SPINE Volume 33, Number 14, pp 1572–1578
©2008, Lippincott Williams & Wilkins
Standing Balance and Sagittal Plane Spinal Deformity
Analysis of Spinopelvic and Gravity Line Parameters
Virginie Lafage, PhD,* Frank Schwab, MD,* Wafa Skalli, PhD,† Nicola Hawkinson, NP,*
Pierre-Marie Gagey, MD,‡ Stephen Ondra, MD,§ and Jean-Pierre Farcy, MD*
Study Design. Prospective study of 131 patients and
volunteers recruited for an analysis of spinal alignment
and gravity line (GL) assessment by force plate analysis.
Objective. To determine relationships between GL, foot
position, and spinopelvic landmarks in subjects with varying
sagittal alignment. Additionally, the study sought to analyze
the role of the pelvis in the maintenance of GL position.
Summary of Background Data. Force plate technology
permits analysis of foot position and GL in relation to
radiographically obtained landmarks. Previous investigation noted fixed GL-heel relationship across a wide age
range despite changes in thoracic kyphosis. The pelvis as
balance regulator has not been studied in the setting of
sagittal spinal deformity.
Methods. The 131 subjects were grouped by sagittal
vertical axis (SVA) offset from the sacrum: sagittal forward (⬎2.5 cm), neutral (⫺2.5 cm ⱕ SVA ⱕ 2.5 cm), and
sagittal backward (SVA ⬍⫺2.5 cm). Simultaneous spinopelvic radiographs and GL measure were obtained.
Offsets between spinopelvic landmarks, heel position,
and GL were calculated. Group comparisons were made
for all offsets to determine significance.
Results. Aside from the offset T9-GL and GL-heels, all
other offsets between spinopelvic landmarks and GL revealed significant differences (P ⬍ 0.001) across the 3
subject groups. However, with increasing SVA, the GL
kept a rather fixed location relative to the feet. A correlation between posterior pelvic shift in relation to the heels
with increasing SVA in this study population was confirmed (r ⫽ 0.6, P ⬍ 0.001).
Conclusion. Increasing SVA in standing subjects leads
to a posterior pelvic shift in relation to the feet. However,
no significant difference in GL-heel offset is noted with
increasing SVA. It thus appears that pelvic shift (in relation to the feet) is an important component in maintaining
a rather fixed GL-Heels offset even in the setting of variable SVA and trunk inclination.
Key words: spinal alignment, force plate, gravity line,
sagittal balance. Spine 2008;33:1572–1578
The aging process and deformity of the spinal column
can lead to altered sagittal plane alignment. To date,
From the *NYU Hospital for Joint Diseases, New York, NY; †Laboratoire de BioMecanique, ENSAM/CNRS URM8005, Paris; ‡Association Française de Posturologie, l’Hay-les-roses, France; and §Northwester Medical Center, Chicago, IL.
Acknowledgment date: March 27, 2007. First revision date: January 8,
2008. Acceptance date: January 8, 2008.
The manuscript submitted does not contain information about medical
device(s)/drug(s). Corporate/Industry funds were received in support of
this work.
No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript.
Supported by Medtronic Sofamor Danek.
Address correspondence and reprint requests to Virginie Lafage, PhD,
Research Scientist, NYU Hospital for Joint Diseases, 380 2nd Ave.,
Suite 1001, New York, NY 10010; E-mail: [email protected]
1572
optimal spinal balance remains poorly defined. However, a number of radiographic parameters have been
developed as guides of sagittal alignment. Values of thoracic kyphosis by Cobb measurements, and lumbar lordosis are commonly used to assess regional alignment.
Global spinal alignment is commonly assessed by a
plumbline method1– 4 drawn onto full-length radiographs. More recently, pelvic parameters5–7 have also
been examined to improve evaluation of sagittal plane
balance. Although ranges of normal values7–11 and series
of patients with specific pathology such as spondylolisthesis12–14 have been published, the correlation between
these radiographic parameters and standing balance is
not yet fully understood.
