Open Journal of Orthopedics, 2013, 3, 199-203
http://dx.doi.org/10.4236/ojo.2013.34036 Published Online August 2013 (http://www.scirp.org/journal/ojo)
199
Effects of Curved Periacetabular Osteotomy on the Stress
Fields of the Pubic Rami and Ischium: A Finite Element
Model Analysis
Norio Imai1*, Yoichiro Dohmae1, Ken Suda2, Dai Miyasaka2, Tomoyuki Ito2, Naoto Endo2
1Department of Orthopedic Surgery, Niigata Prefectural Shibata Hospital, Niigata, Japan; 2Department of Orthopedic Surgery, Nii-
gata University Medical and Dental Hospital, Niigata, Japan.
Received May 18th, 2013; revised June 19th, 2013; accepted July 1st, 2013
Copyright © 2013 Norio Imai et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ABSTRACT
Background: Postoperative pubic or ischial stress fracture may be a complication after curved periacetabular osteotomy
(CPO). The discontinuity of the superior pubic rami is a risk factor for this complication. We investigated the stress
field differences in standing and sitting position s after CPO. Methods: We used finite element analysis to assess the ef-
fects of inferior pubic rami and ischial fractures with or without discontinu ity of superior pubic rami. We used the “un-
ion model”, obtained from a bony union at the osteotomy site of the superior pubic rami from 38-year-old woman who
had undergone CPO for left hip dysplasia. We deleted the bony union region and created a discontinuity in the superior
pubic rami equal to the non-union, creating the “discon tinuity model”. We compared the stress field and stress value in
the simulated standing and half weight-bearing positions on the operative side, one-legged standing position on the
non-operative side, and th e sitting position. Findings: In 4 cases, the inferior rami experienced the highest stress. Stress
values in the discon tinuity model were higher than those in the union model: 1.7 times in the case of one-leg ged stand-
ing on the operative side, 2.4 times in the case of half weight-bearing on the operative side, 3.8 times in the case of
one-legged standing on the non-operative side, and 2.0 times in the sitting position, respectively. Interpretation: We
recommend patients delay weight bearing on the operative side, avoid the sitting position as long as possible, and sit
down slowly to prevent inferior pubic rami and ischial fractures after CPO.
Keywords: Curved Periacetabular Osteotomy; Ischial Fracture; Inferior Pubic Ramifracture; Stress Field; Finite
Element Analysis
1. Introduction
Acetabular osteotomies are useful not only for increasing
the covering of the femoral head, but also for medializa-
tion of the femoral head. Therefore, they are considered a
useful treatment for hip dysplasia in young patients [1-4].
However, the method requires detachment of the gluteus
medius muscle; consequently, the surgery is significantly
invasive. For this reason, curved periacetabular osteot-
omy (CPO) [4], considered less invasive than other ace-
tabular osteotomies, was adopted as the treat ment of first
choice for hip dysplasia in young patients in our institu-
tion. However, due to the technical difficulty of these
procedures, various complications have been noted, such
as collapse of rotational fragment, heterotopic ossifica-
tion, and infection [5-8].
Postoperative pubic or ischial stress fracture has also
been referred as a complication in some reports, and dis-
continuity of superior pubic rami [9,10] and narrowing of
the inferior pubic rami [10,11] have been considered as
risk factors. However, a detailed analysis with regard to
load distribution after CPO has not been reported. More-
over, we employ the same rehabilitation program for pa-
tients at risk as for those who do not exhibit the risk fac-
tors, because the fractures do not significantly influence
walking or hospital stay [9,10].
The purpose of this study was to investigate stress
field differences in standing and sitting positions after
CPO. We used computer simulation with finite element
analysis to assess effects of inferior pubic rami and is-
chial fracture with or without discontinuity of superior
*Corresponding a uthor.

Effects of Curved Periacetabular Osteotomy on the Stress Fields
of the Pubic Rami and Ischium: A Finite Element Model Analysis
200
pubic rami. Moreover, we provide recommendations con-
cerning the rehabilitation program of patients with dis-
continuity of the superior pubic rami.
2. Materials and Methods
2.1. Subject
The model pelvis and proximal femur were obtained
from a 38-year-old woman who had undergone CPO for
left hip dysplasia and who underwent a computed tomo-
graphy (CT) examination 3 months after surgery, when
the bony union was still not complete. In these patients,
bony union was observed at the osteotomy site of the
superior pubic rami (Figure 1). The Institutional Review
Board of Niigata Prefectural Shibata Hospital approved
the study and informed consent was obtained from the
patient.
