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800- to 1000-fold lower radiation dose than a CT study of the same region [16, 20, 21]. Re-cent publications have shown that femoral and tibial torsion angles using 3D models based on biplanar radiography are interchangeable with state-of-the-art CT measurements in adults [22]. In addition, length and angle measure-ment other than torsion measurements were reliable on biplanar radiography imaging in adults [15], children, and adolescents [23].

Secondary 3D reconstruction based on bi-planar radiography relies on the correct identi-fication of bony landmarks (e.g., the posterior aspect of the femoral condyles). In the imma-ture skeleton of children and adolescents, these bony landmarks can be hard to identify because of an increasing amount of not yet mineralized cartilage in younger children [24]. The amount these difficulties hamper torsion measurements based on biplanar radiography is unknown.

The purpose of this study was to assess the interchangeability and reliability of fem-

Femoral and Tibial Torsion Measurement in Children and Adolescents: Comparison of 3D Models Based on Low-Dose Biplanar Radiography and Low-Dose CT

Andrea B. Rosskopf1 Leonhard E. Ramseier2 Reto Sutter1 Christian W. A. Pfirrmann1 Florian M. Buck1

Rosskopf AB, Ramseier LE, Sutter R, Pfirrmann CWA, Buck FM

1Department of Radiology, Balgrist Orthopedic University Hospital, Forchstrasse 340, CH-8008 Zurich, Switzerland. Address correspondence to A. B. Rosskopf (andrea.rosskopf@balgrist.ch).

2Department of Orthopedic Surgery, Balgrist Orthopedic University Hospital, Zurich, Switzerland.

Pediatr ic Imaging • Or ig ina l Research

WEB This is a web exclusive article.

AJR 2014; 202:W285–W291

0361–803X/14/2023–W285

© American Roentgen Ray Society

Quantifying femoral and tibial tor-sion is an essential part of the pre-operative workup for lower limb rotation-correcting surgery in

children and adolescents [1–7]. Femoral and tibial torsions are generally measured on axial CT images. Modern CT scanners provide rap-id determination of femoral and tibial torsion at relatively low radiation doses [8].

Because of the higher radiation vulnerabil-ity in children and according to the ALARA (as low as reasonably achievable) principle, parents and referring pediatric clinicians fa-vor techniques that use even less or no ion-izing radiation. Low-dose simultaneous per-pendicular biplanar radiography scanners are available and used in various fields of mus-culoskeletal radiology [9–19]. These scanners enable examination in an upright weightbear-ing position. Image acquisition is associated with an 8 to 18 times lower radiation dose than conventional radiography scanners and an

Keywords: children, CT, femoral torsion, low-dose biplanar radiography, tibial torsion

DOI:10.2214/AJR.13.11103

Received April 19, 2013; accepted after revision July 1, 2013.

OBJECTIVE. The purpose of this study was to evaluate the interchangeability and reli-ability of femoral and tibial torsion measurements in children using 3D models based on bi-planar radiography compared with CT measurements.

MATERIALS AND METHODS. Femoral and tibial torsion were measured in 50 patients (mean age, 10.9 years; range, 4.7–14.8 years) using 3D models based on low-dose biplanar radiog-raphy by two independent readers. Measurements on transverse CT images by two independent readers served as the reference standard. Intermethod and interreader agreement was calculated using descriptive statistics, intraclass correlation coefficient (ICC), and Bland-Altman analysis.

RESULTS. Femoral and tibial torsion were −6°–65° and 6°–51° for 3D models based on biplanar radiography and −13°–59° and 4°–52° for CT measurements. The average differ-ence (± SD) between the two methods was 4.9° ± 3.8° and 5.5°± 4.1°, respectively. The inter-method ICC for biplanar radiography was 0.90 (95% CI, 0.87–0.92) for femoral torsion and 0.75 (0.68–0.80) for tibial torsion. The interreader ICC was 0.93–0.97. Mean measurement differences between the two biplanar radiography readers were 3.4° (0.0°–11.0°) for femoral torsion and 3.9° (0.0°–15.0°) for tibial torsion. Mean interreader differences at CT were 3.3° (0.0°–9.0°) for femoral and 3.0° (0.0°–10.0°) for tibial torsion. There was no trend for larger intermethod differences with decreasing age of the children.

