The Journal of Arthroplasty 36 (2021) 379e386

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The Journal of Arthroplasty

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Basic Science

Anteversion Angle Measurement in Suspected Torsional

Malalignment of the Femur in 3-Dimensional EOS vs Computed
TomographydA Validation Study
Hermann O. Mayr, MD a, *, Jan-Peter Schmidt, MD b, Florian Haasters, MD c,
Anke Bernstein, PhD a, Hagen Schmal, MD a, d, Wolf C. Prall, MD a, c
a
Department of Orthopedics and Trauma Surgery, Medical Center, Faculty of Medicine, Albert-Ludwigs-University of Freiburg, Freiburg, Germany
b
Department of Knee, Hip and Shoulder Surgery, Schoen Clinic Munich Harlaching, Academic Teaching Hospital of the Paracelsus Medical
University (PMU), Salzburg, Austria
c
Department of General, Trauma and Reconstructive Surgery, Munich University Hospital, Ludwig-Maximilians-University (LMU), Munich,
Germany
d
Department of Orthopaedic Surgery, University Hospital Odense, Odense, Denmark

a r t i c l e i n f o a b s t r a c t

Article history: Background: Computed tomography (CT) scan is the standard for assessment of femoral torsion. This obser-
Received 25 April 2020 vational study was conducted to evaluate the comparability of the EOS radiation dose scanning system (EOS
Received in revised form imaging, Paris, France) and the CT scan in patients with suspected torsional malalignment of the femur.
19 July 2020
Methods: Patients with suspected torsional malalignment of the femur were included in a study for
Accepted 22 July 2020
Available online 30 July 2020
surgical planning. The primary endpoint was to compare the 3-dimensional radiological (EOS) imaging
system with the CT scan to determine femoral anteversion (AV) angle. Three independent raters per-
formed measurements. Comparability of CT scan and EOS values was assessed by Pearson correlation, t
Keywords:
EOS
test, interobserver reliability, and intraobserver reliability (Cronbach alpha).
CT scan Results: About 34 femora were examined. Interobserver reliability/intraobserver reliability was 0.911 of
torsional malalignment hip 0.955 for EOS and 0.934 of 0.934 for CT scan. EOS system revealed an AV angle of 12.2
± 10.0
(15.0
to
rotational alignment femur 32.0
). CT examinations showed an AV angle of 12.6
± 9.2
(3.2
to 35.6
). About 11 hips featured
measurement of rotation physiological AV, 14 hips showed decreased AV (<10
) or retroversion (<0
), and 9 hips showed increased
reliability AV (>20
). Overall, a strong Pearson correlation of t ¼ 0.855 and a highly significant correlation in the t
test for both methods was seen. In patients with decreased AV, retroversion, or increased AV, Pearson
correlation only resulted in a moderate/low correlation of t ¼ 0.495 and t ¼ 0.292. The t test showed no
significant correlation at malrotation.
Conclusion: In torsional malalignment, EOS does not have correlation with CT measurements. In contrast
to CT scan, EOS allows femoral torsion measurement independent of legs' positioning.
© 2020 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/).

The anteversion (AV) angle of the proximal femur is an impor- arthroplasty [5,6]. It is crucial regarding the restoration of an
tant parameter in planning and postoperative assessment of anatomic joint mechanism and the reconstruction of the center of
various surgical interventions [1e4]. Above all, knowledge of the rotation [5]. AV of the femur is also of particular importance when
individual AV of the femur is important for planning a total hip planning and performing corrective osteotomies in congenital hip
dysplasia [7,8]. Gold standard for torsion measurements of the
lower extremity is the computed tomography (CT) scan. Therefore,
One or more of the authors of this paper have disclosed potential or pertinent the CT scan is also used to determine the femoral AV angle [9,10].
conflicts of interest, which may include receipt of payment, either direct or indirect, The biplanar low-dose EOS system (EOS imaging, Paris, France)
institutional support, or association with an entity in the biomedical field which was developed to provide a low-radiation method for analysis of
may be perceived to have potential conflict of interest with this work. For full whole-body and whole-leg angles [11]. The operational principles
disclosure statements refer to https://doi.org/10.1016/j.arth.2020.07.058.
of the EOS 2-dimensional/3-dimensional (3D) system are as fol-
* Reprint requests: Hermann O. Mayr, MD, Department of Orthopedics and
Trauma Surgery, Medical Center, Faculty of Medicine, Albert-Ludwigs-University of lows. Perpendicularly vertical-moving X-ray tubes create very thin
Freiburg, Hugstetter Str. 55, 79106 Freiburg, Germany. collimated X-ray beams, collected by unique line detectors. It is a

https://doi.org/10.1016/j.arth.2020.07.058
0883-5403/© 2020 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
380 H.O. Mayr et al. / The Journal of Arthroplasty 36 (2021) 379e386

