Effect of the screw type (S2-alar-iliac and iliac), screw length, and screw head angle on the risk of screw and adjacent bone failures after a spinopelvic fixation technique: A finite element analysis
Effect of the screw type (S2-alar-iliac and iliac), screw length, and screw head angle on the risk of screw and adjacent bone failures after a spinopelvic fixation technique: A finite element analysis
Jong Ki Shin 0 1
Beop-Yong Lim 1
Tae Sik Goh 0 1
Seung Min Son 1
Hyung-Sik Kim 1
Jung Sub Lee 0 1
Chi-Seung Lee 1
0 Department of Orthopaedic Surgery and Biomedical Research Institute, Pusan National University Hospital , Busan , Republic of Korea, 2 Department of Orthopaedic Surgery, Myung Eun Hospital , Busan , Republic of Korea, 3 Biomedical Research Institute, Pusan National University Hospital , Busan , Republic of Korea, 4 Department of Orthopaedic Surgery and Biomedical Research Institute, Pusan National University Yangsan Hospital , Yangsan , Republic of Korea, 5 School of Medicine, Pusan National University , Busan , Republic of Korea
1 Editor: Jose Manuel Garcia Aznar, University of Zaragoza , SPAIN
Spinopelvic fixations involving the S2-alar-iliac (S2AI) and iliac screws are commonly used in various spinal fusion surgeries. This study aimed to compare the biomechanical characteristics, specifically the risk of screw and adjacent bone failures of S2AI screw fixation with those of iliac screw fixation using a finite element analysis (FEA).
Data Availability Statement: All relevant data are
within the paper and its Supporting Information
Funding: This research was supported by Basic
Science Research Program through the National
Research Foundation of Korea (NRF) funded by the
Ministry of Education (https://www.nrf.re.kr/index)
NRF2016R1D1A1B03934304, CSL). This research was
A three-dimensional finite element (FE) model of a healthy spinopelvis was generated. The
pedicle screws were placed on the L3-S1 with three different lengths of the S2AI and iliac
screws (60 mm, 75 mm, and 90 mm). In particular, two types of the S2AI screw, 15Ê- and
30Ê-angled polyaxial screw, were adopted. Physiological loads, such as a combination of
compression, torsion, and flexion/extension loads, were applied to the spinopelvic FE
model, and the stress distribution as well as the maximum von Mises equivalent stress
values were calculated.
For the iliac screw, the highest stress on the screw was observed with the 75-mm screw,
rather than the 60-mm screw. The bones around the iliac screw indicated that the maximum
equivalent stress decreased as the screw length increased. For the S2AI screw, the lowest
stress was observed in the 90-mm screw length with a 30Ê head angle. The bones around
also supported by the Bio & Medical Technology
Development Program of the National Research
Foundation (NRF) funded by the Korean
government (MSIT) (https://www.nrf.re.kr/index)
(NRF-2016M3AE8942063, TSG). The funders had
no role in study design, data collection and
analysis, decision to publish, or preparation of the
Competing interests: The authors have declared
that no competing interests exist.
the S2AI screw indicated that the lowest stress was observed in the 90-mm screw length
and a 15Ê head angle.
It was found that the S2AI screw, rather than the iliac screw, reduced the risk of implant
failure for the spinopelvic fixation technique, and the 90-mm screw length with a 15Ê head angle
for the S2AI screw could be biomechanically advantageous.