CONCLUSION. Femoral and tibial torsion measurements in children using 3D models based on biplanar radiography are comparable to CT measurement results. Despite skeletal immaturity, torsion measurements in children on biplanar radiography seem to be as reliable as those on CT images.

Rosskopf et al.Femoral and Tibial Torsion Measurements in Children

Pediatric ImagingOriginal Research

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oral and tibial torsion measurements based on low-dose biplanar radiography in children and young adolescents compared with mea-surements on low-dose CT.

Materials and MethodsInstitutional review board waiver was obtained.

From August 2011 until November 2012, 54 consec-utive children and young adolescents were referred for preoperative CT and biplanar radiography. Bipla-nar radiography was performed instead of conven-tional radiography as the preoperative routine. Ado-lescents over 16 years old were excluded. One patient had to be excluded because of a missing lateral bipla-nar radiography image. Three patients were exclud-ed because of incorrect patient positioning on the bi-planar radiography acquisition.

Eventually, 50 patients (mean age, 10.9 years; range, 4.7–14.8 years) were included. The group consisted of 21 boys (mean age, 10.9 years; range, 5.9–14.2 years) and 29 girls (mean age, 10.9 years; range, 4.7–14.8 years).

For CT studies, a 64-MDCT scanner (Brilliance 64, Philips Healthcare) was used. Axial images (pitch factor, 0.426; matrix, 512 × 512; reconstruction thick-ness, 1 mm; and reconstruction increment, 0.45 mm)

of the hip joint (tube voltage, 100 kV; tube current, 100 mAs/slice), knee joint (tube voltage, 100 kV; tube current, 60 mAs/slice), and ankle joint (tube voltage, 80 kV; tube current, 50 mAs/slice) were acquired without moving the patient between these three acqui-sitions. The legs of the patients were stabilized during scanning with tapes around the midfeet. The axial re-constructed bone window images were used for tor-sion measurements of the femur and tibia bilaterally.

According to the established method by Her-nandez et al. [25], femoral torsion was defined as the angle between a line through the femoral neck and a line adapted to the posterior contour of the femoral condyles (Figs. 1A–1C). The line through the femoral neck was defined parallel to the cor-tical borders of bone of the femoral neck and ex-tending through the center of the femoral head.

Tibial torsion was measured according to the method described by Goutallier et al. [26] be-tween a line adapted to the posterior contour of the tibial head and the intermalleolar axis (Figs. 1D and 1E). The intermalleolar axis was defined as a line through the midpoints of the articular surface of the medial and lateral malleolus.

The measurements on CT images were per-formed independently by two musculoskeletal ra-

diologists who were blinded to the results of the measurements on 3D models based on biplanar ra-diography. The overall operator time for the CT ex-amination was about 20 minutes (scanning time, 3 × 8 seconds; positioning time, 5 minutes; recon-struction and measurement time, 15 minutes).

On the same day of the CT examination, all pa-tients underwent examination in a low-dose bipla-nar radiography scanner (EOS Imaging System, EOS Imaging) resulting in an anteroposterior and a lateral image (anteroposterior image: tube volt-age, 80 kV; tube current, 200 mA and lateral im-age: tube voltage, 100 kV; tube current, 250 mA) that were acquired simultaneously. These images were later used for computer-aided 3D modeling of the legs using sterEOS software (EOS Imaging).

SterEOS software was introduced for torsion measurements by Chaibi et al. [27] and is already known for 3D measurements of femoral and tibial torsion in adults. The reconstruction process con-sists of a software-guided step-by-step procedure. The 3D models are created by identifying several anatomic reference points on the anteroposterior and lateral images (Figs. 1F–1K). On the basis of these landmarks, the software produces semiauto-matically a 3D bone model with contours superim-

A

D

Fig. 1—7-year-old boy.A–E, Axial CT images of left leg. Femoral torsion was measured by calculating angle between center of femoral head and femoral neck shaft (A and B) and dorsal contour of femoral condyles (C). Tibial torsion was quantified by calculating angle between dorsal contour of tibial plateau (D) and intermalleolar axis (E). White circle (A) = contour of femoral head. White dot (A) = center of femoral head. White line (B) = femoral neck axis. White line (C) = line adapted to posterior contour of medial and lateral femur condyle. White line (D) = line adapted to posterior contour of tibial head. White dots (E) = center of black lines. Black lines (E) = cartilage covered joint surface of medial and lateral malleolus. White line (E) = intermalleolar axis defined as line connecting center of black lines.

