assessment of pelvic tilt in anteroposterior radiographs...
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Archives of Orthopaedic and Trauma Surgery https://doi.org/10.1007/s00402-018-2931-z
ORTHOPAEDIC SURGERY
Assessment of pelvic tilt in anteroposterior radiographs by means of tilt ratios
T. Schwarz1 · A. Benditz1 · H.‑R. Springorum1 · J. Matussek1 · G. Heers1 · M. Weber1 · T. Renkawitz1 · J. Grifka1 · B. Craiovan1
Received: 21 December 2017 © Springer-Verlag GmbH Germany, part of Springer Nature 2018
AbstractIntroduction In anteposterior (AP) radiographs, cup position in total hip arthroplasty and acetabular anatomy in hip-preserv-ing surgery are highly influenced by pelvic tilt. The sagittal rotation of the anterior pelvic plane is an important measurement of pelvic tilt during hip surgery. Thus, correct evaluation of cup position and acetabular parameters requires the assessment of pelvic tilt in AP radiographs.Methods Changes in pelvic tilt inversely change the height of the lesser pelvis and the obturator foramen in AP radiographs. Tilt ratios were calculated by means of these two parameters in simulated radiographs for ten male and ten female pelvises in defined tilt positions. A tilt formula obtained by exponential regression analysis was evaluated by two blinded investigators by means of 14 simulated AP radiographs of the pelvis with pelvic tilts ranging from + 15° to − 15°.Results No differences were found between male and female tilt ratios for each 5° step of simulated pelvic tilt. Pelvic tilt and tilt ratios correlated exponentially. Using the tilt formula, the two blinded investigators were able to assess pelvic tilt with high conformity, a mean relative error of + 0.4° (SD ± 4.6°), and a mean absolute error of 3.9° (SD ± 2.3°). Neutral pelvic tilt is indicated by a tilt ratio of 0.5 when the height of the lesser pelvis is twice the height of the obturator foramen.Conclusion The analysis and interpretation of cup position and acetabular parameters may be improved by our method for assessing pelvic tilt in AP radiographs.
Keywords Pelvic tilt · Cup position · Anterior pelvic plane · Total hip arthroplasty · Acetabular parameters
Introduction
The analysis of pelvic tilt has become increasingly important for understanding and treating different orthopedic diseases. The position of the pelvis is of high importance during
Electronic supplementary material The online version of this article (https ://doi.org/10.1007/s0040 2-018-2931-z) contains supplementary material, which is available to authorized users.
* T. Schwarz [email protected]; [email protected]
A. Benditz [email protected]
H.-R. Springorum [email protected]
J. Matussek [email protected]
G. Heers [email protected]
M. Weber [email protected]
T. Renkawitz [email protected]
J. Grifka [email protected]
B. Craiovan [email protected]
1 Department of Orthopedic Surgery, University Medical Center Regensburg, Kaiser-Karl-V.-Allee 3, 93077 Bad Abbach, Germany
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surgical interventions on the trunk such as spinal fusion, pelvic osteotomy, and total hip arthroplasty (THA).
The alignment of spinopelvic balance is crucial in the surgical treatment of various vertebral deformities [1–3], and thus necessitates the assessment of key parameters such as pelvic incidence, pelvic tilt, and sacral slope. This statement also applies to THA [4–6]. 1° of pelvic tilt may change cup anteversion by 0.8° [7]. In an own investigation, variation in pelvic tilt of ± 15° resulted in a difference of 25° between the minimum and maximum angles for anteversion and of 8° for inclination [8]. However, not only cup position is highly influenced by pelvic tilt. Even acetabular anatomy in AP radiographs depends on pelvic orientation. Tannast et al. showed relevant changes for many radiographic hip param-eters, influencing the diagnosis of acetabular pathologies and the planning of corrective acetabular surgery (reorientation or rim trimming) [9].
Although pelvic tilt always describes the rotation of the pelvis around its horizontal axis, different definitions have been described for its angular measurement in lateral radio-graphs (Fig. 1). In a first definition by Legaye and Duval-Beaupere [10], the pelvic tilt angle is defined by a vertical line through the femoral head and a line from the mid-sacral plateau and femoral head [11] (Fig. 1a). This definition is predominantly used by spinal surgeons for the individual planning of surgically reconstructing sagittal balance during spinal fusion. The individual pelvic parameter ‘pelvic inci-dence’ can be obtained by adding the angles of pelvic tilt and sacral slope [1–3, 10, 11]. Two other definitions of pelvic tilt are predominantly used during pelvic and hip surgery. In the prior definition, the tilt angle is measured between a horizon-tal line and a line connecting the upper border of the sym-physis with the promontory [12] (Fig. 1b). Because modern methods of hip arthroplasty use more and more intraopera-tive navigation techniques, the anterior pelvis plane (APP) gains increasing importance as a reference plane [13–17].