Although radiographs permit an evaluation of vertebral alignment and pelvic landmarks, current standard
techniques using 36⬙ cassettes do not capture foot position. Analysis of spinal and pelvic offsets from standing
foot position can be captured through forceplate analysis: a forceplate device is made of pressure sensors distributed across a level surface and permit detailed analysis of foot position and pressures of a subject that stands
on such a platform. Given that a freestanding person
must be balanced over a small area between the feet to
avoid requiring support (or falling over),10,15 it would
appear that spinal imbalance or misalignment must lead
to compensatory mechanisms to maintain a center of
force [gravity line (GL)] in close relation to the feet.10
It has been reported that normal global balance in
adults by radiographic plumbline falls within a narrow
range from the pelvis (sacrum, or S1). Jackson3 has reported values in asymptomatic adults with a mean sagittal vertical axis [SVA (distance form a plumbline drawn
form C7 and the posterosuperior corner of S1)] offset of
0.5 cm (SD ⫾ 2.5 cm). According to these data, offset
greater than 2.5 cm anteriorly or posteriorly are considered beyond the normal range.
The purpose of this study was to evaluate the impact
of SVA offset on pelvic parameters and spinopelvic position relative to the feet in standing subjects. This study
also sought to analyze the effects of SVA offset on GL
position. Enrolment was limited to adults without significant coronal deformity or imbalance. A hypothesis of
this study was that healthy volunteers and patients (even
in the setting of marked sagittal plane deformity) maintain a GL within a fixed area relative to the feet. Furthermore, changes in pelvic location (translation and orientation) associated with SVA offset were hypothesized to
occur.
Sagittal Balance and Spinal Deformity • Lafage et al 1573
Table 1. Subjects Distribution Among Groups
No.
Mean age ⫾ 1 SD (yr)
Male/female ratio
Mean SVA ⫾ 1 SD (cm)
SVA range (cm)
Sagittal
Backward (Sb)
Neutral (N)
Sagittal
Forward (Sf)
43
35 ⫾ 16
28/15
⫺4.5 ⫾ 1.6
关⫺9 to ⫺2.6兴
51
51 ⫾ 18
21/30
0.2 ⫾ 1.3
关⫺2.4 to 2.1兴
37
68 ⫾ 17
17/20
8.0 ⫾ 5.0
关2.6 to 18.9兴
Materials and Methods
Patient Selection
This is a prospective, institutional review board approved
study including 131 adult volunteers (66 asymptomatic and 65
patients; age range, 18 –93 years). Volunteers were recruited
through paper advertisement and patients during office consultation. Exclusion criteria included previous spine surgery, frontal deformity (maximal Cobb angle greater than 20 degree or
frontal imbalance greater than 5 cm), pregnancy, or history of
a neurologic disorder. All subjects had radiographic spinal
evaluation in the free-standing position16 with 36⬙ cassette
films. The sagittal vertical axis (SVA), which is the distance
between the C7 plumbline and the posterosuperior corner of S1
in the sagittal plane, was used to subdivide the group of subjects (Table 1). According to SVA value, subjects were classified
as sagittal backward (SVA ⬍⫺2.5 cm), neutral (⫺2.5 cm ⱕ
SVA ⱕ 2.5 cm), or sagittal forward (SVA ⬎2.5 cm), meaning
respectively that C7 plumbline falls behind, over, and in front
of S1.
Balance Evaluation Protocol
For each subject, forceplate technology and radiologic measurements were combined to evaluate their frontal and sagittal
balance. This published protocol17,18 is briefly presented
hereafter. Subjects were asked to stand on a forceplate in a
free-standing position, with elbows flexed at approximately
45 degrees and fingertips on the collar bones. This position
was chosen as it has been shown to be a normal relaxed
position that does not alter spinal posture and allows radiographic evaluation.16,19
These measurements (Figure 1) aim to evaluate the position
of anatomic components (feet, pelvis, and vertebrae) in relation
to the GL at a moment in time. Although the subject kept
his/her free-standing position, frontal and sagittal full length
radiographs were obtained, which included femoral heads and
C7 vertebra. Precise GL location at the time of radiographs was
extracted from forceplate data based on a time stamp placed
during film exposure. As previously reported,17,18 these measurements were postprocessed using a Matlab function in order
to project the gravity line and the heel line (vertical line drawn
from the midpoint between both heels) on each radiograph.