(Fig. 1 continues on next page)

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posed on biplanar radiographs (Fig. 2). For torsion measurements, the identification of the following landmarks is crucial: the greater trochanter of the proximal femur, posterior contour of the femoral

condyles, posterior contour of the tibial head me-dially and laterally, and malleoli of the ankle joint. For easier identification of these landmarks, one foot (generally the right) is placed slightly anterior

to the other (shifted-feet standing position), which results in a slightly more anterior position of one (the right) knee on the lateral radiograph. After 3D modeling, sterEOS software automatically calcu-

F

I

Fig. 1 (continued)—7-year-old boy.F–K, Low-dose biplanar radiography images of left leg. Anteroposterior (F) and lateral (G) images show proximal femur with overlying contours as provided by sterEOS (EOS Imaging) software. Anteroposterior (H) and lateral (I) images show knee joint. Identification of posterior contour of femoral condyles (pink and red semicircles) as well as posterior contour of tibial head medially and laterally (pink and red lines) is crucial. Green line ending in red dot identifies intercondylar groove. Note overlying soft-tissue contours of right leg. Anteroposterior (J) and lateral (K) biplanar radiographs show ankle joint. Green outlines (F and G) = semiautomatically adapted osseous contours of femur and tibia by computer software. Blue dots and circles (F and G) = software-specific lines and points to facilitate fitting process not necessarily representing anatomical axis. Pink and red items (H and I) = software specific to facilitate fitting process. Orange outlines (J and K) = semiautomatically adapted osseous contours of fibula by computer software. Blue lines and dots (J and K) = software-specific items to facilitate fitting process not necessarily representing anatomical axis. Green outlines (J and K) = adapted semiautomatically to osseous contour of tibia by computer software.

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lates femoral and tibial torsion results using a stan-dardized algorithm.

Each 3D model was reconstructed twice by two independent readers—a radiology technician and a radiologist—both trained in the use of sterEOS software. The overall operator time for the bipla-nar examination was about 21 minutes (scanning time, 13 seconds; positioning time, 1 minute; re-construction and measurement time, 20 minutes).

Statistical AnalysisFemoral and tibial torsion measurements per-

formed on CT images were compared with the re-sults of biplanar radiography–based 3D modeling results by calculation of correlation coefficients, descriptive statistics, and Bland-Altman analysis.

Interreader agreements of biplanar radiography measurements as well as CT measurements were also evaluated by intraclass correlation coefficient (ICC). ICC and Bland-Altman analyses were calculated us-ing SPSS statistical software, version 17 (IBM).

ResultsThe range of CT measurements for femo-

ral torsion was −6°–59° for reader 1 (mean (± SD), 25° ± 12.9° SD), and −13°–58° for reader 2 (mean, 24° ± 13.1°). For tibial torsion measure-ments using CT, the range was 8°–52° (mean,

27° ± 8.5°) for reader 1, and 4°–50° (mean, 26° ± 9.0°) for reader 2. The average difference between CT readers was 3° ± 2.3 ° (range, 0°–9.4°) for femoral measurements and 3° ± 2.1° (range, 0°–9.8°) for tibial measurements. Inter-reader agreement of CT measurements was very good and statistically significant for all measure-ments (p < 0.001). ICC was 0.95 (95% CI, 0.93–0.97) for femoral torsion measurements and 0.93 (0.89–0.95) for tibial torsion measurements. The range of femoral torsion measurements based on biplanar radiography 3D models was −5°–65° for reader 1 (mean, 23° ± 13.2°) and −6°–62° for reader 2 (mean, 22° ± 12.8°).

The range of tibial torsion measurements was 9°–48° (mean, 30° ± 7.1°) for reader 1, and 6°–51° (mean, 30° ± 9.0°) for reader 2. The av-erage difference between the two readers us-ing biplanar radiography 3D models was 3° ± 2.5° (range, 0°–11°) for femoral measurements and 4.0° ± 3.5° (range, 0°–15°) for tibial mea-surements. The maximal difference between the two readers using biplanar radiography 3D models was 11° for femoral torsion and 15° for tibial torsion and therefore slightly higher than the maximal difference between the measure-ments of the two CT readers (femur 9° and tibia 10°) (Table 1). Interreader correlation of mea-

surements based on 3D models was very good for femoral torsion measurements (0.95, 0.93–0.97) and good for tibial torsion measurements (0.79, 0.70–0.85).