Thus, a more modern definition of pelvic tilt includes the APP that is defined by the two superior iliac spines and the symphysis. The tilt angle is hereby measured between the APP and a vertical line [14] (Fig. 1c). Intraoperative palpa-tion of the superior iliac spine and the symphysis allows the approximate assessment of pelvic tilt.
In the previous studies, we and other authors have pro-vided mathematical solutions for correcting the high vari-ations in the cup position due to pelvic tilt [5, 6, 8]. These corrective methods require the assessment of the exact angulations between the APP and the vertical line in lateral radiographs of the pelvis, as shown in Figs. 1c and 2a. How-ever, if the central X-ray beam is focused beyond the anterior superior spine or the pelvis is slightly rotated, the assessment
Fig. 1 Different definitions of pelvic tilt: spinal surgery (a), hip surgery (b), and hip arthro-plasty (c)
Fig. 2 Assessment of pelvic tilt on lateral radiographs with the cen-tral X-ray beam (CB) focused exactly (a) and dorsally (b) onto the anterior superior spine. The X-ray projection causes a double contour of the posterior Os ileum (dashed yellow line) when the central beam is not exactly focused onto the anterior superior spine, thus impeding the assessment of pelvic tilt
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of pelvic tilt may be impeded in lateral radiographs because of the double contour of the anterior superior iliac spines (Fig. 2b). In addition, in contrast to AP radiographs, lateral radiographs of the pelvis are not routinely conducted and are thus not available for tilt assessment. Therefore, the aim of this feasibility study was to answer the question if it is pos-sible to assess pelvic tilt (angulation between the APP and the vertical line) in AP radiographs.
This question is not new, as Tannast has already com-pared different pelvic parameters in AP radiographs to assess pelvic tilt in the definition, as shown in Fig. 1b [12]. By means of regression analysis, Blondel has also established a method for determining pelvic tilt in the spinal definition (Fig. 1a) in AP radiographs using the sacro-femoral-pubic angle [18]. However, the reference plane for cup position in total hip arthroplasty is either the functional coronal plane (FCP) or the APP. Thus, for tilt-dependent cup correction and translation of cup position from the FCP to the APP, pelvic tilt angulation must be measured between the APP and the vertical line (FCP) (Fig. 1c) [8]. To the best of our knowledge, no method is yet available for assessing pelvic tilt in AP radiographs using this ‘arthroplasty definition’.
Materials and methods
Patients
The current study is a secondary retrospective analysis of a larger project. In that registered prospective controlled trial (DRKS00000739, German Clinical Trials Register), patients had received THA with the intraoperative use of an imageless navigation device [13]. At a mean of 6 weeks (5–7) after surgery, pelvic and femoral CT scans had been performed (Somatom Sensation 16; Siemens, Erlangen, Ger-many) to verify the position of the prosthetic component. For the pelvic tilt series, ten male and ten female CT scans were randomly selected to simulate radiographs using the newly developed digital hip arthroplasty planning software modiCAS||3D for CT scans (modiCAS GmbH, Erlangen, Germany).
Simulation radiographs showing defined pelvic tilt
The film-object distance was 90 cm and the film-focus dis-tance was about 110 cm in all simulated anteroposterior radiographs, and the central beam was directed at the upper margin of the symphysis. The APP was defined by the two anterior superior iliac spines (ASIS) and the pubic tubercle (PT). In lateral simulated radiographs, the central beam was directed at the APP (the two coinciding lines between the PT and the right and left ASIS). Rotation of the coronal plane around the transversal axis of the upper margin of the
symphysis resulted in pelvic tilt in the simulated anteropos-terior radiographs. Neutral tilt was checked in the simulated lateral radiographs, showing congruent lines between the APP and the coronal plane. We used the Xbox recording tool (Microsoft Corporation, Redmond, USA) to record videos in which we manually simulated pelvic tilt ranging from − 25° posterior to + 25° anterior. By slowly playing the vid-eos, we created screenshots (Snipping Tool Plus, Gladbeck, Germany) of the simulated anteroposterior and lateral radio-graphs in tilt steps of 5°. The 11 pictures of one patient were brought together to one tilt series and the tilt parameters (A and B) were measured as shown below. The ten male and ten female tilt series are added to this publication in the sup-plementary material.