Degenerative spinal diseases are one of the most frequently reported chronic health problems
affecting the adult population owing to aging. In addition, adult spinal deformities have
increased owing to the growing elderly population. These spinal conditions lead to an
imbalance in the structural support of the spine. With the development of surgical techniques and
supporting surgical skills, spinal deformity surgeries have become more frequent. However,
because of the poor bone quality of most elderly patients, implantation-related problems are
frequent, especially in the lumbosacral area. Kim et al. [
] reported a pseudoarthrosis rate of
24% at the L5-S1 junction in adult scoliosis surgery. In addition, many studies have shown that
long instrumentation to the sacrum without pelvic fixation is susceptible to implant failure
To overcome the complications associated with fusions ending at S1, sacropelvic fixation
has been introduced as a safe alternative [5±7]. Spinopelvic fixation with iliac screws has been
used in the correction of various spinal deformities requiring long spinal fusions. Iliac screws
consist of independent anchors that are placed in the ilium and connected modularly with
modern spinal constructs consisting of rods with pedicle screws and hooks [
]. This technique
provides powerful control of the pelvis; however, extensive subfascial dissection to expose the
posterior-superior iliac spine (PSIS) during implant insertion is required and has caused
complications. In addition, long-term problems have occurred associated with implant
prominence related to the PSIS starting point [
]. In one study, 22% of the patients required
screw removal after 2 years , and in another study, the incidence of these problems was
higher after 5 years [
The S2-alar-iliac (S2AI) technique, first proposed in 2007, uses a starting point in the sacral
ala, midway between the S1 and S2 dorsal foramina along a line that connects the lateral edge
of the two foramina [
]. This point is also in line with the S1 pedicle screw starting point. The
S2AI technique does not require the dissection of the subcutaneous tissue over the iliac crest
or the sacral paraspinous muscle, as is required for iliac screws that start at the PSIS. Decreased
implant prominence is one of the advantages of this technique, as the starting point is
approximately 15 mm deeper than that used for the entry to the PSIS [
]. This technique allows a
single rod to be used without the need for cumbersome connectors and has the potential to
minimize the complexity of the procedure . Moreover, S2AI screws have fewer unplanned
reoperations than iliac bolts for instrumentation-related complications, wound infections, and
instrumentation removal owing to pain [
]. However, one recent study reported a high rate
of mechanical failure of S2AI screws in an early/midterm follow-up [
]. The failure rate of
the S2AI screws was 35% compared to 12% for the iliac screws with lateral connectors.
As previously mentioned, many studies have focused on the anatomy, surgical technique,
and risk evaluation during iliac and S2AI screw fixations. However, there are few studies that
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have performed a biomechanical assessment and comparative study of these two fixation
types. Few studies have evaluated the material internal stress in the screw and the bone around
the screw, under various fixations and spine motions. To evaluate the biomechanical and
physical factors, such as the von Mises equivalent stress, cadaver or computational approaches to
the analysis are required.
The finite element method (FEM) of computational analysis is widely used in the study of
many biological systems, especially the musculoskeletal system. Through the FEM, many
complex geometrical and material properties of the biological system can be effectively evaluated,
and many physical variables, such as stress, strain, damage, and fracture, can be quantitatively
In a few studies, the biomechanical ability and stability of the spinopelvic system have been
numerically investigated. Garcia et al. [
] carried out a simulation study to analyze the
functional performance of the pelvis and the stability of different types of fixations for several kinds
of fractures. Zhao et al. [
] produced an FE model of a Tile C pelvic ring injury and compared
the stability of seven types of models that were fixed using normal and lengthened sacroiliac
screws for the treatment of bilateral vertical sacral fractures. Bruna-Rosso et al. [
the biomechanical features of stable sacroiliac joint (SIJ) fixation in the physiological condition
using a detailed FE model. The pre-instrumented and post-instrumented SIJ mobilities were
compared using different implant configurations.
As previously mentioned, many computational studies have focused on the spinopelvic
fixation analysis. However, there is no comparative study regarding the iliac and S2AI screw
fixation technique. Hence, in this study, the biomechanical characteristics of the iliac and S2AI
screw fixed spinopelvic system under various implant configurations were computationally
evaluated, and the simulation results, such as the equivalent stress values in the surrounding
bone and screw, were investigated. Based on the calculated results, an optimal implant
condition was proposed.
Materials and methods
This research was approved by the Institutional Review Board (IRB) of the Pusan National
University Hospital (PNUH). The reference number is PNUH-IRB-E-2016068. An informed
consent statement was signed after receiving oral description of the simulation prior to the
start of simulation.
The software programs Mimics 19.0 (Materialise, Belgium), SolidWorks 2016 (Dassault
Systèmes, USA), and ANSYS 16.1 (ANSYS Inc., USA) were used. A computed tomography
(CT) scanner (GE, USA) was used to collect raw data in the digital imaging and
communication in medicine (DICOM) format with a scan slice of 0.75 mm.
