Wafa Skalli

Radiographic analysis of the sagittal alignment and balance of the spine in asymptomatic subjects

BACKGROUND There is an increasing recognition of the clinical importance of the sagittal plane alignment of the spine. A prospective study of several radiographic parameters of the sagittal profile of the spine was conducted to determine the physiological values of these parameters, to calculate the variations of these parameters according to epidemiological and morphological data, and to study the relationships among all of these parameters. METHODS Sagittal radiographs of the head, spine, and pelvis of 300 asymptomatic volunteers, made with the subject standing, were evaluated. The following parameters were measured: lumbar lordosis, thoracic kyphosis, T9 sagittal offset, sacral slope, pelvic incidence, pelvic tilt, intervertebral angulation, and vertebral wedging angle from T9 to S1. The radiographs were digitized, and all measurements were performed with use of a software program. Two different analyses, a descriptive analysis characterizing these parameters and a multivariate analysis, were performed in order to study the relationships among all of them. RESULTS The mean values (and standard deviations) were 60 degrees 10 degrees for maximum lumbar lordosis, 41 degrees +/- 8.4 degrees for sacral slope, 13 degrees +/- 6 degrees for pelvic tilt, 55 degrees +/-10.6 degrees for pelvic incidence, and 10.3 degrees +/- 3.1 degrees for T9 sagittal offset. A strong correlation was found between the sacral slope and the pelvic incidence (r = 0.8); between maximum lumbar lordosis and sacral slope (r = 0.86); between pelvic incidence and pelvic tilt (r = 0.66); between maximum lumbar lordosis and pelvic incidence, pelvic tilt, and maximum thoracic kyphosis (r = 0.9); and, finally, between pelvic incidence and T9 sagittal offset, sacral slope, pelvic tilt, maximum lumbar lordosis, and thoracic kyphosis (r = 0.98). The T9 sagittal offset, reflecting the sagittal balance of the spine, was dependent on three separate factors: a linear combination of the pelvic incidence, maximum lumbar lordosis, and sacral slope; the pelvic tilt; and the thoracic kyphosis. CONCLUSIONS AND CLINICAL RELEVANCE This description of the physiological spinal sagittal balance should serve as a baseline in the evaluation of pathological conditions associated with abnormal angular parameter values. Before a patient with spinal sagittal imbalance is treated, the reciprocal balance between various spinal angular parameters needs to be taken into account. The correlations between angular parameters may also be useful in calculating the corrections to be obtained during treatment.

Gravity line analysis in adult volunteers: age-related correlation with spinal parameters, pelvic parameters, and foot position.

Study Design. Prospective radiographic and forceplate analysis in adult volunteers. Objective. Assess gravity line (GL) location and foot position regarding anatomic spinal structures to evaluate key correlations and age-related changes in balance. Summary of Background Data. Global spinal balance is commonly assessed by the C7 plumbline. This radiographic parameter does not offer information on foot position or forces transmitted, and poor correlation with the true GL has been demonstrated. Methods. A total of 75 asymptomatic adult volunteers were equally distributed into three age groups. Full length, free-standing spine radiographs were obtained with simultaneous acquisition of GL and feet location (forceplate). GL and heels were projected on each radiograph to compute their distance from anatomic entities and to investigate correlations with radiologic parameters and age-related changes. Results. In this study group, advancing age led to a significant increase in thoracic kyphosis. The plumbline from C7 shifted anteriorly with age. In the sagittal plane, the GL was anterior to the vertebral column for all groups. With age, the GL location regarding the heels remained constant, while the pelvis moved posteriorly toward the heels and underwent a small retroversion (increasing pelvic tilt). The acetabulum was the most reliable radiographic marker of the GL location. Conclusions. This quantitative study in volunteers reveals clear age-related changes in the spino-pelvic association and offers quantitative support to the “cone of economy” concept proposed by Dubousset. The pelvis can be seen as a regulator to help maintain a rather fixed GL-heel association with age-related changes in the spinal column. Further study in patients suffering from deformity can confirm the importance of radiographic-gravity line correlations and enhance our understanding of optimal balance.

Standing Balance and Sagittal Plane Spinal Deformity: Analysis of Spinopelvic and Gravity Line Parameters

