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A monocentric, retrospective radiographic study with 99 asymptomatic volunteers.The authors performed the postural analysis commonly scheduled when evaluating sagittal balance in a vertebra-by-vertebra manner by enrolling an asymptomatic population. They measured the position and angulation of each vertebra to reveal those for which the spatial positioning could be relevant during spinal surgeries.Several recent publications detailed the sagittal alignment parameters and focus on global analysis parameters. Some patients with identical commonly evaluated spinal parameters have exhibited very different profiles, with notable differences in vertebral positions and orientations. Therefore, a fine segmental analysis of position of each vertebra could be interest to gain understanding of spine alignment.The authors obtained full-spine EOS x-rays of 99 volunteers in the standard free-standing position. We used a validated three-dimensional reconstruction technique to extract current spinal parameters and the positions and angulations of all vertebrae and lumbar discs. Particular attention was paid to the positions and angulations of the apical and transitional vertebrae in the general population and in subgroups according to pelvic incidence (PI).T1 was the most common transitional cervicothoracic vertebra (in 89.9% of subjects) and was oriented downwards by an average of 22.0° (SD=7.3°, minimum=2.3°, maximum=40.1°). The thoracic apex trio of T5 (22.2%), T6 (28.3%), and T7 (36.4%) were equally found. The transitional thoracolumbar vertebrae were L1 (39.4%) and T12 (33.3%). The lumbar apex was usually the L3-L4 disc (36.4%). T1 seemed to be the transitional vertebra (90%) irrespective of the PI. For the other relevant vertebrae, the greater the PI, the more cranial the vertebra.We performed a detailed three-dimensional assessment of overall spinal balance using positional and rotational parameters. The positions and orientations of all vertebrae were specified, particularly the apical and transitional vertebrae.Level 3.

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.

Study design A numerical study was conducted to identify the intervertebral stiffness of scoliotic spines from spinal flexibility tests. Objective To study the intervertebral 3-dimensional (3D) stiffness distribution along scoliotic spine. Summary of background data Few methods have been reported in literature to quantify the in vivo 3D intervertebral stiffness of the scoliotic spine. Based on the simulation of flexibility tests, these methods were operator-dependent and could yield to clinically irrelevant stiffnesses. Methods This study included 30 patients surgically treated for severe idiopathic scoliosis. A previously validated trunk model, with patient-specific geometry, was used to simulate bending tests according to the in vivo displacements of T1 and L5 measured from bending test radiographs. Differences between in vivo and virtual spinal behaviors during bending tests (left and right) were computed in terms of vertebral rotations and translation. An automated method, driven by a priori knowledge, identified intervertebral stiffnesses in order to reproduce the in vivo spinal behavior. Results Because of the identification of intervertebral stiffnesses, differences between in vivo and virtual spinal displacements were drastically reduced (95% of the differences less than +/-3 mm for vertebral translation). Intervertebral stiffness distribution after identification was analyzed. On convex side test, the intervertebral stiffness of the compensatory curves increased in most cases, whereas the major curve became more flexible. Stiffness singularities were found in junctional zones: these specific levels were predominantly flexible, both in torsion and in lateral bending. Conclusion The identification of in vivo intervertebral stiffness may improve our understanding of scoliotic spine and the relevance of patient-specific methods for surgical planning.