Fig. 1. Determination of the femoral neck axis using the Lee method [12e14]. The individual angles are determined between the femoral neck axis and the horizontal and between
the femoral condyle tangent of the trailing edge and the horizontal.

slot scanning system using ultralow radiation doses with the pa- planes. There was no additional radiation exposure for the patient
tient in an upright load-bearing position followed by 3D recon- because the EOS whole-leg image was used to determine all axes
struction of the skeletal system. A special software (SterEOS; EOS and angles of the lower extremity, and no additional conventional
imaging, Paris, France) allows the generation of these 3D models X-ray image was necessary. A 3D model was created using the EOS
from the biplanar EOS scanning images. Using these 3D models, software. The AV angles were then determined from this 3D model
torsion parameters such as the AV angle of the proximal femur and of the femur.
other angles can be calculated. There has been no study to date,
validating EOS vs CT scan when measuring the AV angle in patients Exclusion Criteria
with suspected torsional malalignment of the femur. To our
knowledge, this is the first study comparing these 2 measurement Condition after treatment with knee or hip arthroplasty, pro-
methods in case of suspected maltorsion of the proximal femur. nounced acetabular dysplasia, patient age younger than 18 years
and older than 80 years, time after corrective osteotomy of the
Materials and Methods lower extremity and/or the pelvis, pregnant patients, patients who
are unable to receive information or to consent, and pronounced
Patients with a suspected torsional malalignment of the femur post-traumatic malposition.
and an indication for surgical treatment of hip joint osteoarthritis,
patellofemoral instability, or unicompartmental knee joint osteo- CT Measurement Method
arthritis were included in the study. The suspicion of torsional
malalignment of the femur was derived from the gait pattern, The femoral torsion was determined in CT scan as the angle
clinical examination of the hip rotation, patella position, and po- between the axis of the femoral neck and the tangent to the femoral
sition of the foot. If available, existing X-ray images were evaluated. condyles. The Lee method [12,13] (transversal method) was used to
For ethical reasons, only patients with an indication for surgical determine the AV angle (Fig. 1). The axis of the femoral neck was
treatment were included in the study to avoid unnecessary radia- determined as a connecting line drawn from the center of the
tion exposure to other patients. As usual, a CT scan was carried out femoral head to the center of the femoral neck. The first axial
to determine the AV angle of the proximal femur. Then these pa- section of the scan with a visible connection between the femoral
tients underwent a biplanar low-dose EOS X-ray of the pelvis and neck and the greater trochanter was used for this. All CT mea-
leg statics for preoperative clarification and operative planning surements were carried out by 3 independent orthopedic surgeons
instead of the conventional leg full-length image and X-rays in 2 with verified additional qualification in skeleton radiography and
 H.O. Mayr et al. / The Journal of Arthroplasty 36 (2021) 379e386 381

Fig. 2. SterEOS software, completed measurement with measurement parameters plotted. Source: SterEOS workflow hip. AV, anteversion.
(Courtesy of EOS imaging, Paris, France)