Assessment of tilt ratios
In all simulated anteroposterior radiographs, two investiga-tors (TS, BC) defined three pelvic lines and measured the two intermediate parameters A and B, as shown in Fig. 3. The first line touches the two lower margins of the sacroiliac joint, the second line the upper border of the two obturator foramens, and the third line the two lower borders. Distance A is measured centrally between the first line and the sec-ond line, and correlates with the height of the lesser pelvis; distance B describes the height of the obturator foramen and is also measured centrally between the second line and the third line. The tilt ratios (B/A) of the ten male and ten female pelvises were calculated in steps of 5° from − 5° to + 25° by the two investigators. In some cases of extreme pelvic tilt (anterior and posterior), no tilt ratio could be assessed because of the restricted view in either the obturator foramen or the lesser pelvis.
Statistics and the pelvic tilt formula
In each of the 11 pelvic tilts ranging between − 25° and + 25°, we used Student’s t test to compare the male with the female tilt ratios. Because we did not find any differences in the tilt ratios between men and women, we calculated the mean tilt ratios (± SD) of all 40 single values (10 males, 10 females, and 2 investigators) that were summarized in a nomogram. Finally, we used IBM-SPSS Statistics 22 (SPSS Inc., Chicago, Illinois) for exponential regression analysis to obtain an exponential formula that shows the relationship between pelvic tilt and tilt ratio.
Verification of formula for pelvic tilt
As described above, we used the digital planning soft-ware modiCASII3D for CT scans (modiCAS GmbH, Erlangen, Germany) to simulate 14 radiographs (7 male and 7 female pelvises) showing pelvic tilt ranging from
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− 15° to + 15°. Two blinded observers (RS and AB) were instructed on how to measure the three pelvis lines and calculate pelvic tilt with our new formula. We calculated the mean relative error (± standard deviation) from the relative differences between adjusted and measured pelvic tilt. The mean difference and the confidence interval were graphically shown with a Bland–Altmann blot. The mean absolute error (± standard deviation) was calculated from the one-sided absolute differences between adjusted and measured pelvic tilts. To show inter-observer variability, we calculated the interclass correlation coefficient with IBM-SPSS Statistics 22 (SPSS Inc., Chicago, Illinois).
Results
Assessment of tilt ratios and the pelvic tilt formula
In some patients, tilt ratios could not be assessed because of extreme anterior and posterior tilt. The restricted view into the obturator foramen in anterior tilt (+ 25° and + 20°) did not allow the assessment of the tilt ratio in 12/40 (+ 25°) and 4/40 (+ 20°) measurements (Table 1). In posterior tilt of − 25° and − 20°, the height of the lesser pelvis was not measurable in 18 and 6 cases. Between − 15° and + 15°, the three pelvic lines could be measured by both observers in all simulated radiographs. Thus, 40 tilt ratios were assessed for each 5°-step (Table 1).
Male and female tilt ratios were compared for each 5°-step of pelvic tilt. Student’s t test did not yield any
Fig. 3 Assessment of the tilt ratio (B/A). The lower margin of the sacroiliac joint and the upper and lower borders of the obturator foramen have to be identified on both sides (a). The parameters A and B are determined by the three pelvis lines connecting these points, and the tilt ratio (B/A) can be calculated. During anterior (b) and posterior (c) tilt, the height of these two parameters is inversely changed
Table 1 Male, female, and total tilt ratios (TR) from − 25° posterior to + 25° anterior tilt
‘n’ accounts for the number of radiographs analyzed by both observers, in which the three pelvic lines were easy to measure
Pelvic tilt Male Female T test Total
n Mean TR (SD) n Mean TR (SD) P value n Mean TR (SD)
− 25° 7 2.14 (0.45) 15 2.65 (1.17) 0.27 22 2.49 (1.02)− 20° 14 1.48 (0.24) 20 1.64 (0.46) 0.24 34 1.57 (0.39)− 15° 20 1.26 (0.53) 20 1.05 (0.22) 0.11 40 1.16 (0.41)− 10° 20 0.84 (0.22) 20 0.79 (0.15) 0.45 40 0.81 (0.18)− 5° 20 0.61 (0.14) 20 0.60 (0.09) 0.79 40 0.61 (0.12)0° 20 0.48 (0.10) 20 0.48 (0.07) 0.93 40 0.48 (0.09)5° 20 0.38 (0.09) 20 0.38 (0.05) 0.97 40 0.38 (0.07)10° 20 0.30 (0.07) 20 0.30 (0.05) 0.68 40 0.30 (0.06)15° 20 0.22 (0.06) 20 0.25 (0.05) 0.12 40 0.24 (0.06)20° 18 0.18 (0.04) 18 0.20 (0.05) 0.15 36 0.19 (0.05)25° 12 0.15 (0.03) 16 0.14 (0.03) 0.95 28 0.15 (0.03)
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differences between female and male tilt ratios. For this reason, we analyzed both groups together and graphically illustrated the mean tilt ratios (± SD) dependent on pelvic tilt (Fig. 4). Our diagram excludes extreme pelvic tilt positions and focuses on the range of − 15° to + 15°, in which all tilt ratios were easy to measure.
