The Rib Vertebra Angle Difference and its Measurement in 3D for the evaluation of early onset scoliosis

The Rib Vertebra Angle Difference (RVAD) as defined by Mehta (1972) is used to predict the progression of early onset scoliosis. No clear physical significance has been established for this measurement. The purpose of this study was to evaluate the RVAD along the thoracic spine and the equivalent measurement on 3D reconstructions of the spine and rib cage of early onset scoliosis patients in order to determine their relationship with the geometry of the chest wall and evolution along the spine. The RVAD was measured on PA radiographs of 42 infantile scoliotic patients (Cobb >20°) from T4 to T10 according to the method described by Mehta. The RVAD 3D was computed using the same landmarks from the 3D reconstruction generated from the calibrated biplanar radiographs. Cases were divided into Phase I and Phase II using Mehta's classification based on the rib head overlap with the apical vertebral body on coronal plane radiographs. A linear relationship exists between the Metha (2D) and 3D RVAD for both Phase I (r = 0.87) and Phase II (r = 0.78) patients. For more severe deformities (RVAD 3D ≥ 35°), a relationship was found between RVAD 3D and the axial rotation of the thoracic vertebrae (r = 0.51) in Phase II patients. However, no significant relationship exists between axial rotation and RVAD 3D for Phase I patients as well as Mehta's RVAD. Maximal RVAD measurements were located 2 ½ levels above the apical vertebra. Results indicated that RVAD 3D provides additional information to Mehta's RVAD on the torsional nature of the deformity. Considering the importance of clinical indices to assess the progression of early onset scoliosis, this study raises some questions on looking solely at the RVAD measured on radiographs at the apical vertebra of Phase I patients and suggests considering also levels above the apex of the scoliotic curve and 3D measurements. Further investigation is required to fully understand the 3D nature of the spine and rib cage deformities.

Recent advances in the study of candidate genes for adolescent idiopathic scoliosis

We used a microarray approach to evaluate gene expression profiles in human AIS osteoblasts, and to identify genes that are differentially expressed following estrogen exposure in non-AIS and AIS human osteoblasts. We found that more than one gene is likely responsible for AIS. Furthermore, some of these genes are estrogen-regulated, suggesting a possible role of estrogens in the etiology of scoliosis.

Geometric variability of the scoliotic spine using statistics on articulated shape models

This paper introduces a method to analyze the variability of the spine shape and of the spine shape deformations using articulated shape models. The spine shape was expressed as a vector of relative poses between local coordinate systems of neighboring vertebrae. Spine shape deformations were then modeled by a vector of rigid transformations that transforms one spine shape into another. Because rigid transformations do not naturally belong to a vector space, conventional mean and covariance could not be applied. The Fréchet mean and a generalized covariance were used instead. The spine shapes of a group of 295 scoliotic patients were quantitatively analyzed as well as the spine shape deformations associated with the Cotrel-Dubousset corrective surgery (33 patients), the Boston brace (39 patients), and the scoliosis progression without treatment (26 patients). The variability of intervertebral poses was found to be inhomogeneous (lumbar vertebrae were more variable than the thoracic ones) and anisotropic (with maximal rotational variability around the coronal axis and maximal translational variability along the axial direction). Finally, brace and surgery were found to have a significant effect on the Fréchet mean and on the generalized covariance in specific spine regions where treatments modified the spine shape.

Biomechanical modelling of a direct vertebral translation instrumentation system: preliminary results

Many new spine instrumentation concepts were introduced in recent years, like the incremental direct vertebral translation. The objective was to develop a biomechanical model in order to analyze the biomechanics of this instrumentation system. The patient-specific spine model was built using the 3D reconstruction based on bi-planar radiographs of a scoliotic patient (thoraco-lumbar Cobb: 49 degrees ). The mechanical properties were derived from literature, experiments on cadaver spines and patient's side bending radiographs. Each screw construct was modelled by four rigid bodies connected each other by kinematic joints. The screw-vertebra flexible joint was represented by 3 experimentally derived non-linear springs, and the rods by non-linear flexible elements. The correction manoeuvres were simulated by lowering the rod, tightening the crimps (incremental segmental translation) and applying secondary correction manoeuvres (direct vertebra derotation, compression, distraction and construct tightening). The simulations showed that the system allows a good force distribution among implants. The long post pushing and pulling contributed, to a great extent, to a global correction in the coronal plane, while the crimp tightening had more important effect in the sagittal plane. The preliminary results illustrated the effectiveness of local correction by a direct vertebra translation technique. Our next step is to validate the model and compare the performance of this strategy with other spinal instrumentation systems.