CT scan diagnostics. All measurements were repeated at a second calculates the data. The measurement parameters are then available
time by the respective rater, without knowing the previous values. according to the set workflow (Fig. 2). In the eighth step, it is possible
to create a 3D model for visual inspection (Fig. 3). All measurements
were carried out by 3 independent orthopedic surgeons with veri-
EOS Measurement Method fied additional qualification in skeleton radiography. All measure-
ments were repeated at a second time by the respective rater,
A simultaneous scan in the anteroposterior (AP) and lateral di- without knowing the previous values.
rections was carried out in all patients. A whole-leg scan of both Contraindications to the use of the hip and pelvis workflows in
lower extremities was performed. The lower limb or the hip work- the Lee method are pronounced dysplasia, deformities, and the
flow was used to calculate the femoral AV. The following data were presence of a hip or knee arthroplasty. In these cases, a precise
obtained when performing the hip workflow at the end of the assignment of the required bony reference points is not possible.
measurement: femoral offset, cervicodiaphyseal angle (CCD),
femoral neck length, diameter femoral head, femoral torsion, and
femoral length. The workflow of SterEOS software encompasses the Statistics
following steps. In step 1, when using the lower limb workflow, the 3
reference points of the femoral center, trochlea, notch, and femoral Statistical evaluation was carried out using the SPSS statistics pro-
diaphysis are selected. This is done for the AP and lateral planes. In gram (version 26; IBM Corporation, Armonk, NY). Pearson correlation
the case of the hip workflow, only the landmarks of the thigh are coefficient and the 2-sided t test of the mean values for matched
marked. In step 2, the previously selected points are manually fine samples were calculated to compare the 2 measurement methods.
tuned. The lateral and medial femoral condyles are marked. The Based on the power calculation, when there was a correlation of t ¼
tangent to the femoral condyles is then determined to calculate the 0.47, with a desired power of 80% and a bilateral significance level of 5%,
AV angle. In step 3, the trochanter major is first marked in the AP a necessary number of 33 hips was calculated to obtain a statistically
view. In step 4, the femoral neck axis is defined using 2 reference significant result when using the Pearson correlation to discover a
points. In step 5, the last bony reference points are drawn. The lateral correlation coefficient of P s 0. The intraclass correlation coefficient
and medial as well as the ventral and dorsal cortex of the femoral (ICC) was determined to calculate the inter-rater reliability between 3
diaphysis are determined in both planes. In step 6, it is again possible independent raters. The intraobserver reliability was determined by
to manually adjust reference points. Here, improvements can be Cronbach alpha. The primary endpoint of the study was the comparison
made in every area of the femur to define the later 3D model as of the measured AV of the femoral neck between the 2 measurement
precisely as possible. In step 7, the SterEOS software automatically methods; EOS and CT in the Pearson product-moment correlation.
382 H.O. Mayr et al. / The Journal of Arthroplasty 36 (2021) 379e386

Fig. 3. SterEOS software, 3-dimensional model of the hip workflow. Source: SterEOS workflow hip.
(Courtesy of EOS imaging, Paris, France)

Institutional Review Board extremity. Torsional malalignment was not confirmed in 11 hips. 14
hips showed a reduced AV (<10
) or retroversion (<0
), and 9 hips
The study was approved by the Ethics Committee of the Albert- showed an increased AV (>20
) (Table 1).
Ludwigs-University of Freiburg, Germany with confirmation from
June 5, 2018, application number 586/17. All patients were informed Mean Values
that CT scan and EOS in combination are currently being used in
combination with the clinical parameters to plan the optimal therapy Determining the AV angle in the EOS system resulted in a total
and optimal analysis of complex malalignment. The radiation expo- mean value of 12.2
± 10.0
(15.0
to 32.0
). Slight differences
sure of both methods was described. The aim of the study was were found in the comparison of the right and left sides with an
explained. Patients were advised that participation in the study was EOS right mean of 13.4
± 2.4
(12.0
to 32.0
) and an EOS left
voluntary. All information was provided, and consent forms were mean of 11.1
± 2.8
(15.0
to 30.0
). The CT scan examinations
signed by the head of the study and respective patient. The institu- showed overall mean values of 12.6
± 9.2
(3.2
to 35.6
). Dif-
tional review board had been assured that there were no additional ferences from left to right with CT scan were revealed with a CT
risks for patients as these were routine procedures. It had been right mean of 13.5
± 2.2
(3.2
to 35.6
) and a CT left mean of
clarified that it was the objective of the study to establish a method 11.6
± 2.1
(2.8
to 29.6
). A comparison of the CT scan vs EOS
that would reduce the radiation exposure of future patients. showed an average deviation of 0.4
for the total mean value.
Furthermore, it was assured that only patients with clinical suspicion Including all values that showed reduced AV (<10
) or retroversion
of a suspected torsional malalignment of the femur and simultaneous (<0
) in either EOS or CT scan, 14 hips were measured with an EOS
indication for surgical treatment of hip joint osteoarthritis, patello- mean value of 4.2
± 7.4
(15.0
to 13.0
) and a CT scan mean value
femoral instability, or unicompartmental knee joint osteoarthritis of 4.0
± 3.8
(3.2
to 12.5
). Including all values that showed
were included in the study. The power calculation was presented. increased AV (>20
), in either EOS or CT scan, 9 hips were measured
with an EOS mean value of 25.4
± 2.3
(19.0
-32.0
) and a CT scan
Results mean value of 24.2
± 2.3
(17.2
-35.6
).

