SYMPOSIUM: ABJS CARL T. BRIGHTON WORKSHOP ON HIP PRESERVATION SURGERY
Pelvic Morphology Differs in Rotation and Obliquity Between
Developmental Dysplasia of the Hip and Retroversion
Moritz Tannast MD, Peter Pfannebecker MD,
Joseph M. Schwab MD, Christoph E. Albers MD,
Klaus A. Siebenrock MD, Lorenz Bu ¨chler MD
Published online: 14 July 2012
? The Association of Bone and Joint Surgeons1 2012
and acetabular retroversion represent distinct acetabular
pathomorphologies. Both are associated with alterations in
pelvic morphology. In cases where direct radiographic
assessment of the acetabulum is difficult or impossible or
in mixed cases of DDH and retroversion, additional indi-
rect pelvimetric parameters would help identify the major
underlying structural abnormality.
We asked: How does DDH and
retroversion differ with respect to rotation and coronal
obliquity as measured by the pelvic width index, anterior
inferior iliac spine (AIIS) sign, ilioischial angle, and
obturator index? And what is the predictive value of each
variable in detecting acetabular retroversion?
We reviewed AP pelvis radiographs for 51
dysplastic and 51 retroverted hips. Dysplasia was diagnosed
Developmental dysplasia of the hip (DDH)
based on a lateral center-edge angle of less than 20? and an
acetabular index of greater than 14?. Retroversion was
diagnosed based on a lateral center-edge angle of greater
than 25? and concomitant presence of the crossover/ischial
spine/posterior wall signs. We calculated sensitivity, spec-
ificity, and area under the receiver operating characteristic
(ROC) curve for each variable used to diagnose acetabular
We found a lower pelvic width index, higher
prevalence of the AIIS sign, higher ilioischial angle, and
lower obturator index in acetabular retroversion. The entire
innominate bone is internally rotated in DDH and exter-
nally rotated in retroversion. The areas under the ROC
curve were 0.969 (pelvic width index), 0.776 (AIIS sign),
0.971 (ilioischial angle), and 0.925 (obturator index).
Pelvic morphology is associated with ace-
tabular pathomorphology. Our measurements, except the
AIIS sign, are indirect indicators of acetabular retroversion.
The data suggest they can be used when the acetabular rim
is not clearly visible and retroversion is not obvious.
Level of Evidence
Level III, diagnostic study. See
Guidelines for Authors for a complete description of levels
Developmental dysplasia of the hip (DDH) and acetabular
retroversion represent distinct acetabular pathomorpholo-
gies. In DDH, the acetabulum is undercovered and often
excessively anteverted [7, 8, 16, 21]. In acetabular retro-
anteriorly . Both conditions lead to distinct pathome-
chanical problems: a dysplastic acetabulum leads to static
overload of the articular cartilage while a retroverted
One or more of the authors (JMS) has received fellowship funding
from the Maurice E. Mu ¨ller Foundation of North America. Each
author certifies that he or she, or a member of his or her immediate
family, has no commercial associations (eg, consultancies, stock
ownership, equity interest, patent/licensing arrangements, etc) that
might pose a conflict of interest in connection with the submitted
All ICMJE Conflict of Interest Forms for authors and Clinical
Orthopaedics and Related Research editors and board members are
on file with the publication and can be viewed on request.
Each author certifies that his or her institution approved the human
protocol for this investigation, that all investigations were conducted
in conformity with ethical principles of research, and that informed
consent for participation in the study was obtained.
M. Tannast (&), P. Pfannebecker, J. M. Schwab,
C. E. Albers, K. A. Siebenrock, L. Bu ¨chler
Department of Orthopaedic Surgery, Murtenstrasse,
Inselspital, University of Bern, 3010 Bern, Switzerland
Clin Orthop Relat Res (2012) 470:3297–3305
and Related Research®
A Publication of The Association of Bone and Joint Surgeons®
acetabulum leads to dynamic impingement between the
prominent anterosuperior aspect of the acetabulum and the
femoral head-neck junction.
Based on a number of reports and consistent with our
clinical observation, there are indicators that the pathologic
acetabular morphology in DDH and retroversion is asso-
ciated with alterations in pelvic morphology [1–7, 14, 15,
26]. Kojima et al. [14, 15], using three-dimensional (3D)
CT, showed a decrease in the transverse diameter of the
pelvic inlet and outlet in DDH, suggesting a general nar-
rowing of the bony pelvis. Fujii et al.  used CT to
examine rotational deformity of the innominate bone in
DDH. They noted internal rotation of the innominate bone
in dysplastic hips compared with controls. In addition, hips
with acetabular retroversion, both in the control group
(n = 4) and in the setting of DDH (n = 9), had externally
rotated innominate bones. While this suggests opposing
rotational abnormalities of the innominate bone between
acetabular retroversion and DDH, these two groups were
not compared exclusively.