FE models for spinopelvis and implants
An FE model of the spinopelvis (L3-Pelvis), which included three vertebrae, three discs, and a
pelvis, was reconstructed. The geometrical specifications of the spinopelvis were obtained
from 64 spiral CT images of a 27-year-old healthy male without a history of spine injury and
osteoporosis or radiographic evidence of degeneration. The patient underwent a CT
examination for a health checkup at our hospital, and the CT images were used with the patient's
consent. The date of patient recruitment for imaging was August 15, 2016, and the time period for
this research was June 28, 2016 to May 31, 2017.
The CT images were scanned and imported into Mimics 19.0 to construct the
three-dimensional (3D) surface of the spinopelvis. To avoid unexpected stress concentration, the surface of
the spinopelvis was smoothed. The FE model was generated using the ANSYS meshing tool. A
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10-node tetrahedral solid element was adopted to express the cancellous bone, and an 8-node
hexahedral element with a 1-mm thickness was used to represent the cortical bone,
surrounding the cancellous bone. To describe the intervertebral disc in the FE model, the nucleus
pulposus and annulus fibrosus were adopted using an 8-node hexahedral solid element. The
model without implants had a total of 223833 elements and 341569 nodes.
Owing to their important roles in pelvic biomechanics, the anterior/posterior longitudinal,
interspinous, sacroiliac, sacrospinous, sacrotuberous, and pubic ligaments were incorporated
and modeled as the spring elements. The attachment points were ascertained by mimicking
the anatomy as closely as possible. The final FE model of the normal spinopelvis is shown in
Fig 1(A), 1(B) and 1(C).
This 3D geometry of the spinopelvic model was then exported into 2016 SolidWorks
software to generate the fixation models. The polyaxial pedicle screw instrumentation (7.0 × 35
mm) was placed bilaterally from L3 to S1, and two different screws were adopted for
spinopelvic fixation: iliac and S2AI screws. The diameters of the iliac and S2AI screws were 7.0 mm
each, and the length of the three screws varied: 60 mm, 75 mm, and 90 mm. Two types of the
S2AI screw, 15Ê- and 30Ê-angled polyaxial screws, were used.
The iliac and S2AI screws were implanted into the spinopelvic model using the standard
surgical technique. The iliac screws were inserted from the PSIS to the anterior-inferior iliac
spine (AIIS) at each ilium, and the lateral connectors were placed in the iliac screws. The S2AI
screws were placed 1-mm inferior and 1-mm lateral to the S1 dorsal foramen. Angulation of
the screw was directed just above the sciatic notch in the coronal plane.
The threads of the screws were eliminated to simplify the FE models. Based on the
simplified model, the computational time as well as the analysis risk owing to an unrealistic high
stress could be reduced, and a smooth transfer of stress between the screw and bone could be
]. The element type of the implant was the 8-node hexahedral solid element, and
the number of elements for the implants was 42139 for the iliac-screw construct and 24199 for
the S2AI-screw construct. The final FE model of the implant with the iliac and S2AI screws
and two types of head angles of the S2AI screw are shown in Fig 1(D), 1(E) and 1(F).
The material properties of the bones, intervertebral disc, implants, and various ligaments
are listed in Tables 1 and 2. The material of the implant was Ti-6Al-4V.
This study also evaluated the sensitivity of the elements that affected the accuracy of the
results prior to the regular FE analysis (FEA). The coarse, medium, and fine densities of three
representative meshes were selected to determine the number of elements, as shown in Fig 2.
An analysis was performed using the three densities, and the results converged from the
medium- to fine-mesh densities. Table 3 lists the number of elements, number of nodes,
computational time, and equivalent stress on the screw/bone for each density. For the fine
mesh, the convergence and accuracy were increased; however, its use was not practical because
of the increased computational time. Therefore, the medium mesh with a high degree of
convergence was selected for the analysis.