This process permitted an analysis of offsets between spinopelvic structures and these 2 lines as described in the Table 2.
Radiologic Measurements
Each radiographic film was digitized through a Vidar scanner
(Vidar Systems Corp., Herndon, VA) with 75 dpi resolution
and 12 gray levels. Sagittal and frontal spinopelvic parameters
were evaluated using Spineview software (Surgiview, Paris,
France), a validated,17,20 computer based tool, which enables
quantitative measurements of the spine and pelvis. The spinal
sagittal plane (Figure 2) was described by calculating the lum-
Figure 1. Projection of anatomic landmarks, gravity line (doted
line), and heel line on digital radiographs. The gravity line and
heels line are projected to evaluate offsets between these lines
and radiologic landmarks.
1574 Spine • Volume 33 • Number 14 • 2008
investigated using t-test and then subjected to a simple linear
regression analysis and Pearson correlation. The level of significance was set at 0.05.
Table 2. Static Test Parameters Processed on the
Frontal and Sagittal Plane
Parameter
T4–GL
T9–GL
T12–GL
L3–GL
S1–GL
FH–GL
Heel–GL
Heel–S1
Heel–FH
Description
Results
Distance between T4 vertebral body center and the gravity
line
Distance between T9 vertebral body center and the gravity
line
Distance between T12 vertebral body center and the gravity
line
Distance between L3 vertebral body center and the gravity
line
Distance between the anterosuperior S1 corner and the
gravity line
Distance between the center of the bicoxofemoral axis
and the gravity line
Distance between the heels mid-point and the gravity line
Distance between the anterosuperior S1 corner and the
heels
Distance between the femoral heads midpoint and the
heels
Radiologic Parameters
According to the inclusion criteria, the SVA parameter
was significantly different across the 3 groups (P ⬍
0.001) as well as the T1 sagittal offset (P ⬍ 0.001) and
the trunk global inclination (P ⬍ 0.001) which varied
from ⫺12 degree to ⫺6 degree and ⫹3 degree, respectively, for the sagittal backward, neutral, and sagittal
forward group. Concerning the sagittal curves, the sagittal forward group seem to have a significant lower lumbar lordosis than the sagittal backward group and a significant greater thoracic T4 –T12 kyphosis (P ⬍ 0.005)
than the 2 other groups (Table 3).
No significant difference was observed for the sagittal
T9 tilt among these 3 groups of subjects.
Several differences were noted across groups in terms
of pelvic morphology and pelvic orientation (Table 4).
No significant difference was found for the sacral slope,
whereas it appeared to be slightly higher (38 degrees) for
the sagittal backward group than for the sagittal forward
group (35 degrees). However, the pelvic incidence,
which is a morphologic parameter, was different among
the groups: mean value was 52 degrees for the neutral
group, lower (48 degrees) for sagittal backward group,
and higher (56 degrees) for the sagittal forward groups.
Consequently, the pelvic tilt was lower (10 degree) for
bar lordosis, the thoracic kyphosis, the sagittal vertical axis
(SVA), the global trunk inclination (defined as angle between
the vertical and the best fit line across all vertebral centroids
from C7 to S1) and the sagittal tilt of T1 or T9, which is the
angle between the vertical plumb line and the line drawn from
the vertebral body of T1 or T9 and the center of the bicoxofemoral axis. The sagittal pelvic morphology and orientation
were described by the pelvic incidence, pelvic tilt, sacral slope,
and the overhang (Figure 3).