Study Design. Prospective study of 131 patients and volunteers recruited for an analysis of spinal alignment and gravity line (GL) assessment by force plate analysis. Objective. To determine relationships between GL, foot position, and spinopelvic landmarks in subjects with varying sagittal alignment. Additionally, the study sought to analyze the role of the pelvis in the maintenance of GL position. Summary of Background Data. Force plate technology permits analysis of foot position and GL in relation to radiographically obtained landmarks. Previous investigation noted fixed GL-heel relationship across a wide age range despite changes in thoracic kyphosis. The pelvis as balance regulator has not been studied in the setting of sagittal spinal deformity. Methods. The 131 subjects were grouped by sagittal vertical axis (SVA) offset from the sacrum: sagittal forward (>2.5 cm), neutral (−2.5 cm ≤ SVA ≤ 2.5 cm), and sagittal backward (SVA <−2.5 cm). Simultaneous spinopelvic radiographs and GL measure were obtained. Offsets between spinopelvic landmarks, heel position, and GL were calculated. Group comparisons were made for all offsets to determine significance. Results. Aside from the offset T9-GL and GL-heels, all other offsets between spinopelvic landmarks and GL revealed significant differences (P < 0.001) across the 3 subject groups. However, with increasing SVA, the GL kept a rather fixed location relative to the feet. A correlation between posterior pelvic shift in relation to the heels with increasing SVA in this study population was confirmed (r = 0.6, P < 0.001). Conclusion. Increasing SVA in standing subjects leads to a posterior pelvic shift in relation to the feet. However, no significant difference in GL-heel offset is noted with increasing SVA. It thus appears that pelvic shift (in relation to the feet) is an important component in maintaining a rather fixed GL-Heels offset even in the setting of variable SVA and trunk inclination.

3D reconstruction of the spine from biplanar X-rays using parametric models based on transversal and longitudinal inferences

Reconstruction methods from biplanar X-rays provide 3D analysis of spinal deformities for patients in standing position with a low radiation dose. However, such methods require an important reconstruction time and there is a clinical need for fast and accurate techniques. This study proposes and evaluates a novel reconstruction method of the spine from biplanar X-rays. The approach uses parametric models based on longitudinal and transversal inferences. A first reconstruction level, dedicated to routine clinical use, allows to get a fast estimate (reconstruction time: 2 min 30 s) of the 3D reconstruction and accurate clinical measurements. The clinical measurements precision (evaluated on asymptomatic subjects, moderate and severe scolioses) was between 1.2 degrees and 5.6 degrees. For a more accurate 3D reconstruction (complex pathologies or research purposes), a second reconstruction level can be obtained within a reduced reconstruction time (10 min) with a fine adjustment of the 3D models. The mean shape accuracy in comparison with CT-scan was 1.0 mm. The 3D reconstruction method precision was 1.8mm for the vertebrae position and between 2.3 degrees and 3.9 degrees for the orientation. With a reduced reconstruction time, an improved accuracy and precision and a method proposing two reconstruction levels, this approach is efficient for both clinical routine uses and research purposes.

3D reconstruction method from biplanar radiography using non-stereocorresponding points and elastic deformable meshes.

Standard 3D reconstruction of bones using stereoradiography is limited by the number of anatomical landmarks visible in more than one projection. The proposed technique enables the 3D reconstruction of additional landmarks that can be identified in only one of the radiographs. The principle of this method is the deformation of an elastic object that respects stereocorresponding and non-stereo-corresponding observations available in different projections. This technique is based on the principle that any non-stereocorresponding point belongs to a line joining the X-ray source and the projection of the point in one view. The aim is to determine the 3D position of these points on their line of projection when submitted to geometrical and topological constraints. This technique is used to obtain the 3D geometry of 18 cadaveric upper cervical vertebrae. The reconstructed geometry obtained is compared with direct measurements using a magnetic digitiser. The order of precision determined with the point-to-surface distance between the reconstruction obtained with that technique and reference measurements is about 1 mm, depending on the vertebrae studied. Comparison results indicate that the obtained reconstruction is close to the actual vertebral geometry. This method can therefore be proposed to obtain the 3D geometry of vertebrae.

A Biplanar Reconstruction Method Based on 2D and 3D Contours: Application to the Distal Femur

A three-dimensional (3D) reconstruction algorithm based on contours identification from biplanar radiographs is presented. It requires, as technical prerequisites, a method to calibrate the biplanar radiographic environment and a surface generic object (anatomic atlas model) representing the structure to be reconstructed. The reconstruction steps consist of: the definition of anatomical regions, the identification of 2D contours associated to these regions, the calculation of 3D contours and projection onto the radiographs, the associations between points of the X-rays contours and points of the projected 3D contours, the optimization of the initial solution and the optimized object deformation to minimize the distance between X-rays contours and projected 3D contours. The evaluation was performed on 8 distal femurs comparing the 3D models obtained to CT-scan reconstructions. Mean error for each distal femur was 1 mm.

The association of sagittal spinal and pelvic parameters in asymptomatic persons and patients with isthmic spondylolisthesis.

Using a specialized orthopedic software package, the authors investigated the sagittal spinal shape and the position of the pelvis in the space in patients with isthmic spondylolisthesis and in persons with no such symptoms. Digitized lateral spinal radiographs of 30 healthy volunteers and 48 patients were evaluated. The absolute values and significant correlations between parameters were analyzed. The pelvic parameters correlated well with lordosis, which shows sagittal balance in the asymptomatic group. The hyperlordosis and the horizontally positioned sacrum in isthmic spondylolisthesis enlarge the tensile force component of gravity, which may cause the lysis. Finally, the authors developed a new balance between the pelvis and the spine after slipping of the vertebral body. The degree of slipping correlated well with the sacrofemoral anatomic constant (incidence), which is unique in each person.