By directly comparing the pelvic anatomy in DDH and
acetabular retroversion, we can establish association
between pelvic morphology and acetabular pathomor-
phology. This would allow us to diagnose acetabular
pathomorphology in cases where direct radiographic
assessment of the acetabulum is difficult or impossible.
Furthermore, using radiographic measurements on plain
AP pelvis radiographs would eliminate the need for addi-
tional, expensive, and sometimes radiation-intense imaging
Our goal, therefore, was to devise a method for assess-
ing pelvic morphology in the presence of acetabular
dysplasia and DDH using plain radiographs. To this end,
we developed four radiographic parameters: pelvic width
index, radiographic appearance of the anterior inferior iliac
spine (AIIS) sign, ilioischial angle, and obturator index.
For each of these four key measurement variables, we
asked two questions: (1) How do these variables differ
between dysplastic hips and retroverted hips? And (2) what
is the predictive value of each variable to detect acetabular
Patients and Methods
We performed a retrospective comparative study of
radiographic pelvic morphology between dysplastic and
retroverted hips. We reviewed radiographs from our insti-
tutional database of patients who underwent periacetabular
osteotomy (PAO) between March 2004 and March 2011
(n = 86) or surgical hip dislocation between March 2007
and March 2011 (n = 300). We defined the dysplasia
group using the following parameters: lateral center-edge
angle of less than 20?  and acetabular index of more
than 14? . We identified a total of 86 patients fitting
these inclusion criteria. We excluded patients undergoing
anteverting PAO (n = 17), incomplete radiographic doc-
umentation (n = 10), or previous hip or pelvis surgery
(n = 8). This left 51 patients in the dysplasia group. We
defined the retroversion group as a lateral center-edge
angle of more than 25? and the presence of three radio-
graphic signs: the crossover sign [10, 18], ischial spine sign
[12, 13], and posterior wall sign [18, 23]. We identified a
total of 317 patients fitting these inclusion criteria. We
excluded patients in whom not all three radiographic signs
were positive (n = 170) and with a previous history of hip
trauma (n = 39), previous hip or pelvis surgery (n = 25),
incomplete radiographic documentation (n = 14), Legg-
Calve ´-Perthes disease (n = 10), and protrusio acetabuli
(n = 8). This left 51 patients in the retroversion group. The
two groups were similar in terms of age, affected side,
weight, height, and BMI (Table 1). Our hospital’s institu-
tional review board approved the study.
Table 1. Demographic information
Dysplasia groupRetroversion groupp valueDysplasia group Retroversion groupp value
Number of hips392312 28
Age (years)* 29 ± 9.0 (15–45) 29 ± 12.0 (13–59)0.98526 ± 6.8 (16–36)24 ± 8.0 (13–42)0.369
Side (% right
of all hips)
Weight (kg)*64 ± 8.9 (48–82)71 ± 13.9 (51–107)0.09875 ± 11.6 (65–101)80 ± 14.1 (62–107)0.381
164 ± 5.4 (155–178)166 ± 9.1 (140–184)0.452 174 ± 10.0 (157–186) 180 ± 9.1 (165–203)0.180
24 ± 3.3 (18–32) 26 ± 6.1 (18–40)0.21525 ± 2.2 (23–30) 25 ± 3.2 (20–32)0.922
57 ± 13 (35–82) 69 ± 11 (47–93)
\0.001 38 ± 10 (26–55)54 ± 12 (30–75)
* Values are expressed as mean ± SD, with the range in parentheses.
3298Tannast et al.Clinical Orthopaedics and Related Research1
Since a substantial number of dysplastic hips have a
positive crossover sign , we further subdivided our
DDH group into those with and without a crossover sign to
evaluate how this affected our measurements. Twenty of
the 39 (51%) hips in the female DDH group and 11 of the
12 (92%) hips in the male DDH group had a positive
A standardized radiographic technique was performed
for all reviewed AP pelvis radiographs. All radiographs
were performed in the supine position. A film focus dis-
tance of 1.2 m was used with the beam centered between
the pubic symphysis and a line connecting the anterior
superior iliac spine with the pelvis in neutral rotation [12,
22, 23]. The longitudinal rotation of the pelvis was verified
as correct when the tip of the coccyx was in line with pubic
symphysis. Images were not specifically corrected for tilt;
however, we recorded the distance between the superior
pubic symphysis and sacrococcygeal junction for each
patient (Table 1).