Study design A three-dimensional analysis of spino-pelvic alignment in 60 asymptomatic young adult males and females. Objectives To analyze the differences in sagittal spino-pelvic alignment in a group of asymptomatic young adult males and females and describe gender specific reference values. Summary of background data Several spinal disorders like idiopathic scoliosis and Scheuermann's disease have a well-known sex-related prevalence ratio. As spino-pelvic alignment plays an important role in spinal biomechanics, it is imperative to analyze possible differences between the male and female spino-pelvic alignment. Furthermore, in spinal fusion surgery, normal sagittal balance should be recreated as closely as possible. Methods An innovative biplanar ultra low-dose radiographic technique was used to obtain three-dimensional reconstructions of the spine (T1-L5), sacrum, and pelvis in a freestanding position of 30 asymptomatic young male and 30 young female adults. Values were calculated for thoracic kyphosis (T4-T12), lumbar lordosis (L1-S1), total and regional lumbopelvic lordosis (PRT12, PRL2, PRL4, and PRL5), sagittal plumb line of T1, T4, and T9 (HAT1, HAT4, and HAT9), T1-L5 sagittal spinal inclination, T9 sagittal offset, and pelvic parameters (pelvic tilt, sacral slope, and pelvic incidence). In addition, vertebral inclination in the sagittal plane of each vertebra was measured. Differences in spino-pelvic alignment between the sexes were analyzed. Results The female spine was more dorsally inclined (11 degrees vs. 8 degrees ; P = 0.003). High thoracic and thoracolumbar vertebrae were more dorsally inclined in women than in men. Thoracic kyphosis, lumbar lordosis, regional lumbopelvic lordosis, sagittal plumb lines, T9 sagittal offset, and pelvic parameters were not statistically different between the sexes. Conclusion These results indicate that the female spine is definitely different from the male spine. The spine as whole and individual vertebrae in certain regions of the normal spine is more backwardly inclined in females than in males. Based on our previous research this signifies that these spinal regions are subjected to different biomechanical loading conditions. These vertebral segments are possibly less rotationally stable in females than in males.

STUDY DESIGN A conceptual study of the influence of the geometry of an articulated disc prosthesis at the cervical level using a 3-dimensional nonlinear finite element model. OBJECTIVE To investigate how the mechanical behavior of the functional spinal unit is affected by the position of the center and the size of the radius of a ball-and-socket design at the cervical level. SUMMARY OF BACKGROUND DATA Little is known about the changes in kinematics and internal efforts after intervertebral disc replacement at the cervical spine, specifically regarding the influence of the geometry of the articulated prosthesis. METHODS A ball-and-socket artificial disc was integrated in a validated 3-dimensional nonlinear finite element model of the cervical spine (posterior geometric center, large radius). The model was loaded in flexion, extension, lateral bending, and axial torsion. Two variant designs were investigated: anterior center and small radius. The intervertebral range of motion, the mean center of rotation, and the contact forces in the facet joints and in the bearing surface of the prosthesis were investigated. RESULTS The range of motion was similar with all prostheses. The posterior geometric center was associated with an adequate mean center of rotation in flexion/extension. The large radius of curvature was associated with the partial unloading of the facet joints and the redistribution of the constraints at the ball-and-socket interface. CONCLUSION Our data estimate the influence of the geometrical parameters of a ball-and-socket total disc replacement on kinematics and constraints at the cervical functional spinal unit. Under the experimental conditions, the facet forces were kept below their normal range in the case of the posterior center and a large radius.

STUDY DESIGN Prospective randomized study of patients undergoing lumbar arthrodesis. OBJECTIVES To quantify MRI changes of the erector spinae following lumbar surgery through a posterior approach and the possible protection of these muscles during surgery by the use of cholinergic blockade. SUMMARY OF BACKGROUND DATA It has been shown that lumbar spine surgery through a posterior approach can induce iatrogenic lesions in the erector spinae. We have shown in a previous study that histologic changes on muscular biopsy performed in the multifidus at the end of the surgical procedure were not modified by the use of cholinergic blockade during surgery. METHODS Twenty patients scheduled to undergo pedicle-screw enhanced L4-L5 arthrodesis were enrolled in this study. Ten patients received curare during anesthesia and 10 patients did not. MRI was obtained the day before the operation and at 6 months of follow-up on the same MR scanner. T1-weighted images were obtained in the axial plane. The 2 slices immediately proximal and distal to the pedicle screw construct on the postoperative MRI were selected. The corresponding slices were selected on the preoperative MRI. Each erector spinae on the 4 slices was surrounded using a mouse-guided tool. The contractile component of the cross-sectional area (CCSA) was calculated from the number of pixels surrounded and the signal intensity of each pixel. RESULTS There was only slight changes in the erector spinae CCSA proximal to a posterior lumbar arthrodesis. Erector spinae CCSA decreased by 27% distal to the arthrodesis. Curare showed no efficacy in preventing muscle damage. CONCLUSIONS Erector spinae muscle alterations mainly occur distal to posterior lumbar surgical procedures.

