of the local ethical commission.
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