Statistical Analysis
Data were statistically analyzed using SPSS software (SPSS,
Chicago, IL). Significant differences between the 3 groups were
T1
C7
T4
T1
Sagittal
Tilt
Thoracic
Kyphosis
Figure 2. Sagittal spinal radiologic parameters. A, Thoracic kyphosis measured from the superior endplate of T4 to the inferior
endplate of T12. Lumbar lordosis
measured from the superior endplate of L1 to the superior endplate of S1. Sagittal vertical axis
(SVA) defined as the horizontal
offset from the posterosuperior
corner of S1 to the vertebral
body of C7. B, T1 sagittal tilt and
T9 sagittal tilt defined as the angle between the vertical plumbline and the line drawn from the
vertebral body of T1 or T9 and
the center of the bicoxofemoral
axis.
T9
Sagittal
Tilt
T12
L1
Lumbar
Lordosis
A
SVA
T9
B
Sagittal Balance and Spinal Deformity • Lafage et al 1575
Table 4. Pelvic Radiological Parameters (Mean Value ⴞ
1 Standard Deviation)
Sacral
Slope
Sacral slope (deg)
Pelvic Tilt (deg)*
Incidence (deg)†
Overhang (deg)*
Pelvic
Incidence
Sagittal
Backward (Sb)
Neutral (N)
Sagittal
Forward (Sf)
38 ⫾ 9
10 ⫾ 7
48 ⫾ 10
22 ⫾ 16
36 ⫾ 10
16 ⫾ 6
52 ⫾ 9
33 ⫾ 15
35 ⫾ 10
21 ⫾ 8
56 ⫾ 11
45 ⫾ 17
*Significant differences (P ⬍ 0.001) among the 3 groups.
†Significance differences (P ⬍ 0.05) between the sagittal backward group and
the 2 others.
Pelvic
Tilt
Overhang
Figure 3. Pelvic parameters. Pelvic tilt defined by the angle between the vertical and the line through the midpoint of the sacral
plate to femoral heads axis (retroversion is then measured as a
pelvic tilt increase, anterversion as a pelvic tilt decreases). Sacral
slope defined as the angle between the horizontal and the sacral
plate. Pelvic incidence defined as the angle between the perpendicular to the sacral plate at its midpoint and the line connecting this
point to the femoral heads axis. “Pelvic Incidence ⫽ Pelvic Tilt ⫹
Sacral Slope.” Overhang defined as the horizontal offset between the
midpoint of the sacral plate and the femoral heads axis.
crossed the lumbar spine for the sagittal backward group
is in front of the entire spine for the neutral group and
crossed the cervical spine for the sagittal forward spine.
It is notable that aside for the distance between T9 and
the GL (T9 –GL) and the distance between the GL and
the heels (GL-Heel), all the other parameters revealed
significant differences (P ⬍ 0.001) across the 3 groups
(Table 5). When SVA increased, the GL kept a rather
fixed location relative to the feet, while all measured
anatomic components measured (excepted T9) moved
around this GL: as shown on Figure 4, T4 moved closer
to he GL, T9 kept its location, and the lower vertebrae
(T12, L3, and S1) moved away from the GL (toward the
heels). In addition to the spinal curvature changes meaHeel
Line
Gravity
Line
the sagittal backward group than for the neutral one (16
degree), and it was higher 21 degree in sagittal forward
group; these differences were significant (P ⬍ 0.001). In
summary, the sagittal backward group appears to
present a smaller pelvic incidence and a pelvic anteversion regarding the neutral group; in contrast, the sagittal
forward group revealed a greater mean pelvic incidence
and a pelvic retroversion regarding the neutral group.
Forceplate
A qualitative analysis of spine location regarding the heel
line and the gravity line (Figure 4) revealed that the GL
Sagittal Forward
Neutral
Sagittal Backward
T9
Table 3. Spinal Radiological Parameters (Mean Value ⴞ
1 Standard Deviation)
Kypho (deg)*
Lordo (deg)†
SVA (mm)‡
IncliC7S1 (deg)‡
T1 sagittal offset (deg)‡
T9 sagittal offset (deg)
Sagittal
Backward (Sb)
Neutral (N)
Sagittal
Forward (Sf)
⫺41 ⫾ 11
63 ⫾ 12
⫺45 ⫾ 17
⫺12 ⫾ 3
⫺8 ⫾ 2
⫺14 ⫾ 3
⫺42 ⫾ 15
57 ⫾ 11
0 ⫾ 13
⫺6 ⫾ 2
⫺5 ⫾ 2
⫺13 ⫾ 4
⫺51 ⫾ 20
50 ⫾ 13
80 ⫾ 50
3⫾7
1⫾5
⫺10 ⫾ 6
*Significance differences (P ⬍ 0.005) between the sagittal forward group and
the 2 others.
