Three-dimensional stereoradiographic modeling of rib cage before and after spinal growing rod procedures in early-onset scoliosis

Clinical Biomechanics 25 (2010) 284–291

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Three-dimensional stereoradiographic modeling of rib cage before and after spinal growing rod procedures in early-onset scoliosis Marc Sabourin a,b,*, Erwan Jolivet a, Lotfi Miladi b, Philippe Wicart b, Virginie Rampal a,b, Wafa Skalli a a b

Arts et Metiers Paristech, CNRS, LBM, 151 Boulevard de l’hopital, 75013 Paris, France Hopital Saint Vincent de Paul, 82 av. Denfert Rochereau, 75014 Paris, France

a r t i c l e

i n f o

Article history: Received 11 August 2009 Accepted 11 January 2010

Keywords: Scoliosis Surgical treatment Pediatrics Modeling Methods Stereoradiography

a b s t r a c t Background: Early-onset scoliosis frequently leads to major thoracic deformity and pulmonary restrictive disease. Growing rods surgical techniques were developed to achieve a satisfactory correction of the spinal curves during growth. The effect on the rib cage deformity has not yet been documented. The purpose of this study was to analyze the changes of the thoracic geometry after implantation of a growing rod, and to evaluate a stereoradiographic reconstruction method among young scoliotic patients. Methods: Four patients were enrolled in the study, and four additional patients in the reproducibility study. Three-dimensional spine and rib cage models were generated after low-dose stereoradiographic imaging (EOS). Three-dimensional parameters were computed before and after surgery. Intra and inter-observer reproducibility was calculated, and the accuracy was assessed in comparison to volumetric CT-scan. Findings: The average Cobb angle was reduced from 50.8° to 26°. The surgery resulted in a complex 3D effect on the rib cage, combining frontal, lateral, and axial rotation. This effect was dependent of the side (concave or convex), and the position relative to the apical vertebra. Mean errors in comparison to CTscan were 3.5 mm. Interpretation: The results on the spinal deformity are comparable to other series. The effect on the rib cage is of a smaller magnitude than in the case of a spinal arthrodesis. A longer follow-up is necessary to confirm the positive effect on the rib cage deformity. Further research should be performed to improve the reproducibility of 3D parameters. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Pulmonary function is one of the major issues of the management of juvenile scoliosis. In this specific type of scoliosis, the pattern of the curve tends to increase to major magnitudes associated with a pulmonary function impairment, and an above standard mortality rate (James, 1954; Jones et al., 1981; Branthwaite, 1986; Pehrsson et al., 1992). The thoracic deformity consecutive to early-onset scoliosis can lead to a restrictive disease (Muirhead and Conner, 1985). Since the maturation of the lung occurs during the first 9 years of life, one can observe an inhibition of the growth of the alveoli and of pulmonary vessels (Davies and Reid, 1971). The windswept shape of the thorax also leads to a mechanical respiratory inefficiency (Jones et al., 1981). Brace treatment is commonly advised, but some cases will progress despite a well-conducted non-operative treatment. In attempt to stop the curve progression, a short spinal fusion can be * Corresponding author. Address: Arts et Metiers Paristech, CNRS, LBM, 151 Boulevard de l’hopital, 75013 Paris, France. E-mail address: [email protected] (M. Sabourin). 0268-0033/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.clinbiomech.2010.01.007

performed. This procedure is effective on the spinal deformity, but adversely leads to a thoracic growth arrest. At the time of final arthrodesis, the restrictive lung disease is usually persistent in spite of the early surgery (Goldberg et al., 2003; Karol et al., 2008; Vitale et al., 2008). An alternate procedure is to use a spinal instrumentation without arthrodesis, which necessitates periodical lengthening to allow spinal growth. The first mention of this technique is due to Harrington (1962), promoting a subperiosteal approach. The results were not entirely satisfactory, because of implant failures and uncontrolled fusions. Luque’s trolley (Luque, 1977) has also been used, but a subperiosteal approach was necessary, leading to frequent growth arrests. Moe et al. (1984) described a modified technique, with an improved implant, and a subcutaneous placement. Still, the results remained inconclusive, associated with a high complication rate. With the development of modern spinal instrumentation, growing rods techniques gained a new interest. Single-rod and dual-rod variants have been developed, offering satisfactory curve correction and acceptable complication rate (Blakemore et al., 2001; Akbarnia et al., 2005; Thompson et al., 2007). Nevertheless, thoracic changes after surgery are only documented for congenital

