Introduction roentgen stereophotogrammetric, radio-stereometric, and ultrasound [8-11]. Using these

Introduction

Low back pain (LBP) is one of the most common reasons
for primary care visits after the common cold, with approximately 90% of adults
being impacted by this condition at some time in their lives 1, 2. One of the
most overlooked sources of LBP is the sacroiliac joint (SIJ) due to its complex
nature and the fact that the pain emanating from this region can mimic other
hip and spine conditions 1, 3. However, recent studies have reported a higher
prevalence of the SIJ as a source of LBP leading physicians to place greater
focus on the treatment and consideration of SIJ dysfunction as a pain generator
4.

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The SIJ, the largest axial joint in the body, is the
articulation of the spine with the pelvis that allows the transfer of loads to
pelvis and lower extremities 5, 6. Sexual dimorphism exists in pelvis such
that compared to the male sacrum, the female sacrum is generally wider, more
uneven, less curved, and more backward tilted. Males tend to have a relatively
long and narrow pelvis, with a longer and more conical pelvic cavity than those
of females 7.

There are different methods to measure the
SIJ motion such as roentgen stereophotogrammetric, radio-stereometric, and ultrasound
8-11. Using these methods, it is shown that the SIJ rotation and translation
in different planes are not exceeding 2-3° and 2 mm, respectively 12, 13.  To the authors’ best knowledge, there is no study
which discusses the biomechanical differences between male and female SIJs in
terms of ROM, SIJ ligaments strain, stress, and load sharing across the SIJ.
Cadaver studies would be technically demanding due to low motions at SIJ. In
addition, quantifying stresses across the joint is not feasible. Therefore, experimentally
validated finite element analysis approach would be the most practical tool to assess
the ROM, stresses, and strains across the joint. The objective of this study
was to quantify these parameters at the SIJ using gender specific finite
element models of SIJ. The study was aimed to better understand the
biomechanical differences in SIJ between genders in terms of their mobility and
the possible pain sites.

 

Material and Methods

Male Finite Element Model of the
Lumbar Spine-Pelvis-Femur

The previously developed and validated
finite element lumbar spine model 14, 15 was used for the male model. The
3-dimensional (3D) pelvis geometry was generated using a 1 mm slice of computer
tomography (CT) of a 55 year old male pelvis without any abnormalities,
degeneration or deformation of the pelvis. The 3D reconstruction of spino-pelvis
model was done using MIMICS software (Materialise Inc., Leuven, Belgium). After
3D reconstruction of the bones and spinal discs, they were imported into
Geomagic Studio software (Raindrop Geomagic Inc., USA) to reduce
noises, remove spikes, smooth surfaces, and create patches and grids for
meshing. Hypermesh software (Altair Engineering, Inc., USA) was used to create
the mesh structure from the 3D model.

Lumbar spine and pelvis bones were modeled as
trabecular cores surrounded by a cortical layer with a thickness of 1 mm 15,
16. The linear hexahedral element type was utilized for cortical and
cancellous bones of vertebrae and intervertebral discs. Tetrahedral element
type was used for the cortical and cancellous bones of the pelvis. The truss
elements were employed for ligamentous tissues including the SIJ and spinal ligaments.
144,360 elements were generated for the male model.

Female Finite Element Model of the
Lumbar Spine-Pelvis-Femur

Computer tomography (CT) images of a 55
years old female’s spine, pelvis without any abnormalities, degeneration were
used to reconstruct the female spino-pelvis model. MIMICS software
(Materialise, Leuven, Belgium) was utilized to build the 3D geometry of the
bones and then intervertebral discs were made by filling the space between each
two vertebrae of the CT images. Next, smoothing and meshing were carried out by
Geomagic Studio software (Raindrop Geomagic Inc., USA) and the Hypermesh
software (Altair Engineering, Inc., USA). Figure 1 shows the male and female
spine-pelvis-femur FE models. 

The linear hexahedral element type was utilized for
cortical bone of vertebrae and spinal discs. Tetrahedral elements were assigned
to the cancellous bone of both the vertebrae and the pelvis as well as the
cortical bone of the pelvis. The truss elements were employed for ligamentous
tissues. The SIJ ligaments were anterior sacroiliac ligament (ASL),
interosseous ligament (ISL), long posterior sacroiliac ligament (LPSL), short
posterior sacroiliac ligament (SPSL), sacrospinous ligament (SSL), and
sacrotuberous ligament (STL). A detailed view of the pelvis ligaments is shown
in the figure 2. The female model as a whole contained 463,735 elements.

 

Material Properties

The material properties used in the FE
models were extracted from previous studies 14, 17 for cortical and
cancellous bones, annulus, nucleus, ligaments, and joints are summarized in table
1. Similar material properties were used for both male and female models. The SIJs,
spine facets, articular cartilages, and pubic symphysis were modelled as
non-linear soft contact. The femurs were kinematically coupled to the pelvis.