Two of us (PP, CEA) independently evaluated the
following key measurement variables on each radiograph
(Table 2): (1) pelvic width index (Fig. 1A), (2) presence
of the AIIS sign (Fig. 1B), (3) ilioischial angle (Fig. 1C),
and (4) obturator index (Fig. 1D). The two observers
each performed two separate sets of measurements on
deidentified preoperative plain AP pelvis radiographs.
The two sets of measurements were performed a mini-
mum of 7 days apart. Inter- and intraobserver reliabilities
were tested for each parameter using the intraclass cor-
measurements and the kappa value for ordinal measure-
ments (Table 2).
Table 2. Descriptions and reliability/reproducibility of the four key measurement variables
index (Fig. 1A)
A line (Line a) is drawn from the pubic symphysis to the most lateral edge
of the ischial tuberosity, parallel to the interteardrop line. A parallel
line is drawn between the midpoint of the sacrum and the most lateral
point on the iliac wing (Line b). The index is calculated as the ratio of
the length of Line a to the length of Line b and is expressed as a
0.98 (0.96–0.99) 0.89 (0.77–0.95)
Anterior inferior iliac
spine sign (Fig. 1B)
The anterior inferior iliac spine is given a grade of 0 (indicating it is not
fully seen in profile) or 1 (indicating it is seen fully in profile).
0.83 (0.6–1.0)*0.69 (0.42–0.96)*
A line is drawn connecting the base of each radiographic teardrop (the
interteardrop line). A second line is then drawn from the intersection of
the ilioischial line and the iliopectineal line to the lateral-most point on
the ipsilateral obturator foramen. The inner angle between these two
lines is then measured.
0.92 (0.81–0.96) 0.92 (0.81–0.96)
A line (Line c) is drawn to the maximum width of the obturator foramen.
The index is calculated as the ratio of Line c to1
interteardrop line (Line d) and is expressed as a percentage.
2 of the length of the
0.97 (0.94–0.98) 0.97 (0.94–0.99)
* The kappa value was calculated for this variable; the ICC was calculated for other remaining variables; values are expressed as mean, with 95%
CI in parentheses; ICC = intraclass correlation coefficient.
Fig. 1A–D The diagrams illustrate how to (A) calculate the pelvic
width index (a/b), (B) determine the AIIS sign, (C) determine the
ilioischial angle, and (D) calculate the obturator index (c/d). For a
detailed explanation of these calculations, see Table 2.
Volume 470, Number 12, December 2012 Pelvic Morphology in DDH and Retroversion3299
Due to the lack of available comparable data in the lit-
erature, a pilot set of measurements was taken on a subset
of our patients to perform a power analysis. Based on these,
we detected a mean difference of 10% in the pelvic width
index with an estimated SD of 10%. Using these numbers,
a power analysis was performed and a minimum sample
size of 32 hips for each group was required to provide an a
of 0.01 and a b of 0.10.
Results from the complete set of measurements were
collected and stratified by group and sex. Normal distri-
bution was confirmed using the Kolmogorov-Smirnov test.
We compared groups using a paired t-test for continuous
variables and Fisher’s exact test for categorical variables.
To evaluate the overall predictive performance of our key
measurement variables, a receiver operating characteristic
(ROC) curve was calculated for each variable. We then
calculated the sensitivity and specificity of each test based
on thresholds detected for the ROC curve.
The pelvic width
p\0.001) in the retroversion group than in the dysplastic
group (Table 3). There was a higher (females: p\0.001;
males: p = 0.013) prevalence of the AIIS sign in the
index was smaller (both sexes:
retroversion group than in the dysplastic group. The ili-
oischial angle was higher (females: p\0.001; males:
p = 0.009) in the retroversion group than in the dysplastic
group. The obturator index was lower (both sexes:
p\0.001) in the retroversion group than in the dysplastic
group. When we subdivided our DDH hips into those with
and without a crossover sign, there was no difference
between the two groups for females (Table 4).