2.2. Geometry and Material Properties
Geometrical data were obtained using a high-definition
three-dimensional digitizer (Mimics, Materialise Japan
Co. Ltd, Yokohama, Japan). After digitizing the pelvis,
the coordinates were imported in to a commercially avail-
able finite element modeling software package (SIMU-
LIA Abaqus FEA, IDAJ Co. Ltd, Yokohama, Japan) for
three-dimensional finite mesh reconstruction. The model
was divided into a cortical bone and a cancellous bone,
consisting of approximately 800,000 elements and 200,000
nodes. The material properties were as follows: Young’s
modulus and Poisson’s ratio were 17,000 MPa and 0.3,
respectively, in cortical bone and 70 MPa and 0.2, re-
spectively, in cancellous bone [12].
2.3. Loading and Boundary Conditions
In the standing position, the anterior pelvic plane, con-
sisting of the bilateral anterior superior iliac spine and
Figure 1. Union model. Pelvic model after CPO on the left
side. In this model, bony union was obtained at osteotomy
site of superior pubic rami.
pubic symphysis, was parallel to vertical, and the supe-
rior endplate of the sacru m and non-w eigh ted femur w ere
rigidly f ixed. We app lied a fo rce of 435 N for one-legg ed
standing position and 218 N for half weight bearing, re-
spectively, to the femur from distal. These values were
calculated based on the weight of one leg, approximately
15% of a total weight (preoperative body weight) of 50
kg [10]. The friction factor of the hip joint was assumed
to be 0.01 [12]. In the sitting position, th e anterior pelvic
plane was tilted 25˚ posteriorly, based on a previous re-
port [13], and the superior endplate of the sacrum was
rigidly fixed. We applied a force of 174 N from the most
distal point of each bilateral ischium. This value was
calculated based on the weight of two legs and a total
weight of 50 kg (348 N); each leg received half the force
(174 N).
Our study model was originally obtained from a bony
union at the osteotomy site of the superior pubic rami
(Figure 1); this is defined as the “union model”. We de-
leted the bony union region similar to the osteotomy line
by a length of 5 mm, and created a discontinuity in the
superior pubic rami equal to non-union; this is defined as
the “discontinuity model” (Figure 2). Subsequently, we
investigated the difference of load distribution between
these 2 m o d e ls.
3. Results
In all 4 cases the inferior rami experienced the highest
stress (Figures 3-6).
In the case of one-legged standing position , in the dis-
continuity model, the stress field >5.0 MPa was distrib-
uted from the inferior rami to the ischium. By contrast, in
the union model, the stress field >5.0 MPa was not ob-
served (Figure 3). Similarly, in the discontinuity model,
the stress field >2.5 MPa was distributed from the infe-
rior rami to the ischium, whereas, in the union models,
Figure 2. Discontinuity model. In this model, we partially
deleted the bony union and created a discontinuity at os-
teotomy site of superior pubic rami.

Effects of Curved Periacetabular Osteotomy on the Stress Fields
of the Pubic Rami and Ischium: A Finite Element Model Analysis
201
Figure 3. One-legged standing on the operative side. The
highest load was 7.97 MPa in discontinuity model (a) and
4.61 MPa in union model (b).
Figure 4. Half weight bearing on the operative side. The
highest load was 3.99 MPa in discontinuity model (a) and
1.68 MPa in union model (b).
Figure 5. One-legged standing on the non-operative side.
The highest load was 2.69 MPa in discontinuity model (a)
and 0.71 MPa in union model (b).
Figure 6. Sitting position. The highest load was 8.47 MPa in
discontinuity model (a) and 4.18 M P a in union model (b).
the stress field >2.5 MPa was not observed in cases of
half weight-b earing on the operative side and on e-legged
standing on the non-operative side (Figures 4 and 5). In
case of sitting position, in the discontinuity models, the
stress field >5.0 MPa was observed at two places in the
inferior rami. In the union models, the stress field >5.0
MPa was not observed, but stress field was higher than
others, similar to that in the discontinuity model (Figure
6).