The ANSYS 16.1 FEA software program was used to calculate the stress distribution in each
model. As listed in Tables 1 and 2, linear elastic isotropic material properties were assigned to
all tissues and implants [
21, 25, 26
The bilateral acetabulum of the FE model was fixed. The interface condition between the
screw and bone plays a vital role in determining the stress distribution [
]. Accordingly, a
surface-to-surface contact element was adopted to simulate the contact interfacial
characteristics between the screw and bone. In this study, however, a large amount of computational time
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Fig 1. Finite element model and loading/boundary condition of the spinopelvis and implant. Image (a) shows the anterior view
with ligaments, and (b) shows the posterior view with ligaments. Image (c) shows the sagittal plane view, and (d) shows the S2AI
screw. Image (e) shows the iliac screw, and (f) shows the 15Ê head angle. Image (g) shows the 30Ê head angle, and (h) shows the
was required to calculate the stress distribution in the bone-screw interface owing to the large
number of contact interfaces. Moreover, there was a convergence problem with the contact
condition during the calculation since the number of contact nodes/elements was large.
Therefore, the contact condition was postulated as a rigid alternative. The implants were locked to
the bone to describe the fixation [
]. In addition, the contact behavior between the screw and
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10-node tetrahedral solid element
8-node hexahedral solid element
8-node hexahedral solid element
8-node hexahedral solid element
8-node hexahedral solid element
spinal rod interfaces was set as a rigid bond. For the behavior of the spinopelvis, the facet joints
or SIJs were materialized by inserting a contact condition between the abutted bones. The
facet joints were assumed to be frictionless contact because it was routinely removed during
]. The SIJs were considered to be a bonded contact because it was outside the main
concern of this study [
A pure moment of 10 Nm combined with a pre-compressive load of 700 N was applied to
the top surface of L3 and the superior articular processes. 60% (500 N) and 40% (100 N) of the
total load were separately applied to the upper vertebral body and superior articular processes,
respectively, as the facet joints can commonly carry 10% to 40% of the compressive load of the
total force subjected to the vertebrae [27±29].
Through the combination of the moment and compression, four types of loads,
compression, flexion, extension, and rotation, were generated and adopted to the FE model. The
loading and boundary conditions are shown in Fig 1(G).
Validation of the FE model
The proposed FE model was validated by simulating the experimental cadaveric study of
O'Brien et al. [
]. The normalized range of motion (ROM) test at the L3-pelvis in a cadaveric
study was simulated. Then, the three types of screws (65-mm and 80-mm S2AI screws and a
90-mm iliac screw) and two types of the mechanical behaviors (flexion-extension and axial
rotation) were simulated with the FE model. The simulation results were compared with the
experimental results in the literature, as shown in Fig 3.
As shown in Fig 3, the simulation results in the axial rotation case coincided well with the
experimental results. However, some differences in the normalized ROM values were observed
between the simulation and experimental results in the flexion-extension case. The error
Number of springs
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Fig 2. Classification according to the mesh densities to evaluate the sensitivity of the elements. Image (a) shows the coarse mesh, and
(b) shows the medium mesh. Image (c) shows the fine mesh.
ranges were approximately 0.5±3.0% in the flexion-extension case and 6.7±38.0% in the axial
rotation case. This was because of the anatomical shape as well as the material properties of the
spinopelvis for the FE model and the cadaver do not correspond. Because of the limitations of
the cadaveric study of O'Brien et al., the crucial factors, such as the anatomical information of
the spinopelvis, material properties of the hard/soft tissues, and implementation position of
the pedicle screw, as well as the spinal rod, could not be evaluated prior to the simulation.
Despite this, the proposed FE model and FEA procedure in this study might be reasonable for
the computational biomechanical investigation of the spinopelvic fixation technique.
Figs 4, 5 and 6 show the von Mises equivalent stress distribution of the screw in the spinopelvic
model with the iliac and S2AI screws under various loading conditions. Fig 4 shows the iliac
Fig 3. Comparison of the experimental data from the literature with the simulation results of this study.
screw with a 60-mm length, and Figs 5 and 6 show the S2AI screws with a 75-mm length and a
15Ê head angle and a 90-mm length with a 30Ê head angle, respectively.
In addition, Fig 7 shows the maximum equivalent stress in the screw with respect to the
screw type (iliac and S2AI screws), screw length (60, 75, and 90 mm), and head angle of the
S2AI screw (15Ê and 30Ê) under four types of loading conditions.