Mesh Convergence Study

 The
mesh convergence analysis was done on the segregated L4-L5 motion segment of
the female model. An initial seed size was assigned and the model was subjected
to 7.5 N.m bending moment to simulate motions in all planes and the ROM was
measured. The mesh refining was repeated until the difference in the ROM in all
planes was below 4%. The final element size so determined was used to mesh the
other segments of the model. The simulation was run using ABAQUS 6.14 software
(SIMULIA, Inc., Providence, RI, USA). 

Finite Element Model Validation

The intact male model SIJ ROM was previously
validated 14, 15 against study of Miller et al. 18 under the same loading
and posture conditions. Due to lack of data on female specimens SIJ ROM under
two leg stance condition, the validation for the female model was performed under
one leg stance condition. To be consistent, the validation under one leg stance
condition was done for both male and female models. To validate the SIJs ROM
for intact male and female models, loading conditions of the cadaver study done
by Lindsey et al. 19 was simulated. This experiment was carried out for
intact L4 to pelvis of the male and female specimens under single leg stance
condition. A 7.5 Nm pure moment load was applied to the top endplate of L4 to
simulate various spinal motions. The motion at the SIJ was calculated for both
right and left joints.

Loading and Boundary Conditions

In all models, a 400 N compressive
follower load was applied through wire elements which followed the curvature of
the lumbo-pelvis segment to simulate the effect of muscle forces and weight of
the upper trunk. A 10 N.m bending moment was then applied at the superior
surface of the L1 vertebrae to simulate the physiological flexion, extension,
lateral bending, and axial rotation. To constrain the models, femurs were fixed
in all degrees of freedom to prevent relative displacement of the legs in two
leg stance condition 14, 15.

Data
Analysis

The SIJ motion was calculated using the angular
displacements at the sacrum minus those at the ilium for right and left joints.
The maximum von Mises stresses, normal and shear loads across the
SIJ for each of the models were analyzed. The average of the maximum principal
strains were calculated and compared for all ligaments of the pelvis in the
intact male and female models.

 

Results

Model Validations

The predicted data for all physiological loadings fell within one
standard deviation of the experimental data, except for right lateral bending
and right axial rotation for the male data, Figure 3.

Range of Motion

The comparison of range of motion at SIJ
is shown in figures 4 and 5. ROM of SIJ in the female model was the greatest in
extension (1.36° left SIJ, 1.33° right SIJ) followed by flexion (0.50° left
SIJ, 0.50° right SIJ), right rotation (0.44° left SIJ, 0.44° right SIJ), right
bending (0.30° left SIJ, 0.35° right SIJ), left rotation (0.29° left SIJ, 0.33°
right SIJ), and left bending (0.24° left SIJ, 0.30° right SIJ). In the male
model, the maximum ROM of SIJ occurred in left rotation (0.54° left SIJ, 0.58°
right SIJ) followed by right rotation (0.45 left SIJ, 0.48 right SIJ),
extension (0.37 left SIJ, 0.36 right SIJ), flexion (0.28° left SIJ, 0.27° right
SIJ), left bending (0.11° left SIJ, 0.12° right SIJ), and right bending (0.12°
left SIJ, 0.10° right SIJ).  It was found
that in flexion-extension (F-E) movements, SIJ had the highest motion in female
model (1.86°), however, the male model had the greatest motion in axial
rotation (1.07°). The lowest motion occurred in lateral bending in both female
and male models (0.55° vs. 0.24°). According to the predicted motion data the
female model experienced 86% higher mobility in flexion, 264% in extension,
143% in left bending, and 228% in right bending compared to the same motions in
the male model. In left and right rotation, the ROM of the male model was 78%
and 9% greater than the female model, respectively.

Stresses
across SI Joint

 The
maximum stress in the female model occurred during the left rotation, followed
by flexion, right rotation, right bending, left bending, and extension, figure 6.
In the male model, the greatest stress happened during the left rotation,
followed by left bending, extension, flexion, right bending, and right
rotation.

The maximum stresses at the female SIJ
were higher by 27% in flexion, 28% in right bending, 49% in left bending, 45%
in right rotation, and 20% in left rotation compared to those of the male model,
figure 6.

Sacrum had higher stresses compared to the
ilium in both models, figure 6. Stresses at the female sacrum and ilium were up
to 49%, and 29% greater than the male model.

Pelvis
Ligaments Strains

Figure 7 and 8 show the results of the SIJ
ligament strains for female and male models, respectively. The anterior
sacroiliac ligament (ASL) strains were the same during all motions. The long
posterior sacroiliac ligament (LPSL) experienced greatest tension during
extension motion and had no strain under the other motions. The short posterior
sacroiliac ligament (SPSL) was strained maximum during extension, but had
comparable values under the other loads. The interosseous ligament (ISL)
underwent the largest tensile strains during all motions. The sacrospinous
ligament (SSL) and sacrotuberous ligament (STL) ligaments were both mostly
strained during flexion, experienced similar values under other loads, and had
no strain during extension.