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deformities, but not for juvenile scoliosis (Campbell et al., 2004). Moreover, it is commonly accepted that the thoracic deformity is only moderately correlated to the Cobb angle (Kuklo et al., 2005) especially in the case of young children (Grivas et al., 2007). To study a three-dimensional deformity, a voxel-based imaging protocol as Computed Tomography (CT-scan) or Magnetic Resonance Imaging (MRI) is usually required. Both techniques have the disadvantage of being performed supine, which modifies the frontal and transverse spinal curvatures and affects the thoracic shape (Torell et al., 1985; Yazici et al., 2001). In addition, the low resolution of MRI is a limitation for analysis of bony structures, while CT-scan generates a significant radiation exposure. The EOSÒ device is a low radiation bi-planar stereoradiographic system in which the acquisitions are simultaneously performed in a standing position with a radiation dose 10 times lower than standard full spine films (Kalifa et al., 1998; Dubousset et al., 2007). Out of these calibrated images, geometrical 3D models can be generated. Spine models have been previously validated (Champain et al., 2006). A reconstruction method of the rib cage was developed and validated in vitro and in vivo on healthy adult volunteers (Bertrand et al., 2008; Mitton et al., 2008). These data suggested that the chest modeling method was fast and accurate within normal adults. It has not yet been evaluated amongst children, nor in the case of scoliotic thoracic deformity. The purpose of this study was to analyze the early changes of the thoracic geometry after implantation of a growing rod, and to evaluate our stereoradiographic reconstruction method among young scoliotic patients.

The spine was partially exposed through two small midline incisions performed at the top and bottom of the planned construct, under radiological guidance. The dissection was carried out through the paravertebral muscles in order to limit the periosteal damage. Three hooks were used as a proximal anchor: one pedicular, and two supralaminar on the two upper vertebrae. Distally, two screws were used as a foundation for the construct. A 5.5 mm CD LegacyÒ (Medtronic) titanium rod was contoured to the desired profile and inserted throughout the paravertebral muscles. After rod derotation, a slight distraction was applied under the control of somatosensory and motor evoked potentials (Fig. 1).

2. Methods

2.3. Morphometric analysis

The studied population was children suffering of early-onset idiopathic scoliosis, progressive despite a well-conducted orthopedic treatment. We excluded the cases of congenital scoliosis with fused or abnormally segmented ribs. Seven children, aged from 6 to 11 years old, with an average Cobb angle of 65.8° were enrolled in the study (Table 1). Pre- and post-operative bi-planar X-rays were proceeded for four of them (patients 1–4). The post-operative acquisition was performed 1–6 months after surgery. The three other children (patients 5–7) were only studied before surgery; their data were therefore included in the repeatability study. All children and their parents were informed of the nature of the examination, and a signed consent was formally obtained. This study was approved by the national review board (CCPPRB 6001) in accordance to our national laws.

Spinal and thoracic parameters were calculated from the threedimensional objects. The reference mark (x, y, z) was orthonormal and connected to the pelvis: the (y) axis was horizontal and parallel to the bi-acetabular axis. The (x) axis was horizontal and posteroanterior, and the (z) axis was vertical. This reference was independent of the position of the subject inside the radiological device and therefore allowed for pre- and post-operative comparison.

2.1. Surgical technique Growing rods procedures have already been described by other authors (Akbarnia et al., 2005; Blakemore et al., 2001; Thompson et al., 2007). We have been using a slightly different procedure, which is hereby described. The surgery was performed in supine position under general anesthesia, and with a soft bipolar traction.

2.2. Imaging protocol and 3D modeling Pre- and post-operative bi-planar X-rays were performed using the EOSÒ system (Biospace Instruments, France). This technique produces simultaneously a frontal and lateral X-ray of the whole body in a standing position with a very low exposure to radiations (Hoan et al., 1979; Charpak, 1996; Kalifa et al., 1998). The images were computed within a specific software, developed jointly by the Laboratory for Imaging Research and Orthopedics (ETSCRCHUM, Montreal, Canada) and the Laboratoire de Biomécanique (CNRS-Arts et Métiers ParisTech, Paris, France), allowing threedimensional spinal reconstruction (Mitton et al., 2000; Pomero et al., 2004). The rib cage was reconstructed using the semi-automated method previously developed (Bertrand et al., 2008; Mitton et al., 2008). Several parameters were then calculated to quantify the 3D deformity.