Tilt ratios decrease exponentially with increasing pelvic tilt. For this reason, we used IBM-SPSS Statistics 22 (SPSS Inc., Chicago, Illinois) for regression analysis to obtain an exponential formula that shows the relationship between pel-vic tilt and tilt ratio. The formula is shown below, and the graph of the formula is shown in Fig. 4 (red line).
Pelvic tilt formula:
Verification of the pelvic tilt formula
In 14 simulated radiographs, two blinded observers (A.B. and H-R.S.) measured the three pelvic lines and consecu-tively calculated pelvic tilt using a Microsoft Excel sheet (see supplementary material) that implemented our tilt for-mula. The mean relative error between the adjusted (simu-lated) tilt and the calculated tilt is + 0.37° (SD ± 4.6°) and is illustrated in the Bland–Altmann blot (Fig. 5) with its confidence interval (− 8.7° to 9.4°). The mean absolute error was 3.9° (SD ± 2.3°). The interclass correlation coefficient
TILT = −ln(
B
A×
1
0.483
)
0.051.
was 0.94 (confidence interval 0.84 to 0.98), indicating excel-lent agreement between the two observers.
Vertical central beam offset from the symphysis
Our pelvic tilt formula was obtained from simulated radio-graphs, in which the central beam was exactly directed at the upper margin of the symphysis. During clinical routine, however, the central beam may vary vertically, either on pur-pose in case of low-centered radiographs for planning hip prostheses, or unintentionally in the case of obese patients in whom boney landmarks such as the symphysis are hard to define. If the X-ray focus is located above the symphysis, the improved view into the lesser pelvis and the poorer view into the obturator foramen may suggest increased anterior tilt, leading to possible misinterpretation of radiographic find-ings such as a cross over sign (Fig. 6a). In contrast, inferior central beam offset suggests increased posterior tilt (Fig. 6b).
For this reason, central beam offset must be considered, necessitating the assessment of the vertical central beam offset angle (Yangle) (Fig. 7a). This angle can be calculated with the trigonometric relationship (formula below) between the film-focus distance (FF) and the projected central beam offset distance Y′ (positive sign for superior offset and nega-tive for inferior offset). FF must be known (usually 110 or 130 cm in supine or standing radiographs), and the projected central beam offset distance can be measured on calibrated AP radiographs. In standing AP radiographs, in which the X-ray beam is centered at the midpoint of the X-ray film, the central beam may found by two crossing lines from the opposing picture corners, as shown in Fig. 7b. Otherwise, radio-opaque markers can be used to show the central beam.
Fig. 4 Mean tilt ratios (± standard deviation) of the simulated pelvic tilts ranging from − 15° posterior to + 15° anterior. The red line cor-responds to the graph of the function obtained by means of the pelvic tilt formula
Fig. 5 Bland–Altmann blot of the relative error (difference between adjusted and calculated tilt) for pelvic tilt assessment by the two blinded observers A.B. and H.-R.S
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For central beam corrected tilt calculation (TILTcorr), the Yangle must either be subtracted or added in relation to the superior or inferior distance of the central beam offset Y′.