In the female model, ASL, LPSL, SPSL and
STL underwent larger strains compared to the corresponding male model
ligaments, while SSL had similar values for both genders, and ISL showed
greater strains in the male model.

Load Sharing across SI Joint

Computed load values, tables 2 and 3
represent the greater average force magnitudes on the ilium and sacrum of both
sides.

 Female SIJs experienced higher loads across the
joints compared to the male SIJs under the similar loading conditions. In both
models, the shear loads were higher than normal forces acting on the SIJ
surfaces.

 

Discussion

Although
there are many studies which have quantified the range of motion, the
literature on the biomechanical differences between male and female SIJ is rare
with only one study comparing the range of motion differences of SIJ between genders
20. However, they did not provide load sharing and stress data across the SI
joint. The current study showed that SIJ had higher mobility in females
compared to males which is in agreement with the literature. In both male and
female models, the motion was minimum in lateral bending. The greatest difference
of the SIJ motions between male and female models occurred during extension in which
the female model showed significantly higher motion than the male model. The
increased mobility in the female SIJ can be attributed to a lesser pronounced
curvature of the SIJ surfaces, a larger gap (2 mm) at the SIJ, and a greater
pubic angle (111°) compared to the male model which had 1 mm gap at SIJ and
pubic angle of 76° 12, 21.

Anatomical study
by Ebraheim et al. 22 revealed that the SIJ surface area is relatively
greater in adult males than females. The smaller joint surface area in female
SIJ can result in higher local stresses across the joint. The current study
showed that the maximum stress values in the female model were 49% higher than
male model.  The results of current study
also showed that greater motion at the SIJ results in higher loads leading to
higher stresses across the joint under different motions, especially, on sacrum
which experienced higher stresses compared to the ilium in both genders. This
higher stress in females can result in higher risk of SIJ pain and higher risk
of sacral stress fracture.

            Obtaining more quantitative
information may be essential to recognize SIJ dysfunction in both genders due
to the lack of quantitative relationship between the physiological spine motion
and the biomechanical factors such as ligament strain that may be associated
with pain in the SIJ. In the current study, the authors illustrated that
depending on the spine motion, the ligament strains varied. The anterior
sacroiliac ligament (ASL), short posterior sacroiliac ligament (SPSL), sacrotuberous
ligament (STL), sacrospinous ligament (SSL), and interosseous sacroiliac
ligament (ISL) underwent tension to constrain the SIJ during flexion. Long
posterior sacroiliac ligament (LPSL) is one of the posterior ligaments which
only functions during extension. SSL and STL seem to have no function in extension;
however, SSL serves as a main constraint in other motions.  Janssen et al. have shown that by sectioning
sacrospinous and sacrotuberous ligaments, SIJ stability decreased 23. The
posterior sacroiliac ligaments contributed most to the SIJ mobility, while the
anterior sacroiliac ligaments had little influence 41. Resisting the nutation
and counternutation of the joint were done by ISL, STL, SSL, and LPSL 43,
44.  The major role in stabilizing the
SIJ was due to ISL, one of the strongest ligaments in the body. Interestingly,
ASL, LPSL, SPSL and STL underwent higher strains in the female model and ISL
stretched more in the male model. These high strains on certain ligaments in both
models can be explained by these anatomical differences which females have
smaller ASL, LPSL, and SPSL and males have smaller ISL compared to each other
7. Although in our models, ligaments had same properties in both models, but
depending on the gender, the strain exerted on the ligaments were different.

To
our knowledge, this is the first study which investigated the difference
between SIJ ROM of female and male as well as stresses, load sharing, and
ligaments strain across the joint in different motions. The presented data can
be used to address various critical questions regarding the anatomical
differences between male and female SIJ. For example, compared to men, who have
a more ventral center of gravity, in females the center of gravity commonly
passes in front of or through the SIJ 24, 25. This difference implies that
men would have a greater lever arm than women, accounting for the stronger SI
joints in males 13. This characteristic may explain why males have more
restricted mobility.

One of the
limitations of this study is the use of similar bone and ligaments material
properties for the female and male models due to the lack of experimental data.

In
conclusion, this study found that the female SIJ had relatively higher range of
motion than male model at both sides of the SI joint. Also, the female SIJ
experienced higher stresses across the joint especially on the sacrum compared
to males which implies that the females are at a higher risk of stress fracture
injury. The major role in stabilizing the SIJ was performed by ISL which is one
of the strongest ligaments in the body.  The shear loads are higher across the female
SIJ, compared to male SIJ.  These
differences may contribute to higher incidence of LBP in females, including
during pregnancy.  We are presenting the
details of model formulations, both for a male and a female sample which can
now be expanded and used to study gender differences in other postures and
other conditions like pregnancy.