Prediction of the T2-T12 kyphosis in adolescent idiopathic scoliosis using a multivariate regression model

The paper presents a nonlinear regression model built on the coronal thoracic curvature, the lumbar lordosis and the slope of the first lumbar vertebra in order to estimate the thoracic kyphosis measure between T2 and T12. To train the proposed model, a large database containing scoliotic spines demonstrating several types of scoliotic deformities was used to train the proposed system by a cross-validation method. Validation was performed on patients exhibiting three different types of sagittal thoracic profiles: normal, hypo-kyphotic, and hyper-kyphotic. Results show that a multivariate regression model based on dependent variables is able to predict with a reasonable accuracy the sagittal thoracic kyphosis for the automatic assessment and classification of the spinal curve.

The relationship between hip flexion/extension and the sagittal curves of the spine

The objective of this study was to develop a finite element model (FEM) in order to study the relationship between hip flexion/extension and the sagittal curves of the spine. A previously developed FEM of the spine, rib cage and pelvis personalized to the 3D reconstructed geometry of a patient using biplanar radiographs was adapted to include the lower limbs including muscles. Simulations were performed to determine: the relationship between hip flexion / extension and lumbar lordosis / thoracic kyphosis, the mechanism of transfer between hip flexion / extension and pelvic rotation, and the influence that knee bending, muscle stiffness, and muscle mass have on the degree to which sagittal spinal curves are modified due to lower limb positioning. Preliminary results showed that the model was able to accurately reproduce published results for the modulation of lumbar lordosis due to hip flexion; which proved to linearly decrease 68% at 90 degrees of flexion. Additional simulations showed that the hamstrings and gluteal muscles were responsible for the transmission of hip flexion to pelvic rotation with the legs straight and flexed respectively, and the important influence of knee bending on lordosis modulation during lower limb positioning. The knowledge gained through this study is intended to be used to improve operative patient positioning.

Influence of correction objectives on the optimal scoliosis instrumentation strategy: a preliminary study

In three recent studies we have shown how different correction objectives from a group of experienced spine surgeons add to the variability in AIS instrumentation strategies. This study examined the effect of correction objectives of three surgeons on the optimal instrumentation strategy. An optimization method using six instrumentation design parameters (e.g. limits of the instrumented segment, number, type and location of implants and rod shape) that were manipulated in a uniform experimental design framework was linked to a patient-specific biomechanical model to analyze the effects of a specific instrumentation configuration. The optimization cost function was formulated to maximize correction in the three anatomic planes and with minimal number of instrumented levels. Three surgeons from the Spinal Deformity Study Group provided their respective correction objectives for a single patient (56 degrees thoracic and 38 degrees lumbar Cobb angle). For each surgeon, 702 surgical configurations were iteratively simulated using a biomechanical model. The influence of the three different correction objectives on the optimal surgical strategy was evaluated. The resulting optimal fusion levels were T2-L4, T4-L2, and T4-L1. A Wilcoxon non parametric test analysis showed that fusion levels and the location of implants significantly were influenced by the correction objectives strategies (p<0.05). The optimal number of implants although different (12 vs.11 vs.10) was not statistically significant (p>0.1). Thus different surgeon-specified correction objectives produced different optimal instrumentation strategies for the same patient.

A 3D visualization tool for the design and customization of spinal braces

A new tool was developed and validated on an X-ray dummy to allow personalized design and adjustment of spinal braces. The 3D visualization of the external trunk surface registered with the underlying 3D bone structures permits the clinicians to select pressure areas on the trunk surface for proper positioning of correcting pads inside the brace according to the patient's specific trunk deformities. After brace fabrication, the clinicians can evaluate the actual 3D patient-brace interface pressure distribution visualized simultaneously with the 3D model of the trunk in order to customize brace adjustment and validate brace design with respect to the treatment plan.

Intra and interobserver variability of preoperative planning for surgical instrumentation in adolescent idiopathic scoliosis

Surgical instrumentation planning for the correction of scoliosis involves many difficult decisions, especially with the introduction of multi-segmental and other instrumentation technologies. A preliminary study has shown a high variability in planning among a small group of surgeons. The purpose of this paper was to evaluate and analyze the selection of fusion levels and instrumentation choices among a more extended group of scoliosis surgeons. Thirty-two experienced spinal deformity surgeons were asked to provide their preferred posterior instrumentation planning for five patients with adolescent idiopathic scoliosis (AIS) using a graphical worksheet and the usual preoperative X-rays. Overall, the number of implants used ranged from 8 to 30 per patient (mean 16; SD 6): 71% of these were mono-axial screws, 20% multi-axial screws, and 9% hooks. The selected superior and inferior instrumented vertebrae varied up to six levels. The following significant groups of strategies were identified: A- "All Pedicle Screws Constructs" [N(A) = 103; 66%]; B- "All Hooks constructs" [N(B) = 5; 3%]; C- "Hybrid Constructs" [N(C) = 48; 31%]. A top-to-bottom attachment sequence was selected in 49% of all cases, a bottom-up in 46%, and an alternate order in 4%. A large variability in preoperative instrumentation strategy exists in AIS within an experienced group of orthopedic spine surgeons. The impact of such choices on the resulting correction is questioned and will need to be determined with adequate clinical, biomechanical, and computer simulation prospective studies.