2.4. Spinal deformity The frontal Cobb angle was calculated after projection of the 3D object, and was compared to Ferguson angle. The sagittal T9 offset, T1–12 kyphosis and L1–L5 lordosis were calculated. The balance was assessed by the T1 plumbline in reference to the middle of the sacral plate in the frontal and sagittal plane. Vertebral axial rotation and frontal tilt were also calculated at each spinal level. 2.5. Ribs orientation To estimate the spatial orientation of each rib, a ‘‘best-fit plane” was calculated using the least squares method. Frontal and lateral

Table 1 Clinical data. Patient nos.

Age (years)

Gender

Main curve

Apex

Preop frontal Cobb angle (limit vertebrae)

Instrumentation

1 2 3 4 5 6 7

6 7 11 9 8 11 8

F F F F M F M

Right thoracic Right thoracic Left thoracic Right thoracic Left thoraco-lumbar Right thoracic Right thoraco-lumbar

T8 T8 T9 T8 T11 T9 T10

33° 68° 73° 29° 84° 80° 94°

T3–L2 T3–L3 T4–L2 T4–L1 Preop only Preop only Preop only

Subjects characteristics. Patients nos. 5–7 were included in the reproducibility study only. The limit vertebrae of the main curve are displayed in brackets in column 6 (preop = preoperative).

(T4–T12) (T5–T12) (T6–T12) (T5–T11) (T8–L2) (T7–L1) (T5–L2)

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Fig. 1. Pre-operative and post-operative view of patient no. 3.

angle were defined by the orientation of this plane relative to the horizontal (Fig. 2).

To calculate the thoracic rotation in the transverse plane and quantify the rib hump, each pair of ribs was first projected to the

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transverse plane, then the ‘‘double tangent” line was drawn. It was defined as the line joining the most posterior point of each hemithorax. The horizontal angle between this line and the pelvis defined the axial rotation angle. The mean inter-rib distance was computed as the average point-to-surface distance between each rib and the best-fit plane of the adjacent rib. For each patient, the ribs were re-numbered in relation to the apex of the main curvature (from proximal to distal: A 7, A 6, . . . , Apex, . . . , A + 2), and the side was defined as concave or convex relative to the main curvature. Therefore, the average post-operative changes for the whole series could be calculated.

2.6. Spine/ribs relationship We adapted two previously reported 2D measurements to our 3D objects. Mehta’s rib-vertebra angle (Mehta, 1972) was calculated by projecting the ‘‘best-fit planes” in the vertebral referential. The Rib Vertebra Angle-Difference (RVA-D) was defined as the angular difference between the concave and the convex side. To determine the global asymmetry between the two hemithoraxes, we also adapted the Apical Vertebral Body-Rib Ratio (AVB-R) (Kuklo et al., 2005). In our study, the distance is measured between the barycenter of the vertebral body and the most lateral point of each hemithorax on the y axis. The value corresponds to the concave/ convex ratio in the horizontal plane at the level of the apical vertebra.

Fig. 2. Three-dimensional orientation parameter of the ribs. For each rib, the ‘‘bestfit plane” was calculated using the least squares method. Frontal (a) and lateral (c) angles are defined after projection of this plane on the orthonormal reference. To calculate the axial rotation angle (b), we first projected each rib pair on the horizontal plane. A line joining the most posterior points of each rib was generated. The axial angle (b) was defined by the intersection of this line and the line joining the center of the acetabulums.