About 34 femora measured from 19 patients (15 women and 4 Pearson Correlation Coefficient, Inter-Rater Reliability, and
men) with an average age of 45.5 ± 19.8 years took part in this Intrarater Reliability
study. Both hips were measured in 15 patients. One-sided mea-
surement was performed in 4 patients. In these patients, there were Pearson correlation coefficient for all hips was included in the
exclusion criteria for determining AV using EOS on the other study, and the hips with regular AV shows a strong correlation
 H.O. Mayr et al. / The Journal of Arthroplasty 36 (2021) 379e386 383

Table 1
Individual AV Angles for EOS and CT Scan Measured by 3 Raters Each in 2 Laps.

Patient No. CT Scan EOS

Rater 1 Rater 2 Rater 3 Mean Rater 1 Rater 2 Rater 3 Mean

Lap 1 Lap 2 Lap 1 Lap 2 Lap 1 Lap 2 Lap 1 Lap 2 Lap 1 Lap 2 Lap 1 Lap 2

Left hip
1 1.0 5.9 4.4 0.5 5.2 2.1 3.2 3.0 0.0 1.0 4.0 4.0 5.0 2.8
2 12.0 15.1 14.6 10.5 15.6 14.2 13.7 12.0 10.0 10.0 17.0 19.0 12.0 13.3
3 2.0 5.4 2.3 1.2 5.2 2.1 3.0 16.0 14.0 14.0 12.0 17.0 14.0 14.5
4 29.3 31.4 34.4 29.0 31.2 35.6 31.8 30.0 28.0 31.0 27.0 28.0 26.0 28.3
5 21.1 17.2 20.4 21.5 17.8 22.4 20.1 27.0 31.0 31.0 27.0 27.0 25.0 28.0
6 10.9 14.2 15.3 10.8 10.9 16.3 13.1 12.0 14.0 14.0 12.0 13.0 11.0 12.7
7 15.8 11.2 18.6 15.1 15.1 18.4 15.7 14.0 11.0 11.0 16.0 19.0 14.0 14.2
8 3.7 0.3 5.4 2.7 2.3 0.4 2.5 3.0 5.0 5.0 2.0 0.0 3.0 3.0
9 17.6 22.9 19.1 15.3 22.0 19.4 19.4 23.0 19.0 15.0 15.0 21.0 23.0 19.3
10 11.9 7.7 10.3 11.8 9.7 10.9 10.4 10.0 12.0 12.0 14.0 13.0 10.0 11.8
11 N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A
12 10.7 8.2 10.4 7.6 9.2 11.2 9.6 14.0 13.0 13.0 12.0 14.0 11.0 12.8
13 9.4 13.5 11.1 9.7 12.4 10.1 11.0 10.0 12.0 13.0 10.0 5.0 7.0 9.5
14 8.7 11.4 14.5 7.9 10.4 14.1 11.2 16.0 12.0 4.0 7.0 12.0 10.0 10.2
15 35.5 30.1 32.2 35.6 30.4 28.9 32.1 32.0 28.0 27.0 25.0 25.0 22.0 26.5
16 11.0 13.2 15.9 12.4 13.2 10.5 12.7 12.0 10.0 10.0 13.0 13.0 13.0 11.8
17 0.1 2.9 0.4 2.1 3.2 0.5 0.5 3.0 8.0 12.0 10.0 6.0 5.0 7.3
18 N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A
19 18.9 24.2 19.1 18.1 23.9 20.1 20.7 21.0 23.0 21.0 24.0 20.0 21.0 21.7
Right hip
1 N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A
2 1.0 2.8 1.3 2.2 1.6 1.5 0.3 2.0 1.0 1.0 3.0 5.0 6.0 3.0
3 1.0 0.1 2.7 1.9 0.1 3.7 0.6 2.0 5.0 4.0 3.0 0.0 2.0 2.7
4 N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A
5 21.0 21.2 18.6 22.1 21.2 17.4 20.3 30.0 28.0 28.0 30.0 30.0 26.0 28.7
6 13.6 10.2 15.5 13.9 11.9 14.7 13.3 13.0 14.0 20.0 16.0 16.0 13.0 15.3
7 8.1 12.5 10.3 8.1 7.6 9.6 9.4 8.0 6.0 2.0 6.0 13.0 11.0 7.0
8 8.8 10.2 11.2 7.4 10.1 12.8 10.1 14.0 12.0 11.0 11.0 11.0 14.0 12.2
9 2.8 5.3 5.7 2.1 3.0 7.7 4.4 3.0 0.0 2.0 1.0 2.0 4.0 1.3
10 2.2 6.5 2.0 0.2 3.5 2.9 2.9 0.0 2.0 4.0 5.0 8.0 6.0 4.2
11 1.1 2.8 2.1 1 1.8 2.9 0.4 16.0 14.0 14.0 13.0 13.0 11.0 13.5
12 11.7 12 13.5 10.7 12.2 15.5 12.6 16.0 12.0 6.0 10.0 16.0 13.0 12.2
13 21.2 20.5 17.6 20.1 21.5 23.6 20.8 19.0 14.0 10.0 8.0 6.0 10.0 11.2
14 18.0 21.0 24.4 19.8 21.5 25.9 21.8 27.0 25.0 27.0 22.0 22.0 24.0 24.5
15 28.1 25.8 27.9 28.7 25.9 29.6 27.7 27.0 23.0 23.0 19.0 19.0 23.0 22.3
16 17.4 19.4 20.4 16.1 19.8 23.4 19.4 16.0 16.0 19.0 15.0 16.0 12.0 15.7
17 3.5 0.2 0.8 4.5 2.7 4.8 2.8 1.0 5.0 15.0 8.0 2.0 2.0 5.5
18 5.0 7.9 9.9 4.3 7.0 10.6 7.5 8.0 12.0 14.0 12.0 10.0 9.0 10.8
19 23.5 21.2 28.5 23.4 23.1 27.5 24.5 25.0 22.0 20.0 22.0 27.0 25.0 23.5