†Significant difference (P ⬍ 0.001) between the sagittal forward group and the
sagittal backward group.
‡Significant differences (P ⬍ 0.001) among the 3 groups.
S1
Figure 4. Mean location of the gravity and heel lines in regard to
spine.
1576 Spine • Volume 33 • Number 14 • 2008
Table 5. Static Test Paramerters Among Groups (Mean
Value ⴞ 1 Standard Deviation)
T4–GL (mm)*
T9–GL (mm)
T12–GL (mm)*
L3–GL (mm)*
S1–GL (mm)*
FH–GL (mm)*
Heel–GL (mm)
Heel–FH (mm)*
Heel–S1 (mm)*
Sagittal
Backward (Sb)
Neutral (N)
Sagittal
Forward (Sf)
109 ⫾ 21
105 ⫾ 26
66 ⫾ 27
15 ⫾ 23
5 ⫾ 20
⫺5 ⫾ 19
120 ⫾ 28
127 ⫾ 30
115 ⫾ 35
90 ⫾ 26
101 ⫾ 30
74 ⫾ 28
31 ⫾ 26
23 ⫾ 22
3 ⫾ 20
113 ⫾ 23
109 ⫾ 31
90 ⫾ 32
61 ⫾ 27
99 ⫾ 32
85 ⫾ 39
56 ⫾ 35
58 ⫾ 28
30 ⫾ 26
114 ⫾ 25
84 ⫾ 32
56 ⫾ 38
*Significant differences among the three groups.
sured on radiographs, the current results suggest a rotation of the spine (sagittal trunk forward inclination)
around the T9 vertebra.
Analysis of pelvic location regarding the heel line and
GL (Table 5 and Figure 5) revealed that when SVA increased the pelvis shifted posterior towards the heels.
The combination of the pelvic shift and pelvic retroversion lead to a decrease of the heel-S1 offset from
115 to 90 mm and 56 mm, respectively, for the sagittal
backward, neutral, and sagittal forward (total translation ⫽ ⫺58 mm) and to a decrease of the heel-FH
offset from 127 to 109 mm and 84 mm (total translation ⫽ ⫺43 mm).
The correlation analysis on the entire study group (all
subjects) between forceplate parameters and radiologic
parameters revealed a number of interesting relationships. An increase in SVA offset anteriorly was correlated
with an increase of the S1–GL offset (r ⫽ 0.762) and
FH-GL offset (r ⫽ 0.622): in simple terms, as C7 moves
anteriorly the pelvis shifts posteriorly regarding the feet
and gravity line. Conversely, a posterior displacement of
C7 leads to an anterior pelvic shift.
The findings across the study group also revealed the
T9 vertebra to have a rather fixed location regarding the
gravity line; moreover, this T9 –GL offset was highly correlated with the T9 sagittal inclination from the pelvis
(r ⫽ ⫺0.706).
Discussion
The purpose of this study was to evaluate the impact of
SVA offset on pelvic parameters and spinopelvic position
relative to the feet in standing subjects. SVA offset was
applied to define 3 subject groups given the wide use of
this parameter in the radiographic analysis of spinal balance. Marked differences between groups were identified
on a number of levels: regional and global spinal alignment, pelvic alignment, GL offsets from spinopelvic
landmarks. Quite notably, however, the GL-heels and
T9 –GL offset remained surprisingly constant across the
study groups.