Formula for pelvic tilt (TILTcorr) with central beam correction:
Discussion
Information on pelvic tilt is crucial when interpreting either cup position after total hip arthroplasty or acetabu-lar parameters in the case of femoro-acetabular impinge-ment and hip dysplasia. Using easily measurable pelvic
TILTcorr = TILT − Yangle = TILT − arctan (Y �∕FF).
parameters, this study describes a method for assessing pelvic tilt in AP radiographs in a definition predominantly used in hip arthroplasty.
The method of assessing pelvic tilt in AP radiographs is not new. With a moderately strong correlation, Tan-nast et al. analyzed different pelvic parameters by linear regression analysis to assess the pelvic tilt angle � (angle between a horizontal line and a line connecting the pubic symphysis with the sacral promontory; see Fig. 2a). With a probability of 65%, pelvic tilt could be estimated with an accuracy of 3.8° for men and 4.2° for women [12]. For our study, we used two of these parameters (height of the lesser pelvis and height of the obturator foramen) that change their length inversely during tilting of the pelvis. From these inversely changing parameters, we calculated the tilt ratios that are more precise than single distances, because individual pelvic size, magnification, and focus-object distance can be neglected. Finally, we used expo-nential regression analysis to obtain our mathematical for-mula that assesses pelvic tilt in a more modern definition, i.e., tilting of the APP versus the vertical coronal plane (FCP).
To our surprise, we did not find any differences between the pelvises of men and women when analyzing the tilt ratios, although the two sexes are known to have geometri-cal differences. This finding was fundamental for obtaining only one formula that applies to both sexes. In our validation of this formula, the relative error was + 0.37° (SD ± 4.6°) and the absolute error was 3.9° (SD ± 2.3°), suggesting an average error that may level at less than 1° when analyzing bigger collectives. For single patients, however, inaccuracy of approximately 4° must be assumed. This correlation is not perfect, but a higher correlation is nearly impossible due to the large variation in individual pelvic geometries. This high inter-individual variation is already known from epidemiological investigations on the ‘pelvic incidence’, a pelvic parameter that is unique to each individual [10, 19].
Fig. 6 AP pelvic radiographs with a film-focus distance of 130 and 18 cm superior (a) and 4 cm inferior (b) central beam (CB) offset from the symphysis. Superior central beam offset suggests increased anterior (a) tilt and inferior central beam offset suggests posterior (b) tilt. Anterior tilt may lead to misin-terpretation of a cross over sign
Fig. 7 Inferior and superior view of the pelvis during high-centered (blue) and low-centered (green) radiographs (a). Yangle (vertical offset angle), Y (central beam offset distance), Y′ (projected central beam offset distance), FO (Focus-Object distance), OF (Object-Film distance), and FF (Film-Focus distance). Measurement of the pro-jected central beam offset distance (b) in an AP low-centered radio-graph of the pelvis
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Despite this potential source of mismeasurement and the inaccuracy of approximately 4°, our method is suitable for assessing individual pelvic tilt in AP radiographs in daily clinical routine. Common clinical problems do not address pelvic tilt of ± 4° but rather focus on significantly increased anterior and posterior tilt that can easily be detected by means of our formula. In doubtful cases, a lateral radiograph may be indicated, but accuracy may also be limited due to technical difficulties when the central beam is not exactly focused on the anterior iliac spines, as shown in Fig. 2a. In these cases, pelvic tilt should be measured to the midpoint of the connection line between the two anteroposterior iliac spines [14, 20].
This study has some limitations. Our method is not suit-able for patients with extreme anterior tilt or posterior tilt. In epidemiological investigations, Kanawade et al. reported nearly neutral pelvic tilt (mean 0.4°, SD ± 7.4) in 85 lateral radiographs with the patients in standing position [21]. In a comparable setting, Pierrepont also showed nearly neutral tilt of − 1.3° in 1517 patients but simultaneously described the high range of pelvic tilt (− 30.2° posterior to + 27.9° anterior tilt) in his large collective. Our tilt formula was obtained from tilt series of − 15° to + 15° and thus only vali-dated for this range. In the case of more extreme pelvic tilt, the restricted view into either the lesser pelvis or the obtura-tor foramen did not allow the calculation of a tilt formula for a larger range. Extreme tilts beyond ± 15° are rather rare, and we highly recommend additional lateral radiographs, if exceeding tilts are detected by means of our tilt formula.