AV, anteversion; CT, computed tomography; N/A, not available.

between the 2 measurement methods with t ¼ 0.855 (N ¼ 34) values with torsional malalignment, neither decreased AV and
despite some deviations. The correlation is significant at the level of retroversion (N ¼ 14; P ¼ .072) nor increased AV (N ¼ 9; P ¼ .446)
.01 (2-sided). The correlation of the measurements between EOS showed a significant correlation between the measured AV angles
and CT for the determination of the AV angle with reduced AV in EOS and CT scan.
(<10
) including retroversion (<0
) resulted in a moderate effect of
t ¼ 0.495 (N ¼ 14) in the Pearson product-moment correlation. The
correlation of the measurements between EOS and CT for the
determination of the AV angle in increased AV (>20
) gave a low
effect of t ¼ 0.292 (N ¼ 9) in the Pearson product-moment corre-
lation (Fig. 4; Table 2). The inter-rater reliability by calculating the
ICC for all 3 examiners when measuring the AV angle with the EOS
system showed a strong agreement with an ICC of 0.911. The ICC for
all 3 examiners when measuring the AV angle with the CT scan also
showed strong agreement with 0.934. The ICC for all 3 examiners
showed a strong intraobserver reliability with Cronbach alpha of
0.955 for EOS and 0.934 for CT scan.

The 2-Sided t Test

The calculation of the correlation of the mean values for

matched samples in the 2-sided t test showed a highly significant Fig. 4. Graphical representation of the correlation of the measurements between EOS
correlation considering all values (N ¼ 34; P < .001) and the values and CT scan for the determination of the AV including all cases, N ¼ 34 with t ¼ 0.855.
CT, computed tomography.
with physiological AV (N ¼ 11; P ¼ .001). Taking into account the
384 H.O. Mayr et al. / The Journal of Arthroplasty 36 (2021) 379e386

Table 2
AV Angles for EOS and CT With Mean Values, Range, and SD, Pearson Correlation, and t Test.