Conversely, Figs 8, 9 and 10 show the equivalent stress distribution of the bone around the
iliac and S2AI screws under various loading conditions, respectively. Fig 8 shows the bone
around the iliac screw with a 60-mm length, and Figs 9 and 10 show the bone around the S2AI
screws with a 75-mm length and a 15Ê head angle and a 90-mm length with a 30Ê head angle,
Moreover, Fig 11 shows the comparison results of the maximum equivalent stress in
the bone around the screws regarding the screw type, screw length, and head angle of the
S2AI screw under four types of loading conditions. Since the S2AI screw penetrates the
SIJ, the stress in the sacrum, as well as that in the ilium, were addressed in the graph
All simulation results are listed quantitatively in Table 3. In this table, the maximum
equivalent stress on each screw and surrounding bone are listed. In addition, the increase/decrease in
the quantity of the stress in all cases relative to those in the reference cases were identified. In
this study, the reference cases were the 60-mm screw length and the 15Ê head angle in the S2AI
screw and the 60-mm screw length in the iliac screw.
The simulation results according to the screw length, head angle, and screw type are listed
below. For the iliac screw, the maximum equivalent stress decreased in the order of 75-mm,
90-mm, and 60-mm screw lengths in all loading conditions, as shown in Fig 6 and listed in
Tables 4±7. The highest stress was observed in the 75-mm screw length and not in that of the
60-mm screw length. The bones around the iliac screw showed that the maximum equivalent
stress decreased in the screws in the order of 60-mm, 75-mm, and 90-mm screw lengths in all
loading conditions, except compression with the flexion case, as shown in Fig 11 and listed in
Tables 4 to 7.
Conversely, for the S2AI screw, the highest and lowest maximum equivalent stresses
occurred in the 60-mm screw length with a 15Ê head angle and the 90-mm screw length
with a 30Ê head angle, respectively, in all loading conditions, as shown in Fig 7 and listed in
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Fig 4. von Mises equivalent stress distribution of the screw in the spinopelvic model with the iliac screw under various loading
Tables 4 to 7. The maximum equivalent stress on the sacrum and ilium was found in the
S2AI screw, as shown in Fig 11 and listed in Tables 4 to 7. The bones around the S2AI screw
indicated that the lowest stress was observed in the 90-mm screw length with a 15Ê head
angle on the sacrum and ilium. Although there was no large variation (or percent of change)
of the stress in the sacrum, a large variation of stress was observed in the ilium.
The stress ranges of the iliac screw and surrounding iliac bone were approximately
111±178 MPa and 40±51 MPa, respectively. In addition, the stress ranges of the S2AI
screw and surrounding sacrum/iliac bones were approximately 54±110 MPa and 28±63
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Fig 5. von Mises equivalent stress distribution of the screw in the spinopelvic model with the S2AI screw (75-mm screw length and
15Ê head angle) under various loading conditions.
The aim of this study was to evaluate the biomechanical characteristics of two types of
spinopelvic fixation systems under various implant configurations and loading conditions. To
obtain the stress contour of spinopelvic fixation using FEM, 3D FE models for the cortical and
cancellous bones and the intervertebral disc, including the nucleus pulposus and annulus
fibrosus, which have a significant influence on the mechanical behavior of the spinopelvis,
were fabricated. Moreover, 11 types of lumbar and pelvic ligaments were considered to
accurately represent the human spinopelvic state. Two types of implants for iliac and S2AI fixation
were also generated and inserted into the spinopelvic FE model. Based on a series of
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Fig 6. von Mises equivalent stress distribution of the screw in the spinopelvic model with the S2AI screw (90-mm screw length and 30Ê
head angle) under various loading conditions.
simulations under compression, flexion, extension, and axial rotation, the von Mises
equivalent stress values in the screws and bony structures were calculated, and the simulation results
An ideal internal fixation method should provide a maximum rigidity between the bone
segments and a minimum stress on the surrounding tissues for healing. Excessive stress
around the fixation devices can cause gradual resorption of the surrounding bone and
loosening of the screws. This is an important clinical aspect that must be considered when choosing
the appropriate rigid fixation system [
]. The simulation results in this study showed that the
maximum equivalent stress on the screw and bone around the screw in S2AI screw fixation
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Fig 7. Maximum equivalent stress in the two types of screws. Image (a) shows the iliac screw, and image (b) shows the S2AI screw.
was lower than that in iliac screw fixation. Thus, the S2AI screw method was a more suitable
spinopelvic fixation technique than that of the iliac screw method.