Moreover, stress values in the discontinuity model
were higher than those in the union model: 1.73 times
higher in the case of one-legged standing on the opera-
tive side (7.97 MPa vs. 4.61 MPa in average; Figure 3),
2.38 times higher in the case of half weight bearing on
the operative side (3.99 MPa vs. 1.68 MPa in average;
Figure 4), 3.79 times higher in the case of one leg stand-
ing on the non-oper ative side (2.69 MPa vs. 0.71 MPa in
average; Figure 5), and 2.02 times higher in the sitting
position (8.47 MPa vs. 4.18 MPa in average; Figure 6).
4. Discussion
Stress fractures in the pubic and ischial bones are rela-
tively rare, and published reports have mostly concen-
trated the sports-related injuries in young patients [14-16].
A few reports have described the pubic or ischial fracture
as a complication of acetabular osteotomies, including
CPO [9-11]. In these reports, fractures of the inferior
pubic rami and ischial fractures were observed in ap-
proximately 3% - 20% of patients who had undergone
acetabular osteotomies [9-11].
With regard to etiological factors for these fractures,
[11] demonstrated that cases with a discontinuity at the
site of the superior pubic osteotomy accounted for as
many as 80% of all cases with inferior pubic or ischial
fractures, a statistically significant increase compared to
cases without pubic or ischial fracture (4.5%). They in-
ferred that the fractures derive from disruption of conti-
nuity at the site of pubic osteotomy, which alters load
transfer from the lower extremities, based on past reports
[17,18]. Our model demonstrates that with discontinuity
at the site of osteotomy of the superior pubic rami, the
stress field is concentrated from the inferior pubic rami to
ischial bone, especially on the proximal inferior pubic
rami. These results are consistent with the hypothesis of
Tsuboi et al. Postoperative inferior pubic rami fractures
and ischial fractures are relatively rare and do not affect
the clinical outcome; only 0.3% of cases required further
surgical treatment for painful non-unions. Therefore,
most surgeons do not change the rehabilitation schedule,
because there is no adequate treatment to help prevent
these fractures if a discontinuity at the osteotomy site is
observed. In our institution, patients are permitted to use
a wheelchair 2 days after surgery. Ten-kilogram partial
weight bearing is permitted with 2 crutch es after 2 weeks.
Subsequently, an increase in weight bearing of 10 kg per
week is permitted, and crutches are removed between 3
and 6 months after the operation if the patient can stand
on one leg.
According to several reports [9-11], most fractures
occurred as early as 3 months after surgery. The osteot-
omy site of the inferior rami and ischium may become
stronger with a gradually increasing load over a long

Effects of Curved Periacetabular Osteotomy on the Stress Fields
of the Pubic Rami and Ischium: A Finite Element Model Analysis
202
period of time. For a patient with discontinuity at the
osteotomy site, we suggest that weight bearing in the
operative side be delayed, and we especially recommend
non-weight bearing on the operative side. Moreover, we
also recommend that patients avoid the sitting position
for as long as possible, and take care to sit down slowly.
A limitation of this study is the examination of the
only one pelvic model. The load distribution may be dif-
ferent from other pelvic models because of variation in
pelvic morphology, especially in the width of the infe-
rior pubic rami or in bone mineral density. Moreover, the
discontinuity model in this current study was recon-
structed by virtual deletion and not by using an actual CT
scan from a patient before healing of superior pubic rami.
However, if the model was used before healing of supe-
rior pubic rami, other osteotomy sites, such as ilium
around the acetabulum will not join, therefore, the dis-
tribution may be different. Furthermore, the deletion
width in this study was defined as 5 mm, but the actual
width will be irregular. Therefore, the stress locations or
value may be affected by the deletion width. However,
the location of the stress field was similar to that in our
previous study [10] in a different pelvic model; therefore,
the results of this current study are valid at least with
regard to the stress field. Furthermore, in this study, we
applied a force of 174 N from most distal point of each
bilateral ischium in the sitting model. There are many
muscles and soft tissues around the buttocks, so the load
distribution of the inferior pubic rami and ischium may
be smaller in actuality.
In conclusion, we demonstrated that discontinuity at
the osteotomy site of the superior pubic rami increased
the load on the inferior pubic rami and ischium on the
osteotomy side. Surgeons should be aware of the fact that
these fractures are complications that occur at a relatively
early stage after acetabular osteotomies, including CPO.
Furthermore, we recommend that weight bearing on the
operative side be delayed, and that patients avoid the
sitting position for as long as possible, taking care to sit
down slowly to prevent inferior pubic rami and ischial
fractures after acetabular osteotomies.
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