EOS CT Scan Pearson Correlation Two-sided t Test

AV angle including all hips (N ¼ 34) 12.2
± 10.0
(15
to 32
) 12.6
± 9.2
(3.2
to 35.6
) t ¼ 0.855 P < .001
AV angle in reduced AV or retroversion (N ¼ 14) 4.2
± 7.4
(15
to 13.0
) 4.0
± 3.8
(3.2
to 12.5
) t ¼ 0.495 P ¼ .072
AV angle in increased AV (N ¼ 9) 25.4
± 2.3
(19.0
-32.0
) 24.2
± 2.3
(17.2
-35.6
) t ¼ 0.292 P ¼ .446

AV, anteversion; CT, computed tomography; SD, standard deviation.

Discussion on the CCD angle. Other studies have shown that fluctuations up to
13
can result from the height of the CT scan and CCD angle when
The present study reveals a high correlation of the AV angle of measuring the same femur [17,23]. In the head-shaft-axis method,
the femur determined by the EOS measurement method and CT the average AV angles seem lower compared with the head-neck
scan. However, in patients with torsional malalignment, the cor- method [13,14]. Considering these aspects, the method of Lee has
relation is weaker. The calculated Pearson correlation coefficient a proximal definition and seems a reliable method for measuring
comparing EOS and CT scans was t ¼ 0.855 including all mea- cases with torsional malalignment [12e14,21,22,24].
surements (N ¼ 34). In patients with reduced AV (<10
) and The setting of bony landmarks on CT slice images is known to
retroversion (<0
) or increased AV (>20
), the Pearson product- depend on the position of the patient. This is particularly important
moment correlation only resulted in a moderate effect of t ¼ for the tangent to the femoral condyles. In addition to measure-
0.495 (N ¼ 13) and t ¼ 0.292 (N ¼ 9), respectively. The ICC also gives ments of AV, patellofemoral measurements (tuberosity tibial tu-
valid results in the inter-rater agreement with 0.911 in the EOS bercle groove distance) that depend on the tangent to the femoral
measurements and 0.934 in the CT scan measurement. Cronbach condyles are highly dependent on the position of the leg in the CT
alpha showed a strong intraobserver reliability of 0.955 for EOS and scan. Furthermore, defining the femoral neck axis in slice images is
0.934 for CT scan. The 2-sided t test showed a significant correlation known to have high inter-rater variability. For 3D bone models
between the measured AV angles in EOS and in the CT scan reconstructed from CT data, validations for AV measurements have
considering all values (N ¼ 34; P < .001). But there was no signif- only recently become available. Unfortunately, these studies were
icant correlation in case of torsional malalignment. predominately conducted based on total femur scans of cadavers or
saw bone models. Total femur CT scans require high radiation
Radiation Dose dosages, which definitively limits the application in a clinical
routine. Furthermore, recent publications regarding anatomy
According to previous studies [14,15], EOS showed significantly assessment in 3D state that true 3D angle measurement for AV is
lower radiation dose compared with conventional pelvic X-rays not useful [25,26]. This is due to fact that the 3D angle between the
with a factor of 0.6 for body mass index <25 (7 vs 10.7 dGy/cm2) femoral neck axis and tangent to the femoral condyles is not only
and a factor of 0.3 for body mass index 30-40 (9.7 vs 30.1 dGy/cm2). determined by the AV but also strongly influenced by the CCD
The average radiation exposure of an EOS leg full-length image on angle. To measure only the AV, the angle must be calculated in the
both sides is many times lower than that of the CT scan needed to XY plane of the 3D model, and the z value is set to 0. In the future,
determine the torsion. The radiation exposure for the EOS is 3D modeling and measurement using torsional CT scan will be
approximately 0.63 ± 0.13 mGy. The radiation exposure during the possible, but there is still a lack of a general definition of landmarks,
CT examination varies more, depending on how large the area to be angle measurements, and scientific validation. Therefore, 3D
scanned is set. Radiation levels of 8.4-15.6 mGy have been reported measurement using torsional CT scan was not included in the
[16]. present study.
A weakness of the CT scan is that positioning of the lower ex-
Technical Aspects of CT Scan tremity should be in a neutral position. In cases with pronounced
torsional malalignment however, it is not very reliable to hold this
In the present study, the proximal femoral AV angle was position [22].
determined on CT scan as the angle between the femoral neck axis
and the most prominent aspect of the posterior knee condyles by Technical Aspects of EOS
the method of Lee (transversal method) [12]. According to this
method, the femoral neck axis is determined by a connecting line A major advantage of the biplanar EOS imaging to create a 3D
between the center of the femoral head and the center of the model is the ability to determine a large variety of anatomic pa-
femoral neck. The most proximal axial slice of the CT scan on which rameters. In conventional preoperative planning, a full-leg X-ray in
a connection between the femoral neck and the greater trochanter 2 planes (AP and lateral) and a CT scan are necessary to determine
(neck confluence) is visible is used for the purpose. In contrast to the length, axis, and hip parameters including the angles. With EOS
this single-slice approach, other methods use multiple slices to however, all these desired measurement parameters can be
determine the femoral neck axis and are therefore more sophisti- calculated using 1 X-ray scan. Because it is a spatial scanning sys-
cated in terms of creating a spatial construction [17e20]. Most tem, it is not important to maintain a neutral rotation of the leg. On
methods including the Lee method give the AV angle in relation to a one hand, given the software capability of creating a 3D model, no
tangent to the most prominent posterior knee condyles. In com- further radiologic examinations are necessary for the planning of
parison to the values derived from conventional radiographs as hip arthroplasties or correction osteotomies. In addition, recent
described by Rippstein [21], the mean values of the femoral AV as studies demonstrated the potential value of EOS measurement in
determined by CT scan analyses are significantly higher. Further- patients after total hip arthroplasty [4,5,27]. EOS is superior to plain
more, when applying different measuring methods for CT scans, X-rays and standing X-rays in the angle measurement of the
there sometimes are large deviations especially in patients with acetabular cup and AV of the femoral component even after hip
increased or excessive femoral torsion [22]. In the head-neck arthroplasty [28,29]. The EOS system, on the other hand, features
method, AV values vary depending on the height of the scan and limitations in patients who are unable to stand or hold the
 H.O. Mayr et al. / The Journal of Arthroplasty 36 (2021) 379e386 385