According to previous studies on the stability of spinopelvic fixation, S2AI screw fixation
can lead to a more reliable stability compared to iliac screw fixation owing to its longer screw
]. Conversely, Chang et al.  reported that the maximum mean iliac length
from the PSIS to the AIIS was 118 mm, which was longer than the maximum mean length
based on the ideal trajectory for the S2AI pathway (106 mm). Recent literature indicated that
iliac screw lengths chosen by surgeons, depending on the size of the pelvis, varied from
approximately 50±75 mm [
]. Thus, although the S2AI trajectory allows a shorter anchor
length, it still exceeds the length typically used. In one biomechanical study comparing S2AI
and iliac screws, it was explained that iliac fixation is generally a cancellous bone bed, while
S2AI screws have cortical purchase in the SIJ articulation and may offer additional strength
despite the shorter length [
]. In their study, the 65-mm S2AI screws were not
biomechanically different from the 80-mm S2AI screws or the 90-mm iliac screws. Moreover, they
postulated that the quad-cortical S2AI screw placement may improve its biomechanical property.
However, in this study, the difference in the maximum equivalent stress on the screw and
the bone around the screw owing to the screw length was confirmed. For the S2AI screw, as
the screw length increased, the magnitude of the stress on the screw decreased, and the
maximum equivalent stress value on the ilium around the screw decreased as well. There was no
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Fig 8. von Mises equivalent stress distribution of the bone around the iliac screw under various loading conditions.
large variation in the stress on the sacrum based on the screw length. Conversely, for the iliac
screw, there was no proportional relationship between the screw length and the maximum
equivalent stress on the screw. As described in the Results section, the maximum equivalent
stress occurred in the 75-mm screw length. Nevertheless, as the screw length increased, the
maximum equivalent stress on the ilium decreased, except in the flexion state.
Guler et al. [
] reported that the failure rate of the S2AI screws was 35%, and that of the
iliac screws was 12%, with lateral connectors. There were three cases implanted with S2AI
screws, in which the polyaxial screw head disintegrated from the screw shaft in 20 patients.
Compared to the monoaxial screw, the polyaxial screw is more widely used because the
surgeon can easily insert the rod into the screw head owing to its degrees of freedom. In many
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Fig 9. von Mises equivalent stress distribution of the bone around the S2AI screw (75-mm screw length and 15Ê head angle) under
various loading conditions.
studies comparing the biomechanical effects of monoaxial and polyaxial pedicle screw
fixations, it has been concluded that the load at the bone and implant interface may decrease as
the degree of freedom of the implant increases [35±39]. Fogel et al. [
] reported that the
polyaxial head coupling to the screw is the first to fail and may be a protective feature of the pedicle
screw, preventing pedicle screw breakage. In this study, the maximum equivalent stress values
on the S2AI screw and the bone around the screw were compared while changing the screw
head coupling angle to 15Ê and 30Ê. With respect to the stress, the 30Ê and 15Ê head angles for
the S2AI screw were advantageous to the screw and bone, respectively, since the low stress in
the material and/or structure implied a higher safety. The calculated maximum equivalent
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Fig 10. von Mises equivalent stress distribution of the bone around the S2AI screw (90-mm screw length and 30Ê head angle)
under various loading conditions.
stress ranges for the S2AI screw and ilium were approximately 54±110 MPa and 28±63 MPa,
respectively. Moreover, the yield stresses for Ti-6Al-4V and the cortical bone were 874 MPa
and 135 MPa, respectively [
]. Since the maximum equivalent stress of the screw was
lower than the yield stress of the screw, the head angle of the S2AI screw should be chosen
based on the bone material capacity. Therefore, the screw with a 15Ê head angle was more
suitable for the fixation method.