287

2.7. Method validation 2.7.1. Accuracy The direct comparison of 3D data from CT-scan or MRI with EOSÒ models is not possible because the EOSÒ radiographs are performed in an upright position and CT is performed supine (Torell et al., 1985). To circumvent this problem, we constructed bi-planar projections out of CT-scan (Fig. 3), and then used these projected X-rays as a material for stereoradiographic reconstructions with our specific software. These models were then compared with volumetric objects built using the AvizoÒ software (Mercury Computer Systems) from native CT-scan, and the rib’s midlines were extracted. A direct comparison between the objects was done by calculating the point-to-surface distance, and the parameters previously described were compared. The mean and maximum errors were calculated, and 95% confidence interval (95% C.I.) was computed as 2  RMSSD (Root Mean Square Standard Deviation), according to current guidelines (Glüer et al., 1995). For ethical reasons relative to the amount of radiation required, CT scanning was not performed for all children. We analyzed the chest CT-scan of one of the study patients (patient no. 7) which was performed because of the importance of the deformity. We also used the chest CT-scan of another 10 years old non-scoliotic girl, which was performed after a blunt thoracic trauma without bony lesion. 2.7.2. Reproducibility Intra and inter-observer reproducibility was evaluated for all the pre- and post-operative X-rays of the whole group. The spine, pelvis and chest were reconstructed twice by an orthopaedic surgeon trained to spinal surgery (M.S.), and a third time by a non-clinician expert operator (C.F.).

Fig. 3. Bi-planar Digitally Reconstructed Radiographs (DRR). We used chest CTscans to build DRR. These DRR were used as a material for our stereoradiographic modeling method. The models were then compared to volumetric models issued of CT-slices to assess the accuracy of the method.

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Fig. 4. Frontal, lateral, axial angles and Rib Vertebra Angle-Difference (RVA-D). The pre- and post-operative frontal, lateral and axial angles of the convex ribs, concave ribs, and vertebrae are displayed, a well as the pre- and post-operative Rib Vertebra Angle-Difference. (The level is defined relative to the apex of the main scoliotic curvature. A 3, A 2, and A 1 are supra-apical, and A + 1 and A + 2 are infra-apical.)

Reproducibility was assessed for the previously detailed parameters, as well as for the point-to-surface distance. The 95% C.I. was calculated as 2  RMSSD. 3. Results 3.1. Spinal deformity The post-operative changes in spinal parameters are detailed in Table 3. Table 2 Accuracy and reproducibility of chest walls models. Errors/CT-scan

Point-to-surface (mm) Ribs orientation Frontal angle (°) Lateral angle (°) Axial rotation (°)

Reproducibility (95% C.I.)

Mean

Max

95% C.I.

Intraobserver

3.5

13.6

4.2

2.6

3.8

4.6 1.7 2.2

6.5 3.2 1.9

4.6 8.4 3.2

9.9 10.4 5.2

0.2 0.5 0.3

Interobserver

Errors between volumetric CT-scan reconstruction and stereoradiographic reconstructions (mean error, max error, and 95% C.I.). Intra and inter-observer reproducibility is also exposed. Reproducibility is expressed by the 95% C.I. which is calculated as 2  RMSSD (Glüer et al., 1995).

In the frontal plane, the Cobb angle decreased from 50.8° to 26° ( 49%) in average. The overall balance modification was variable amongst the series, with an improvement for patients 1 and 3. The frontal tilt of superior and inferior uninstrumented vertebrae was improved for patients 1, 3 and 4, but deteriorated for patient 2. The Ferguson angle had a smaller variation range but presented a better repeatability. In the sagittal plane, patients 2 and 4 presented an increase in thoracic kyphosis while patients 1 and 3 post-operative values were similar to preoperative. The T9 sagittal offset decreased for patients 1 and 3, but increased for patient 4. The derotation of the apical vertebra was variable. There was a post-operative improvement for patients 1 and 3. On the whole series, the average was 2.7°. The mean increase in Sacrum-T1 height was 2 cm. 3.2. Chest wall deformity (Fig. 4) 3.2.1. Frontal orientation of the ribs  Before surgery, we observed that the convex rib had a greater frontal obliquity than the concave ribs. The post-operative values had an opposite variation on the concave and convex side. On the convex side, the frontal angle increased in the

For each patient, the pre-operative and post-operative values are exposed, as well as the difference observed between these two values. The average value among the whole series is reported. The last columns reports the intra and inter-observer reproducibility (95% C.I. = 2  RMSSD) for each parameter (T1 plumbline = distance from T1 plumbline to the center of the sacral plate; SUV = superior uninstrumented vertebra; IUV = inferior uninstrumented vertebra; intra = intra-observer; inter = inter-observer).