Second, although assessment of the three pelvic lines is easy, the application of our formula may be difficult during clinical routine. For this reason, we designed a Microsoft Excel sheet ‘Tiltometer’ (see supplementary material), in which the two parameters A and B are inserted and pelvic tilt is calculated by means of our formula. However, prior to any measurement, we recommend an approximate orientation by visually comparing the height of the lesser pelvis (A) with the obturator foramen (B). If the height of the lesser pelvis is twice the height of the obturator foramen (tilt ratio B/A is 0.5), the calculated pelvic tilt is − 0.7°, levelling the height of neutral tilt as described in epidemiological investigations [20, 21]. Tilt ratios lower than 0.5 indicate anterior tilt and posterior tilt must be assumed at higher tilt ratios. However, in every AP radiograph, vertical central beam offset should be taken into consideration. In radiographs with a film-focus distance of 130 cm, the superior and inferior central beam offset caused divergent projections of the obturator foramen and the lesser pelvis (Fig. 7), suggesting increased anterior tilt and posterior tilt. Because of lower film-focus distances, this effect even increases in radiographs in supine position or intraoperative fluoroscopy. Therefore, if the symphysis is not at the level of the central X-ray beam, pelvic tilt should be corrected as described above. The central beam offset may
be automatically corrected in our Excel sheet ‘Tiltometer’, if the projected central beam offset distance (Y′) is inserted and the correction tool is activated.
Finally, this investigation is a feasibility study. Validation for accuracy was done with simulated radiographs. Although the tilt angles were adjusted with very high precision by means of modiCAS||3D (modiCAS GmbH, Erlangen, Ger-many), the correlation between our method and pelvic tilt in true lateral radiographs is pending. A higher absolute error must be expected, because assessment of pelvic tilt in lateral radiographs already bears the risk of potential mismeasurement.
To obtain information on the pelvic position is undoubt-edly important when measuring acetabular parameters and cup position in AP radiographs during hip-preserving sur-gery and total hip arthroplasty. Especially, hip parameters that describe exceeding acetabular coverage and pincer impingement are susceptible to relevant changes with pel-vic tilt [9, 22, 23]. Anterior tilt has significantly increased crossover signs in all investigations, which suggests exceed-ing ventral acetabular coverage and pincer impingement. In contrast, in case of acetabular dysplasia, anterior rotation of the pelvis may compensate deficient ventral acetabular coverage of the femoral head. In these cases, the crossover sign might be a secondary sign for dysplasia and can only be recognized as such when pelvic tilt is taken into account.
In total hip arthroplasty, cup position is highly dependent on pelvic tilt and should be taken into consideration by the surgeon [4–8]. In an own investigation, we showed varia-tions of 25° between minimum and maximum cup antever-sion and of 8° for inclination in ± 15° of pelvic tilt. In pre-operative AP radiographs of the pelvis in standing position that are used for arthroplasty planning, enhanced anterior or posterior tilt may be detected if the tilt ratio (B/A) diverges from 0.5. If cup position is intraoperatively referenced to the APP, the surgeon should consider higher anteversion during cup implantation if enhanced anterior pelvic tilt has been measured in preoperative radiographs. To avoid exceed-ing postoperative anteversion, lower anteversion should be considered when preoperative tilt ratios indicate enhanced posterior tilt. For quality measure and cup assessment in postoperative AP radiographs, we developed a mathemati-cal correction method for central beam offset, pelvic tilt, and pelvic rotation [8, 24]. Combining the formula and the method of this study, it is possible to assess tilt-corrected cup position in AP radiographs, enabling transformation of cup position that is referenced to the FCP (functional coronal plane) to the APP (anterior pelvic plane).
In this study, we describe a novel method for assessing pelvic tilt in AP radiographs in a definition predominantly used in total hip arthroplasty. Evaluation of pelvic tilt in AP radiographs can be simplified in daily clinical routine by means of tilt ratios. Information on pelvic tilt is fundamental
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and enhances the correct interpretation of acetabular param-eters and cup position in AP radiographs.
Funding There is no funding source.
Compliance with ethical standards
Conflict of interest The authors declare that they have no conflict of interest.
Ethical approval This study is a retrospective analysis of data obtained in a registered, prospective, controlled trial (DRKS00000739, Ger-man Clinical Trials Register). This investigation was approved by the local Ethics Committee (No. 10-121-0263). All procedures were in accordance with the ethical standards of the responsible committee on human experimentation and with the Declaration of Helsinki of 1975, as revised in 2000. The study collective including 3D CT of all patients was anonymized using numbers.
Informed consent Informed consent was obtained from all individual participants included in the study.
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