The average difference between the two methods was 4.9° ± 3.8° for the femoral measurements and 5.5° ± 4.1° for the tibial measurements (Table 1). The differences be-tween the methods showed no trend to get larger with decreasing age of the children.

Overall, measurements acquired by CT were comparable to those measured by bipla-nar radiography, shown by an intermethod correlation coefficient of 0.90 (0.87–0.92) for femoral torsion measurements and of 0.75 (0.68–0.80) for tibial torsion measurements.

Bland-Altman plots of femoral and tibial torsion measurements are shown in Figure 3. There was an average difference between the two methods of 2.1° (range, −11° to 17.3°) for femoral measurements and −3.4 ° (range, −16 to 20°) for tibial measurements. All results of 3D modeling measurements except for six of 200 femoral torsion measurements and of six of 200 tibial torsion measurements were found within the ± 1.96 SD interval. With in-creasing mean values of the CT and biplanar radiography torsion measurements, no trend

A

Fig. 2—6-year-old girl.A–D, Three-dimensional reconstruction of both lower extremities based on biplanar radiography data and overlaid on biplanar radiography source images shown in contours (A and B) and surface (C and D) rendering modes. Red outline = adapted osseous contours. Yellow outline = 3D models of bones calculated by software.

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for an increase or decrease of the differenc-es between the two methods could be found. The scatter did not increase with increasing average of CT and 3D torsion measurements, and the variability of results was consistent across the graph.

DiscussionOur results show good reliability, inter-

reader agreement, and intermethod agree-ment of femoral and tibial torsion measure-ments using 3D modeling based on low dose biplanar radiography compared with the ref-erence standard torsion measurements on CT of the leg in children and young adolescents from 5 to 15 years old. Despite skeletal im-maturity, it was possible to reliably calculate 3D models based on biplanar radiography. Maximal measurement differences between the two readers of the reconstructed 3D mod-els were slightly larger than maximal differ-ences between the two CT readers. However, interreader agreement of 3D model measure-ments was in the order of magnitude of inter-reader agreement of CT measurements. Even

in young children (under the age of 10 years) with less ossification of epiphyses and apoph-yses and more variability in configuration of the growing skeleton, we did not find any evi-dence for larger inaccuracy of 3D model mea-surements. Therefore these two measurement methods seem well comparable.

One of the first studies evaluating fem-oral torsion measurements in a pediatric study group with CT [25] showed a similar interobserver correlation (0°–8°; mean, 3°) for femoral CT measurements to our results (0–9 °; mean 3°). An article by Buck et al. [22] examined femoral and tibial torsion mea-surements in an adult study group with osteo-arthritis. Interreader agreement of these CT measurements was similar to ours. Howev-er, the correlation between the readers of the biplanar radiography–based 3D models was slightly superior to our results, possibly be-cause of the immature skeletons of our study group. As mentioned, there are crucial land-marks for the calculation of the torsion mea-surements using 3D models—especially the posterior contour of the femoral condyles

and the medial and lateral aspects of the tib-ial head. In younger children, the epiphyses are much wider and the posterior contours of the femoral condyles less round than in older children. Biplanar radiography 3D software fits better in round-shaped posterior contours of the femoral condyles. Therefore, the user needs to adapt the circles to ovoid bone struc-tures, potentially leading to minor measure-ment errors. This may be the reason the man-ufacturer recommends the use of the software only for patients 15 years old or older [28].

Additionally the lower density of bones in the growing skeleton leads to consider-ably more difficulty in identifying the cor-rect condyle or side of the tibial head (medi-al or lateral) in lateral views. Our data show reliable measurements as long as the circles to identify the femoral condyles are adjust-ed correctly on the anteroposterior view and adapted to the posterior contour of the femo-ral condyles on the lateral view.