The greatest area under the ROC curve was found for
the ilioischial angle with a cutoff of 100? (0.971), followed
by the pelvic width index\56% (0.969) and the obturator
index\40% (0.925) (Fig. 2, Table 5). Compared to the
other key measurement variables, the pelvic width index
had the highest sensitivity (100% when pelvic width
index\56%), and the AIIS sign had the lowest sensitivity
(59%). The highest specificity was found in the AIIS sign
(96%), and the lowest specificity was found in the obturator
DDH and acetabular retroversion represent two distinct
acetabular pathomorphologies. As these disease processes
have become better understood, it is clear they include
not just alterations to the acetabulum but also distinct
Table 3. Results comparing the four key measurement variables for the dysplasia and retroversion groups
Dysplasia group Retroversion groupp value Dysplasia groupRetroversion group p value
Pelvic width index (%)*62 ± 5 (54–81)50 ± 6 (30–56)
57 ± 4 (50–61)49 ± 3 (44–55)
0.013Anterior inferior iliac spine
sign (% positive)
Ilioischial angle (?)*
Obturator index (%)*
3 578 50
96 ± 4 (84–102)104 ± 3 (97–109)
97 ± 3 (92–102)104 ± 3 (98–110)0.009
45 ± 7 (32–69) 28 ± 10 (7–44) 44 ± 5 (37–50)33 ± 6 (21–44)
* Values are expressed as mean ± SD, with the range in parentheses.
Table 4. Results comparing the four key measurement variables for dysplasia group with and without the COS
Dysplasia + COS
(n = 19)
Dysplasia ? COS
(n = 20)
p value Dysplasia + COS
(n = 11)
Dysplasia ? COS
(n = 1)
Pelvic width index (%)* 62 ± 4 (54–73) 63 ± 6 (56–81) 0.68957 ± 4 (50–61)52 NA
Anterior inferior iliac spine
sign (% positive)
Ilioischial angle (?)*
Obturator index (%)*
95 ± 5 (84–102)96 ± 3 (89–100)0.274 97 ± 3 (92–102)98NA
44 ± 8 (32–69)46 ± 5 (35–55)0.77144 ± 5 (37–50)38NA
* Values are expressed as mean ± SD, with the range in parentheses; COS = crossover sign; NA = not applicable.
3300Tannast et al. Clinical Orthopaedics and Related Research1
morphologic changes to the entire pelvis. To date, no direct
comparison has been made between the morphologic pel-
vic changes in these two diseases. However, there is
anthropologic evidence that hip function and pelvic mor-
phology are directly related . We observed a similar
pattern of pelvic morphology in our human subjects,
Fig. 2A–D The
used to determine the predictive
value for (A) the pelvic width
sign, (C) an ilioischial angle
[100?, and (D) an obturator
index\40%, are shown.
Table 5. Predictive value of the key measurement variables for
detection of acetabular retroversion
100 (85–100)90 (79–97)0.969
59 (36–79)96 (87–100)0.776
95 (77–100) 94 (84–99)0.971
89 (78–97) 86 (73–94)0.925
Values are expressed as mean, with 95% CI in parentheses; ROC =
receiver operating characteristic.
Fig. 3A–B (A) The right
dysplastic hip is compared to (B) an AP pelvis radiograph of a left hip
with acetabular retroversion. In both radiographs, the coccyx is in line
with the pubic symphysis, indicating no pelvic malrotation. In
addition, the vertical distance between the symphysis and the
sacrococcygeal joint is similar in both patients, indicating no
substantially different pelvic tilt. The right hemipelvis appears similar
to an obturator oblique view while the left hemipelvis appears similar
to an iliac oblique view.
2 of an AP pelvis radiograph of a
Volume 470, Number 12, December 2012Pelvic Morphology in DDH and Retroversion 3301
Table 6. Pelvic measurement data
Flu ¨ckiger et al. 
Kojima et al. 
Trousdale et al. 
Fujii et al. 
DDH (n = 17)
DDH (n = 40)
DDH (n = 7)
DDH (n = 50)
DDH (n = 51)
Retroversion (n = 51)
Widest area of pelvis between iliac
Bilateral centers of the internal
surface of the acetabulum
Line between the two ischial
Bilateral internal surface of
inferior ischial tuberosity
AP inlet (cm)*
symphysis to sacral promontory
AP midpelvis (cm)*
Posterior-inferior pubic symphysis
to sacrum, passing through
midpoint of bispinous line
of the pelvic
S2-S3 vertebral junction and the
middle posterior pubic
Superior iliac angle (?)?
Opening angles of the ilium at the
anterior superior iliac spine
57.0 ± 6.1
Inferior iliac angle (?)?
Opening angles of the ilium at the
anterior inferior iliac spine
72.0 ± 4.3
Ischiopubic angle (?)?
Closing angle of the ischiopubic
30.6 ± 2.7
* Values are expressed as mean, with the range in parentheses;
?values are expressed as mean ± SD; DDH = developmental dysplasia of the hip; NA = not available; 3D = three-
3302Tannast et al.Clinical Orthopaedics and Related Research1
specifically associated with their hip disease. Our goal was
to directly compare four key measurement variables (pelvic
width index, presence of AIIS sign, ilioischial angle,
obturator index) and evaluate their predictive value for
these two different morphologies.