SVA offset defined 3 groups in this study. This was
based on a common notion that increasing SVA occurs
with age and sagittal plane pathology. To limit con-
Figure 5. Relative location of the pelvis, GL, and heel line.
founding factors, and given the limited information on
age effect on the spinopelvic relationship with GL offsets,
adjustment to groups were made to eliminate significant
differences in age means. In this additional analysis, subject numbers were limited but numerous differences between groups remained statistically significant. The findings of a constant GL-heel offset, a pelvic shift and
retroversion between the sagittal backward and sagittal
forward groups were maintained. Those parameters that
lost significant difference (compared the initial total
study group comparisons) may be attributable to sample
size, although age related effects (CNS, muscle tension,
force, mass . . .) cannot be dismissed. One can note that
age appears to have only medium correlations with L3–
GL, S1–GL, S1-heel, SVA, and pelvic tilt-overhang (all
⬍0.62).
Although current standards in patient evaluation with
radiographs can permit quantification of the plumbline
Sagittal Balance and Spinal Deformity • Lafage et al 1577
offset, position of the lower extremities and feet remains
elusive. Most notably, the posterior pelvic shift in relation to the heels with increasing SVA in a patient population with spinal deformity has not been previously reported (r ⫽ 0.6, P ⬍ 0.001). Group sagittal forward
versus sagittal backward demonstrated significant differences (P ⬍ 0.001) in the pelvic shift but also in the pelvic
tilt. Thus, an increase of the SVA leads to a posterior shift
of S1 toward the heels and perhaps an increase of the
pelvic tilt (retroversion). Findings presented in the current study are consistent with the ones focusing on an
adult asymptomatic population,10,21 meaning the same
global mechanisms of compensation take place in
asymptomatic and patient population.
Analysis of data from this study revealed a higher
pelvic incidence, a supposed fixed morphologic parameter, with higher SVA. This raises the question whether
pelvic morphology can predispose to sagittal plane imbalance, whether changes can occur in the sacral endplate, or as suggested by Skalli et al22 sagittal imbalance
can lead to compensatory motion in the sacroiliac joint.
It would appear that even wide variation in SVA can
be tolerated so long as compensatory mechanisms come
into play to maintain standing balance. It is evident from
the data in this study that the role of the lower extremities and the pelvis are critical in understanding spinal
balance as well as imbalance observed in pathologic
states. As SVA offset increased a posterior shift of the
pelvis occurred. This would require a change in the degree of hip flexion/extension given the noted posterior
translation of the pelvis in relation to the feet with increasing SVA offset. Furthermore, changes in knee and
ankle joint position may come into play to maintain a GL
relationship to the feet.
This study confirms the role of pelvis as an equalizer
(compensatory mechanisms) in the sagittal balance. It
also underlines that the pelvis translation is a parameter
as important as the pelvis rotation (measured by the pelvis tilt). Ideally, one should analyze our patients from
head to feet or at least include the pelvis.
Although this study focused on the static aspects of
the GL in relation to spinal and pelvic landmarks, force
plate analysis also permits dynamic analysis of sway.
Such investigation would permit quantification of GL
movement patterns in standing subjects. Next steps in
the comprehension of standing balance and compensatory mechanisms in the setting of spinal deformity
should include work in the area of dynamic aspects of the
gravity line.
Conclusion
This study analyzed the impact of SVA on spinopelvic
radiographic parameters and forceplate acquired GL and
foot position in adult subjects. Three distinct subject
groups were created by varying SVA values. Significant
differences between groups were noted for a number of
radiographic and GL parameters. Increasing SVA leads
to a posterior pelvic shift in relation to the feet. Increas-
ing pelvic incidence was also noted in the sagittal forward displaced SVA group. However, no significant difference in GL-heels offset is noted across the subject
groups. It appears that pelvic shift is a key component in
maintaining a rather fixed GL-heels offset even in the
setting of variable SVA and trunk inclination.
Key Points
● Increasing SVA offset leads to a posterior pelvic
shift in relation to the feet.
● No significant difference in GL-heels offset is
noted across groups of varying SVA offset.
● Pelvic shift is a key component in maintaining a
rather fixed GL-heels offset (GHO) even in the setting of variable SVA and trunk inclination.
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