Our intermethod correlation was similar to the results of the adult patient group men-tioned previously [22]. In some patients, the

TABLE 1: Measurement Results

Parameter

Femoral Torsion (°) Tibial Torsion (°)

CT Biplanar Radiography CT Biplanar Radiography

Reader 1 24.7 ± 12.9 (−6 to 59) 22.7 ± 13.2 (−5 to 65) 27.5 ± 8.5 (8–52) 30.1 ± 7.1 (9–48)

Reader 2 23.9 ± 13.1 (−13 to 58) 21.7 ± 12.8 (−6 to 62) 25.9 ± 9.0 (4–50) 30.0± 9.0 (6–51)

Both readers 24.3 22.2 26.7 30.1

Averaged difference for reader 1 and reader 2 3.3 (0–9) 3.4 (0–11) 3.0 (0–10) 3.9 (0–15)

Average difference for CT and biplanar radiography 4.9 (0–22) 5.5 (0– 20)

Note—Data are averaged measurement results ± SD. Range of measurement results is shown in parentheses.

–10

–150 20

Average of CT and Biplanar Radiography Torsion

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Fig. 3—Bland-Altman plots.A and B, Bland-Altman plots of femoral (A) and tibial (B) torsion show good correlation of CT and biplanar radiography measurements.

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results of the two measurement methods were not within the 95% limits of agreement. In these specific cases, the landmarks needed for 3D modeling were difficult to identify and led to divergent measurement results. Therefore, in children it is critically important to obtain the correct positioning of the patient before image acquisition with biplanar radiography scanners. Furthermore, we recommend con-sidering CT measurements if 3D modeling is difficult because of bad recognizability of the landmarks needed for 3D reconstruction.

A French group [23] examined length mea-surements of the lower extremity in a pediat-ric study population with an identical biplanar low-dose scanning system. The children were a little older (11–17 years) than in our study. The authors examined the comparability of lower limb length measurements (tibial and femoral length) as well as tibial and femoral angle measurements (tibial mechanical angle, femoral mechanical angle, frontal and lateral knee angulation, and femoral neck shaft an-gle) performed on CT images and using 3D measurements based on biplanar radiography. The results of that study showed similarly re-liable measurements with biplanar radiogra-phy compared with CT images.

Similar to the results of Buck et al. [22], our measurements on CT images showed a higher tibial retrotorsion (average difference, 3°) than the measurements on 3D models based on bi-planar radiography. This finding is explained by a known overestimation of tibial torsion us-ing the intermalleolar axis method on CT im-ages of about 5° [29]. We chose the intermalle-olar method for this study because of its known superior interreader agreement compared with other measurement techniques [29]. In our hospital, pediatric orthopedic surgeons only measure the torsion angles in children with a clinically suspected difference to the contra-lateral side of more than 10°. Intraoperatively, they try to achieve a torsion difference of 5° or less between both legs. So at least an accu-racy of 5° is expected of the radiologic exami-nation. Therefore, the small average difference between the two methods of 4.9° for femoral and 5.5° for tibial torsion measurements seems to be acceptable. Measurements can be per-formed by trained radiology technicians irre-spective of the modality. A physician should be available in case of problems.

In accordance with the ALARA princi-ple, torsion measurements using 3D mod-eling based on low-dose biplanar radiogra-phy provide a huge reduction in radiation dose—making this technique a favorable

alternative, especially in pediatric patients [16, 20, 30, 31]. Radiation doses associated with CT are considerably higher (up to fac-tors of 1000), depending particularly on the pelvic area covered by scanning [16, 20]. Some studies have shown reliable femoral and tibial torsion measurements using MRI scanners. In contrast to biplanar radiography, MRI examinations use no ionizing radiation but take much more time, have higher exam-ination costs, have known limitations in pa-tients with metal implant devices [32–34], and show a significant underestimation of leg length in the upright position [35].

Our study is limited in that chronologic age of patients was used instead of skeletal age. In addition, only 16 patients (32%) were younger than 8 years old. On the other hand, most derotational osteotomies are performed in older children and adolescents because of the potential for spontaneous remodeling un-til the 8 years [36].

In conclusion, femoral and tibial torsion measurements using 3D models based on bi-planar radiography are comparable to those with standard CT measurements in children and young adolescents. Despite skeletal im-maturity, torsion measurements in children on biplanar radiography seem to be as reli-able as those on CT.

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