appropriate position for the time of the examination. Movement Weakness of the Study
artifacts do not allow precise evaluation of the EOS images.
It could not be ensured that the neutral position of the leg was
The Present Findings fully maintained during the CT scan image acquisitions, which to
some extent limits the precision and reliability of the CT scan values
In patients featuring physiological AV angles of the femur, the in patients with pronounced torsional malalignment.
present results reflect the findings by others in the current available
literature. In the study by Folinais et al [30], the correlation showed Conclusion
a value of t ¼ 0.93 in a total of 43 measurements of the AV angle.
The inter-rater correlation also revealed very similar values. The Measuring the femoral AV angle, EOS imaging system enables a
inter-rater correlation was at 0.93 in the study by Folinais et al with strong overall correlation with CT scan measurements as well as
a total of 3 examiners. This is nearly identical to the findings of the high intrarater and inter-rater reliability. In torsional malalignment,
present study. Further similar values for a population of 35 patients EOS does not have correlation with gold standard CT measurements.
were shown in the study by Buck et al [31]. The mean values for the Although CT scans require a neutral positioning of the legs, which
femoral AV was 11.6
for EOS and 11.5
for CT scan; similar to values could not accurately enough be assured in patients with pronounced
in the present study of 12.6
for EOS and CT scan. The study by torsional malalignment, EOS allows femoral torsion measurement
Rosskopf et al [32] also shows results similar to the present trial independent of legs' positioning. This independency may represent
regarding the comparability of EOS and CT scan. The results can also an advantage in patients with pronounced torsional malalignment.
be confirmed in the study by Pomerantz et al [33]. The determi- In addition, radiation dose of an EOS image is many times lower than
nation coefficient of both measurement methods was 0.83-0.84. A that of the CT scan needed to determine the torsion.
better agreement among the investigators was found with an ICC
value of 0.98. Even after total hip arthroplasty, reliable results are
Acknowledgments
achieved by EOS measurement. The average absolute differences
between EOS and CT measurements were 4
± 4
for femoral AV.
The work group thank the Auguste-Schaedel-Dantscher Foun-
Comparable to the current trial, intraobserver agreement was
dation, Garmisch, Germany for its support of the present study.
>0.75, respectively (Cronbach alpha, 0.90) for measurement of
Furthermore, special thanks to T. Reid Prall for English editing of the
femoral AV using EOS imaging [27,29]. Similar observations were
manuscript.
made by Tokunaga et al [34].
To date, high correlations of EOS measurements and the existing
gold standard of CT scans have only been shown for hips featuring a References
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