For the accuracy and verification of the simulation results, it was necessary to specify the
maximum von Mises equivalent stress point of each case. In the iliac screw, the maximum
equivalent stresses of the screws were at the point where the screw and iliac cortical bone met,
and the maximum equivalent stresses of the bones were in the iliac cortical bone at the SIJ.
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Fig 11. Maximum equivalent stress in the adjacent bone of the screws. Image (a) shows the iliac screw, and image (b) shows the sacrum with
the S2AI screw. Image (c) shows the ilium with the S2AI screw.
The maximum equivalent stresses occurred at the head portion of the screws in a few cases;
however, these were beyond the scope of this study. Therefore, the point measured was the
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one at which the SIJ was not in contact with the head. When the S2AI screw was inserted, it
was passed through the inside of the sacrum and ilium. At this time, the SIJ was penetrated
through the cortical bone, and the maximum equivalent stress point was in the sacral and iliac
The limitations of this study are addressed below. First, the threads of the pedicle, iliac, and
S2AI screws in the spinopelvic FE model were simplified to reduce the computational time
and analysis error owing to the stress concentration. However, to obtain an accurate stress
distribution on the screw and the bone around the screw, the threads of the screw should be
considered in the FE model. Specifically, the contact condition between the bone and implant
should be precisely considered during the FEA.
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Second, the spinopelvic FE model in this study was asymmetrical. Accordingly, the stress
distribution as well as the occurrence position of the maximum equivalent stress were not
even. To acquire precise simulation results, this problem should be solved.
Third, 11 types of ligaments were modeled as a spring element, and the intervertebral disc,
including the nucleus pulposus and annulus fibrosus, was simply fabricated using a solid
element. In addition, the endplate of the vertebra was ignored. Since the ligaments, intervertebral
disc, and endplate play a vital role in the biomechanics of the spinopelvis, it is necessary to
model these tissues in detail. For example, the ligaments could be modeled as a shell/solid
element, and the endplate and annulus ground substance should be considered.
Fourth, the pubic symphysis was modeled with 24 spring elements that could match the
equivalent stiffness of the symphysis owing to the convenience of FE modeling [
this tissue is a cartilaginous joint that is thick and stiff. Hence, it might be possible to obtain
the precise calculation results if this part was modeled as 3D solid or shell elements. Similarly,
the bone-screw interface and SIJ were postulated as a bonded contact; however, this is a
limitation of this study as well.
Fifth, this study required various parametric analyses to obtain highly accurate results. A
wide range of parametric analyses and numerical experiments using an advanced FE model
and FEA should be helped comparative analysis and improvement, provided to further the
optimal surgical plan and implant configuration.
In conclusion, by comparing the biomechanical characteristics of the S2AI screw and Iliac
screw, the S2AI screw showed a lower risk of implant failure than that of the iliac screw for
the spinopelvic fixation technique. From analyzing the length and angle characteristics, the
90-mm screw length as well as the 15Ê head angle for the S2AI screw were considered
Conceptualization: Jong Ki Shin, Tae Sik Goh, Seung Min Son, Jung Sub Lee.
Data curation: Jong Ki Shin, Tae Sik Goh, Seung Min Son, Hyung-Sik Kim.
Formal analysis: Beop-Yong Lim, Chi-Seung Lee.
Funding acquisition: Tae Sik Goh, Jung Sub Lee, Chi-Seung Lee.
Investigation: Jong Ki Shin, Hyung-Sik Kim, Chi-Seung Lee.
Methodology: Beop-Yong Lim, Chi-Seung Lee.
Project administration: Jong Ki Shin, Jung Sub Lee, Chi-Seung Lee.
Resources: Jong Ki Shin, Chi-Seung Lee.
Software: Beop-Yong Lim, Chi-Seung Lee.
Supervision: Jung Sub Lee.
Validation: Jong Ki Shin, Beop-Yong Lim, Chi-Seung Lee.
Visualization: Beop-Yong Lim, Chi-Seung Lee.
Writing ± original draft: Jong Ki Shin, Beop-Yong Lim, Chi-Seung Lee.
Writing ± review & editing: Tae Sik Goh, Seung Min Son, Hyung-Sik Kim, Jung Sub Lee.
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