The major limitation to our study is that we only com-
pared two morphologic extremes. Clearly DDH and
acetabular retroversion exist on a spectrum, and mixed-type
pathology is common . While our key measurement
variables show good predictive value and reproducibility
radiographic crossover sign, we have not explicitly evalu-
ated them in the setting of mixed morphology. Second, we
CT, and specifically 3D CT, can provide precise measure-
ments to determinerotational properties of the pelvis inboth
to measure rotation directly, plain radiographs are most
frequently the first,and sometimes the only, imagingstudies
performed on patients seeking treatment for hip disease.
While using plain radiographs limits our ability to directly
compare our values to previously published rotational
measurements performed on CT or MRI [7, 11, 20], we
believe the use of an APpelvisradiograph isfaster and more
for tilt when evaluating these radiographs. When we eval-
uated each group (DDH and retroversion) subdivided by
sex, we saw the difference in means for the symphysis-
sacrococcygeal distance was 12 mm in females and 16 mm
in males (Table 1). This translated to a difference in pelvic
extent to which this variation in tilt affected our measure-
Our results support the theory that the entire hemipelvis
is involved in both acetabular dysplasia and retroversion. In
DDH, the pelvis appears to be internally rotated around a
sagittal axis. This leads to the radiographic appearance of
the hemipelvis mimicking an obturator oblique view
(Fig. 3A). Focal acetabular dysplasia has been described in
cases of neuromuscular hip disease , but our cohort of
patients and multiple other reports [2, 7, 11, 20] confirm
the entire hemipelvis is involved. Even in cases of hip
dysplasia with a concomitant crossover sign, our mea-
surements do not show any difference from those
performed in hips without a crossover sign . In acetab-
ular retroversion, the hemipelvis appears to be externally
rotated around a sagittal axis, leading to the radiographic
appearance of an iliac oblique view (Fig. 3B). Previously,
only the inferior hemipelvis was implicated in acetabular
As part of our radiographic review protocol, we also
measured several standard pelvic measurements that have
been reported previously (Table 6). The fact that our
increases the validity of our key measurement variables.
Therefore, we believe these key measurement variables
allow detection of hemipelvis version on a plain AP pelvis
radiograph, independent of sex.
Based on the predictive value of each key measurement
variable, and independent of sex, a retroverted hip is likely
to be present if the pelvic width index is less than 50%, the
AIIS outline is clearly visible, the ilioischial angle is
greater than 100?, and the obturator index is less than 40%.
These guidelines are helpful for several reasons. First,
these measurements allow one to infer acetabular mor-
phology from existing pelvic morphology, especially when
clear radiographic visualization of the acetabulum may be
difficult. It has been established that pathomorphologic
pelvic changes relate to acetabular disease manifesting at
an early age before ossification of the acetabulum is
complete [2, 20]. Therefore, it is reasonable to assume a
pediatric patient who does not yet display a fully ossified
acetabular rim may have occult acetabular retroversion that
can be detected by our pelvic measurements (Fig. 4). It
should be noted, however, we have not specifically studied
these measurements in the setting of skeletally immature
pelvises. Similarly, in the setting of a THA, where the
normal acetabular rim may be obscured by osteophytes,
these measurements may help preoperatively identify a
retroverted native acetabulum (Fig. 5). Finally, these
measurements can assist in decision making between
reorientation and rim-trimming procedures. For instance,
when ourfour keyvariables
Fig. 4A–B (A) A radiograph of the left hip of a child at age 13 years
shows indistinct anterior and posterior acetabular walls, an elevated
pelvic width index, positive AIIS sign, elevated ilioischial angle, and
decreased obturator index, all indicating likely acetabular retrover-
sion. (B) By age 15 years, the acetabular walls have ossified and
retroversion is plainly evident.
Volume 470, Number 12, December 2012Pelvic Morphology in DDH and Retroversion 3303
retroversion with associated pelvic pathomorphology, one
should consider acetabular reorientation over a rim-
trimming procedure . In our experience, we use these
measurements as an adjunct to the standard radiographic
parameters to identify the major pathology to avoid
inappropriate treatment, but further clinical studies could
help clarify an algorithm for using these parameters.
Distinct pelvic morphology is present in both DDH and
acetabular retroversion. Based on our measurements, the
sagittal rotation and coronal obliquity of the entire innomi-
nate bone are directly related to these two acetabular
pelvimetric parameters indicating DDH and acetabular ret-
roversion on an AP pelvis radiograph. Recognition of these
parameters will help to understand complex morphology in
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