5.49 4.57 3.57 2.84 5.43 3.39 11.56 13.56 3.89 3.09 6.96 8.89

3.07 5.8

6.13 3.31

13.32 4.65

7.19 1.34

15.19 5.75

3.63 7.8

8.75 2.41

3.33 5.8

9.26 3.44

5.84 0.21

3.42 3.65

6.22 4.8 11.33 14.04 12.23 15.07

2.84

21.87

17.28

39.14

0.41

14.45

5.79

5.55

7.69

5.83

13.52

9.68 9.62 5.15 1.16 3.9 3.79 8.4 7.23 3.25 1.15 0.82 2.43 25.85 9.26 1.87 1.39 6.76 9.26 3.33 2.8 4.29 1.48 2.6 28.57 4.84 5.26 7.39 2.24 4.74 13.91 53.04 44.62 28.24 2.56 17.77 30.03

48.2 39.35 35.63 0.33 13.03 16.12

26.37 27.14 36.59 1.37 10.88 17.89

44.12 21.7 21.85 0.48 9.58 1.13

17.75 5.44 14.74 0.89 1.29 16.76

52.98 40.4 27.27 3.3 19.81 24.72

49.66 43.2 31.56 1.82 17.22 53.29

17.74 42.4 49.04 1.65 1.75 24.95

43.6 51.66 47.16 0.27 8.51 34.21

37.53 38.64 35.28 2.22 12.55 24.4

46.39 38.98 34.05 0.72 12.08 26.19

8.86 0.34 1.23 1.5 0.47 1.79

9.55 3.55 17.78 3.58 2.44 6.19

Inter Intra

24.76 18.38 2.68 26.02 23.58 15.96 50.78 41.96 18.64 5.42 5.16 2.86 23.91 21.47 13.77 29.33 26.62 10.9 38.38 23.61 5.9 34.71 29.66 15.55 73.09 53.27 21.45 36 26.2 0.97 31.79 34.02 23.17 67.79 60.22 22.21 19.26 18.57 8.65 13.66 9.16 11.35 32.92 27.72 20

Cobb angle Ferguson angle Apical vertebral rotation Kyphosis T1/T12 (°) Lordosis L1/L5 (°) Sacral slope (°) Pelvic frontal tilt (°) T9 sagittal tilt (°) T1 plumbline frontal (mm) T1 plumbline sagittal (mm) Frontal angle SUV (°) Frontal angle IUV (°)

Average Patient 4 Patient 3 Patient 2

Preop Diff. Posop Preop

Patient 1

Table 3 Spinal parameters value, post-operative correction, and reproducibility.

Posop

Diff.

Preop

Posop

Diff.

Preop

Posop

Diff.

Preop

Posop

Diff.

Reproducibility (95% C.I.)

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supra-apical position (max increase: +5.1°) and decreased in infra-apical position (max decrease: 4.34°). On the concave side, the angle increased mainly in infra-apical position.  When comparing concave and convex sides, the Rib Vertebral Angle-Difference was maximal in the upper thoracic region (max = 42.3°), was null for the apical vertebra, and was negative for the lower levels of the major curve (min = 27.6°). After surgery, the pattern was somehow similar, but with a smaller magnitude (max = 38.4°, min = 21.7°). 3.2.2. Lateral orientation of the ribs  On the convex side, the obliquity was maximal for the apical rib and the two adjacent superior (53°). After surgery, these ribs maintained the same angle to the horizontal. This angle decreased postoperatively for the proximal ribs ( 3.6° for A 3), and increased below the apex (+5.6° for A + 1).  On the concave side, the angle was smaller for the proximal ribs (28.2° for A 3), and greater distally (A + 2). After surgery, the slope increased in the upper part of the main curvature (A 4 to A). 3.3. Thoracic axial rotation The thoracic rotation was maximal one level below the apex, averaging 10.8°. In the upper thoracic region, the angle was opposite and of a smaller magnitude, averaging 5.7°. After surgery, it appeared that thoracic rotation (hence the rib hump) was only slightly modified. Thus, the maximum correction was found at the apical +2 level, with an average correction of 1.6°. 3.4. Thoracic asymmetry The asymmetry between the two hemithorax was evaluated by the modified AVB-R. At the apical level, its preoperative value is 1.72, and the post-operative ratio is 1.6. 3.5. Accuracy and reproducibility of chest wall models Among the two subjects enrolled in the accuracy study, the mean difference between volumetric CT and EOS model was 3.5 mm when using a direct point-to-surface comparison. The precision was ±4.2 mm, as defined by 2  RMSSD. Among all the patients enrolled in the repeatability study, the reproducibility of EOS reconstruction showed a good concordance when using a direct point-to-surface comparison (±2.6 mm for intra observer and 3.8 mm for inter-observer repeatability). The least reproductible parameter was the lateral angle (±8.4°), and the most reproductible was the thoracic rotation (±3.2°). The details of the accuracy and repeatability study are exposed in Table 2. 4. Discussion This preliminary study of post-operative changes after growing rod instrumentation is mainly limited by the sample size, and the heterogeneity of the deformities. Nevertheless, this study is at our knowledge the first attempt to quantify the chest wall deformity of juvenile scoliosis and the corrective effect of fusionless surgical techniques. After surgery, the asymmetry between the two hemithorax was reduced: in the main curve, the frontal angle of the ribs increased on the concave side, and decreased below the apex on the convex side. The frontal convex angle variation was in the same direction as the vertebral frontal tilt, whereas it was in the opposite direction for the concave side. After surgery, the lateral angle