Study design A reliability study was conducted in quantitative 3-dimensional (3D) measurements for mild scoliosis. Objective To evaluate the intrarater and interrater reliability of a computer tool used for 3D reconstruction of the spine. Summary of background data No reliability study of spinal in vivo 3D medical imaging measurements has been performed in the literature. Methods This study included 30 patients (mean age 13 years) with mild idiopathic scoliosis. Spinal 3D reconstruction was performed using a new technique called semiautomatic 3D reconstruction, which requires only the location of the corners of each vertebral body on 2 orthogonal views. Three raters performed the 3D reconstruction procedure on the 30 pairs of radiographs in random order. One of the raters repeated the procedure for the 30 patients 15 days later. Inter-reliability and intra-reliability were estimated for different parameters: thoracic kyphosis, lumbar lordosis, Cobb's angle, pelvic morphologic and positional parameters, and axial rotation. Results Intraclass correlation coefficient showed good or very good agreement for most of the measurements. The 95% prediction limits are approximately 4 degrees for the measurements of spinal curves, 2 degrees for pelvic parameters, and axial vertebral rotation. Conclusions The reliability of 3D reconstruction of the spine is acceptable, and this technique can be used for clinical studies.

STUDY DESIGN A 3-dimensional (3-D) analysis of asymptomatic spinal and pelvic alignment. OBJECTIVE To obtain 3-D reference values of spinal and pelvic parameters, vertebral and intervertebral orientations. SUMMARY OF BACKGROUND DATA Referential values of spine and pelvis alignment are essential for the assessment of posture and balance. However, only 2-D referential values have been reported using standing sagittal radiographs, and, to our knowledge, no 3-D referential values have been reported to date. METHODS A biplanar radiographic technique was used to obtain the 3-D reconstruction of the spine and pelvis of 34 asymptomatic standing subjects. The 3-D values were calculated for most of the spinal and pelvic parameters. In addition, 3-D vertebral and intervertebral orientations were computed, and the apical and junctional zones were investigated. RESULTS As reported in 2-D, a large variability and particular correlations were observed for the 3-D spinal and pelvic parameters. However, significant differences were found between 3-D and 2-D values. The 3-D vertebral and intervertebral sagittal rotations showed specific features in the apical and junctional zones of the asymptomatic spine. CONCLUSION These data may be used as 3-D referential values of spinal and pelvic alignment.

STUDY DESIGN This is a prospective radiographic and force plate analysis involving adult volunteer and patients with scoliosis. OBJECTIVE To assess accurately the center of pressure in standing volunteers and patients with scoliosis, and correlate these finding with radiographic data. SUMMARY OF BACKGROUND DATA A simple and commonly applied parameter of global balance is the plumbline offset. This radiographic measurement refers to the center of C2 (or C7) drawn vertically downward. Although this measurement is simple, it may not accurately reflect the balance of the spine. METHODS This study included adult volunteers (n = 41) and patients with scoliosis (n = 45). Full-length, freestanding spine radiographs were obtained with subjects on a force plate. Simultaneous assessment of the radiologic spinal posture and the floor projection of the center of pressure (gravity line) was possible. The latter was projected on the full spine images and correlated to common radiographic parameters. RESULTS The position of the gravity line differed significantly from the plumbline in frontal and sagittal planes (P < 0.001). This difference was maintained in both study populations. The mean frontal plane alignment of the gravity line was consistently to the right of the plumbline. The mean sagittal plane alignment of the gravity in relation to the plumbline revealed an offset anteriorly. CONCLUSIONS The data analysis of offsets between the gravity line and radiographic parameters revealed a frontal plane mean displacement of the gravity line to the right. In the sagittal plane, a highly significant lack of correlation between the gravity line and plumbline was noted. The plumbline represents a common and convenient visual display of apparent sagittal plane imbalance, but its value as a marker of true postural balance must be questioned.

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.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.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.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.