Quantification of Three-dimensional Vertebral Rotations in Scoliosis: What Are the True Values?

Study Design. The aim of 1 his study Is to quantity cliffs run ces between three dimensional rotations in space and their calculated values, either on two-dimensional projections (radiographs or computed tomographic scans) or three-ctimensicnal calculations using various mathematical procedures. Objective. To use a vertebral model to quantify differences between three-dimensional rotations and their calculated values, using two-dimensional projections or various three-dimensional mathematical procedures. Methods. A specific program allowed us to move a geometric vertebral model in space using given values and sequences of lateral, sagittal, and axial rotations. Differences in positions due to differen t sequences were visualized and quantified. Differences due to rotation around global or vertebral axes were considered. Results. For rotations of about 10º, differences are about 2º between three-dimensional and projected angles. Differences increase when combined rotations are large, as generally occurs in a scoliotic spine. They reach 16º for lateral and sagittal rotations of 30º. Conclusion. Axial rotation measured on transverse projection is misleading for vertebrae rotated in space. Moreover, dealing with large three-dimensional rotations is meaningful only if the used mathematical convention is given.

Surgical correction of scoliosis by in situ contouring: A detorsion analysis

Study Design. A detorsion analysis of the scoliosis surgical correction by means of in situ contouring technique (ISC). Objective. To describe the technique of ISC. To measure the vertebral and intervertebral axial rotation in thoracic and lumbar curves and their correction obtained by ISC. Summary and Background Data. The vertebral and intervertebral axial rotation allows to evaluate the severity of the curves. However, the intervertebral axial rotation is barely studied and the vertebral axial rotation is a controversial point of the surgical correction. Methods. Twenty patients with thoracic and lumbar scoliosis were operated on with ISC. Vertebral axial rotation at the apex and the sum of intervertebral axial rotations all along the curve were computed before and after surgery from the three-dimensional stereoradiographic reconstruction of the spine and the pelvis. All the measurements were made in the standing position. Results. Correction of the axial rotation was obtained at the apex of both thoracic and lumbar curves of idiopathic and degenerative scoliosis. The mean values of correction (in terms of axial rotation) were 8° to 19° (62%–67%). The percentage of correction of the sum of intervertebral axial rotations all along the curve, proposed as a “detorsion index” (preoperative − postoperative/preoperative), was found at 57% to 92%. No significant differences were found for the correction (in terms of axial rotation and detorsion) between idiopathic and degenerative curves. Conclusions. The axial rotation was measured in clinics on standing patients with scoliosis from three-dimensional stereoradiographic reconstruction and demonstrated a reliable detorsion obtained by ISC.

Angle Measurement Reproducibility Using EOSThree-Dimensional Reconstructions in Adolescent Idiopathic Scoliosis Treated by Posterior Instrumentation

Study Design. A reproducibility study was conducted in preoperative and postoperative three-dimensional (3D) measurements for patients operated for adolescent idiopathic scoliosis (AIS). Objective. To assess the reliability of preoperative and postoperative 3D reconstructions using EOS in patients operated for AIS. Summary of Background Data. No prior reliability study of 3D measurements has been performed in the literature for severe scoliosis and for operated patients. Methods. This series included 24 patients (62° ± 11) operated for Lenke 1 or 2 AIS, using either all-pedicle screw constructs (group 1) or hybrid constructs, with universal clamps at thoracic levels (group 2). All patients underwent low-dose standing biplanar radiographs, pre- and postoperatively. Three operators performed the 3D reconstruction process two times preoperatively and two times postoperatively (total 288 reconstructions). Intraoperator repeatability and interoperator reproducibility were calculated and compared between groups. Results. The preoperative reproducibility was between 4° and 6.5° for parameters dedicated to scoliosis (Cobb and apical vertebral rotation), between 4° and 7° for kyphosis and lordosis values, and between 1° and 5° for pelvic measurements. The postoperative reproducibility was between 5° and 8° for values of kyphosis and lordosis, between 1° and 5.5° for pelvic parameters, and between 6.5° and 10.5° for the scoliotic parameters. The reproducibility of the scoliotic parameters was slightly better in the hybrid construct group, but the difference was not significant (P = 0.8). No difference was found between groups for the other parameters. Conclusion. 3D postoperative reconstructions are as reproducible as preoperative ones. The reproducibility is not influenced by the type of implant used for correction. Mean difference between operator was higher than previously reported for the apical rotation measurement, but this difference can be explained by the severity of the curves and the lower visibility of the anatomical landmarks due to the implants.