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decreased in supra-apical position for convex ribs and in infra-apical position for concave ribs. We have also used a previously described 2D measurement, the AVB-R ratio (Kuklo et al., 2005) to our 3D data, instead of surfacic thoracic measurements because of two main problems. First, the modelisation of soft tissues is not yet fully implemented in the EOSÒ system. Research is actively conducted in this way, but there is no soft tissue mediastinal validated model available at this time. Second, the calculation of a surface is dependant of the square of the distances and could lead to increased errors. A study of three dimensional chest wall changes after spinal fusion has been published (Delorme et al., 2001). Their global results were somehow similar to ours, but of greater magnitude. It is noticeable that in spite of a non-segmental approach, the effect of growing rods on chest geometry can be qualitatively compared to the one of segmental correction. Concerning the spinal correction, the average improvement of Cobb angle was 25° (48% of the initial value). This result is equivalent to other growing rods series (Akbarnia et al., 2005; Thompson et al., 2005). We have chosen to use a single-rod technique, even if a comparative study indicated a greater corrective strength for dual-rod construct (Thompson et al., 2005). This choice was made because the use of a single rod reduces the operative time of the first procedure, and makes the periodical lengthening procedures easier. To reduce the implant-related failures reported with single rods, we have been using a greater diameter (5.5 mm) titanium rod. The foundation technique is also stronger, using two distal pedicular screws and three proximal hooks: one pedicular, and two supralaminar. In our opinion, these modifications to the original techniques should improve the 3D corrective strength as well as the loading resistance. Long-term follow-up is necessary to prove these assumptions. Regarding the sagittal curves, we report an overall increase in thoracic kyphosis. The thoracic kyphosis was closer to normal values for every patient, especially in case of lordoscoliosis (patients 2 and 4). There was an improvement in apical rotation, even if it was lower than in the case of a segmental instrumentation associated with arthrodesis (Dumas et al., 2005). This short-term improvement in apical rotation is encouraging, but it should be emphasized that an increase in apical rotation has been reported at late followup, despite the growing rod treatment (Acaroglu et al., 2002). The other goal of this study was the validation of our low-dose stereoradiographic technique for the study of chest wall deformities among young scoliotic patients. As a matter of fact, X-rays exposure is a great matter of concern for those children who will undergo numerous spine and chest radiographs during their whole life. The previously reported in vitro precision (Mitton et al., 2008) was ±10% for the rib’s length. In vivo on non-scoliotic adults (Bertrand et al., 2008) and using a point-to-surface comparison, the intra and inter-observer differences were respectively ±3.5 mm and ±5.1 mm. We hereby report ±2.6 mm and ±3.8 mm, as using the same comparison protocol, and ±4.2 mm in comparison to the ‘‘gold standard” 3D CT-scan. The three-dimensional orientation parameters were less reproducible than the shape itself. We supposed that the approximation method (i.e. the ‘‘best-fit plane” calculated with the least squares method) could be responsible for these errors. The least reproducible parameter was the lateral orientation, which can be explained by the superimposition on the lateral X-rays. Nevertheless, the values are provided with a 95% confidence interval ranging from ±1.9° to ±6.5° in comparison to 3D CT-scan, and from ±5.15° to ±10.4° for inter-observer reproducibility. These values can be considered as acceptable, considering that the spinal deformities reported in our study are important, ranging from 29° to 94°. Even if the corrective effect of surgery can be clearly appreciated on the radiographs and the 3D models, the changes in the ori-