STUDY DESIGN Morphometric analysis of anatomic scoliotic specimens. OBJECTIVE The objective of this study was to identify a typical deformation pattern for thoracic and lumbar vertebrae in idiopathic scoliosis. SUMMARY OF BACKGROUND DATA Idiopathic scoliosis is a three-dimensional deformity affecting the orientation and position of the spine in space. The regional deformity has been studied extensively, but most of the knowledge we currently have regarding the local deformity is the result of isolated observations made on rare scoliotic specimens with severe deformities. MATERIALS AND METHODS Thirty scoliotic specimens from two major osteologic sources were studied using a three-dimensional digitizing protocol developed by our research group creating a precise three-dimensional reconstruction of the vertebrae. Parameters were then calculated for each vertebra from these reconstructions. Every scoliotic specimen was then matched with a normal specimen to provide for a representative control group. RESULTS A total of 984 vertebrae (472 scoliotic and 512 normal vertebrae) were measured, creating the largest database of normal and scoliotic vertebral specimens. A characteristic deformity pattern was identified consisting of progressive vertebral wedging, decreased pedicle width on the concave side of the curve, and articular facet surface varying greatly with all findings increasingly more important toward the apex of the curve and as curve severity increased. All findings were statistically significant with P< 0.05. CONCLUSION These results are of critical importance for the understanding of the local and regional deformity and in understanding curve progression. Our results also advocate caution in the use of pedicle screws in the thoracic spine, especially on the concave side of the curve.

STUDY DESIGN This biomechanical study using cadaver thoracic spines evaluated the initial stiffness of two different fixation constructs using a new spinal implant: the bipedicular spinal fixation device (BSF). OBJECTIVE To compare the biomechanical stiffness of a new construct using BSF with a regular construct using pedicular and laminar hooks. SUMMARY OF BACKGROUND DATA Disadvantages of thoracic posterior implants and developments in in situ rod contouring led to the creation of a new implant for spine deformity surgery that would provide immediate stiffness to preserve spine correction, allow efficient postoperative rehabilitation, and obtain a good fusion rate. METHODS Two age-paired groups of six human thoracic spines each (T3-T12) were compared: a regular group whose construct was in accordance with the Cotrel-Dubousset technique and the BSF group. In both groups, the spines were tested intact and then after injury. An injury was induced by transections of interspinous and anterior longitudinal ligaments and anterior discectomies. A three-dimensional ultrasonic measurement device, the Zebris 3D Motion Analyzer, was used to record the motion of the T6 relative to the T8 vertebra under loads, and to determine the ranges of motion (ROMs) between intact spines and the spine construct. RESULTS In flexion-extension, the regular construct showed a significantly greater mean of relative ROMs than the BSF construct for principal rotation (88% and 69% respectively, P = 0.015). However, no significant differences were demonstrated in any of the other motions. CONCLUSION The BSF construct showed stiffness similar to that of the regular construct, encouraging clinical investigation.

Vertebral stabilization using spinal fixation devices is a widely used technique. A three-dimensional geometric and mechanical finite element model has been used as a simulation tool for the evaluation of the mechanical behavior of spinal devices. The geometry of lumbar vertebrae was parameterized, which allows the construction of the geometric model for a given lumbar segment from the digitization of two roentgenographs. This procedure was used to construct a finite element model for a three-vertebra segment with simulation of fractures in the middle vertebra, and with simulation of a restoration using an osteosynthesis device, implemented in a frame fashion with four screws and two rods linked by two transverse rods, and/or an anterior bone graft. Compression force and torsion moment were considered, and different cases were investigated, by varying the severity of the fracture, the geometric characteristics of the device, and the mechanical characteristics of the material joining the two intact vertebral bodies. Results were analyzed considering the mobility of the vertebral segment, which indicates the ability of the restoration system to stabilize the vertebral segment, and considering the forces and moments distribution in the device, which gives information on part of the forces that pass through the device in each situation. Results show that maximum values of forces and moments in the device are more important in compression than in torsion. Adding an anterior bone graft has an effect mainly for compression, whereas in torsion its effect is negligible. For a rigid fixation device, no significant difference was found between different fracture models, indicating that the posterior arch does not play an important role for an instrumented segment. For compression, a rigid posterior wall, or the presence of a bone graft, reduces greatly the mobility of the instrumented segment. For torsion, suppressing the two transverse rods in the device greatly increases the mobility of the instrumented segment. Using a finite element model of a lumbar vertebral segment appears to be an interesting tool to analyze the behavior of an instrumented spine and to compare between different stabilization systems.