entation parameters are smaller than the 95% C.I. hereby reported. Further research is presently performed in order to improve the reproducibility of the current quantitative analysis method. 5. Conclusion Growing rods techniques have a corrective effect on chest wall geometry. This effect is quantitatively lower than when performing a segmental correction and fusion. The use of a low-dose bi-planar radiographic system can provide valid 3D spine and chest wall models in the case of major deformities. A greater sample and a longer follow-up are necessary to prove the long-term effectiveness of the correction and the improvement of the respiratory function, and new quantitative 3D parameters should be developed to better quantify the chest wall scoliotic deformity. Conflict of interest The authors acknowledge that we do not have any financial or personal relationships with other people or organizations that could inappropriately influence (bias) the work described in the current manuscript. Funding sources This work was funded by a research grant from the ‘‘Association Française pour le Materiel d’Ostéosynthèse” (AFMO) and the ‘‘Société Française de Chirurgie Orthopédique et Traumatologique” (SOFCOT). This work was supported by the ‘‘pôle de compétitivité MEDICEN”. AFMO, MEDICEN and SOFCOT did not have any personal or commercial interest in this research. Acknowledgement We would like to thank Cédric Fedelich for his technical support and for his conscientious work during the reproducibility study. References Acaroglu, E., Yazici, M., Alanay, A., Surat, A., 2002. Three-dimensional evolution of scoliotic curve during instrumentation without fusion in young children. J. Pediatr. Orthop. 22 (4), 492–496. Akbarnia, B.A., Marks, D.S., Boachie-Adjei, O., Thompson, A.G., Asher, M.A., 2005. Dual growing rod technique for the treatment of progressive early-onset scoliosis: a multicenter study. Spine 30 (17 Suppl.), S46–S57. Bertrand, S., Laporte, S., Parent, S., Skalli, W., Mitton, D., 2008. Three-dimensional reconstruction of the rib cage from biplanar radiography. IRBM 29, 278–286. Blakemore, L.C., Scoles, P.V., Poe-Kochert, C., Thompson, G.H., 2001. Submuscular Isola rod with or without limited apical fusion in the management of severe spinal deformities in young children: preliminary report. Spine 26 (18), 2044– 2048. Branthwaite, M.A., 1986. Cardiorespiratory consequences of unfused idiopathic scoliosis. Brit. J. Dis. Chest 80 (4), 360–369. Campbell Jr., R.M., Smith, M.D., Mayes, T.C., Mangos, J.A., Willey-Courand, D.B., Kose, N., Pinero, R.F., Alder, M.E., Duong, H.L., Surber, J.L., 2004. The effect of opening wedge thoracostomy on thoracic insufficiency syndrome associated with fused ribs and congenital scoliosis. J. Bone Joint Surg. Am. 86 (A(8)), 1659–1674. Champain, S., Benchikh, K., Nogier, A., Mazel, C., Guise, J.D., Skalli, W., 2006. Validation of new clinical quantitative analysis software applicable in spine orthopaedic studies. Eur. Spine J. 15 (6), 982–991. Charpak, G., 1996. Prospects for the use in medicine of new detectors of ionizing radiation. Bull. Acad. Natl. Med. 180 (1), 161–168 (discussion 168). Davies, G., Reid, L., 1971. Effect of scoliosis on growth of alveoli and pulmonary arteries and on right ventricle. Arch. Dis. Child. 46 (249), 623–632. Delorme, S., Violas, P., Dansereau, J., De Guise, J., Aubin, C.E., Labelle, H., 2001. Preoperative and early postoperative three-dimensional changes of the rib cage after posterior instrumentation in adolescent idiopathic scoliosis. Eur. Spine J. 10 (2), 101–107. Dubousset, J., Charpak, G., Skalli, W., Kalifa, G., Lazennec, J.Y., 2007. EOS system: whole-body simultaneous anteroposterior and lateral radiographs with very low radiation dose. Rev. Chir. Orthop. Reparatrice Appar. Mot. 93 (6 Suppl.), 141–143.

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