A Novel System for the 3-D Reconstruction of the Human Spine and Rib Cage From Biplanar X-Ray Images

The main objective of this study was to develop a 3-D X-ray reconstruction system of the spine and rib cage for an accurate 3-D clinical assessment of spinal deformities. The system currently used at Sainte-Justine Hospital in Montreal is based on an implicit calibration technique based on a direct linear transform (DLT), using a sufficiently large rigid object incorporated in the positioning apparatus to locate any anatomical structure to be reconstructed within its bounds. During the time lapse between the two successive X-ray acquisitions required for the 3-D reconstruction, involuntary patient motion introduce errors due to the incorrect epipolar geometry inferred from the stationary object. An approach using a new calibration jacket and explicit calibration algorithm is proposed in this paper. This approach yields accurate results and compensates for involuntary motion occurring between X-ray exposures.

Simulation of progressive spinal deformities in Duchenne muscular dystrophy using a biomechanical model integrating muscles and vertebral growth modulation

BACKGROUND

Ninety percent of Duchenne muscular dystrophy patients develop scoliosis in parallel with evident muscular and structural impairment. Altered muscular spinal loads acting on growing vertebrae are likely to promote a self-sustaining spinal deformation process. The purpose of this study was to simulate the effect of asymmetrical fat infiltration of the erector spinae muscles combined with vertebral growth modulation over a period of growth spurt.

METHODS

A finite element model of the trunk was built. It integrates (1) longitudinal growth of vertebral bodies and its modulation due to mechanical stresses, (2) muscles and control processes generating muscle recruitment and forces. Three different impairments of the erector spinae muscles were considered and their actions over 12 consecutive cycles representing a span of 12 months were analyzed.

FINDINGS

When asymmetrical muscle degeneration was simulated and weaker erector spinae muscles were located on the convex side of the curve, mild scoliosis (Cobb angle of 8-19 degrees ) was induced in the frontal plane and the kyphosis increased from 72 degrees to 110 degrees in all simulations. Those changes were accompanied by a substantial increase of muscle activity in the Rectus Abdominus and Obliquus Internus.

INTERPRETATION

Scoliosis as documented in the literature were induced through an asymmetrical activity in the erector spinae muscles and it can be hypothesized that the Rectus Abdominus and Obliquus Internus have a role in maintaining balance and counteracting against spine torsion. This study demonstrated the feasibility of the modeling approach to investigate a musculo-skeletal deformation process based on a neuromuscular deficit.

Optimization of rib surgery parameters for the correction of scoliotic deformities using approximation models

BACKGROUND

As opposed to thoracoplasty (a cosmetic surgical intervention used to reduce the rib hump associated with scoliosis), experimental scoliosis has been produced or reversed on animals by rib shortening or lengthening. In a prior work (J. Orthop. Res., 20, pp. 1121-1128), a finite element modeling (FEM) of rib surgeries was developed to study the biomechanics of their correction mechanisms. Our aims in the present study were to investigate the influence of the rib surgery parameters and to identify optimal configurations. Hence, a specific objective of this study was to develop a method to find surgical parameters maximizing the correction by addressing the issue of high computational cost associated with FEM.

METHOD OF APPROACH

Different configurations of rib shortening or lengthening were simulated using a FEM of the complete torso adapted to the geometry of six patients. Each configuration was assessed using objective functions that represent different correction objectives. Their value was evaluated using the rib surgery simulation for sample locations in the design space specified by an experimental design. Dual kriging (interpolation technique) was used to fit the data from the computer experiment. The resulting approximation model was used to locate parameters minimizing the objective function.

RESULTS

The overall coverage of the design space and the use of an approximation model ensured that the optimization algorithm had not found a local minimum but a global optimal correction. The interventions generally produced slight immediate modifications with final geometry presenting between 95-120% of the initial deformation in about 50% of the tested cases. But in optimal cases, important loads (500-2000 N mm) were generated on vertebral endplates in the apical region, which could potentially produce the long-term correction of vertebral wedging by modulating growth. Optimal parameters varied among patients and for different correction objectives.

CONCLUSIONS

Approximation models make it possible to study and find optimal rib surgery parameters while reducing computational cost.

A variability study of computerized sagittal spinopelvic radiologic measurements of trunk balance

OBJECTIVE

The accurate measurement of spinal and pelvic alignment in the sagittal plane is of prime importance for various disorders. Pelvic incidence (PI) is a fundamental anatomic parameter that is specific and constant for each adult individual and is related to pelvic orientation as well as to the size of lumbar lordosis (LL). It is the summation of the sacral slope (SS) and pelvic tilt (PT), two position-dependent variables that determine pelvic orientation in the sagittal plane. The authors have proposed a computer software designed to measure PI, SS, PT, LL, and thoracic kyphosis (TK) on standardized standing lateral digitized x-rays of the spine and pelvis. The purpose of this study was to evaluate the inter- and intraobserver variability of measurements using this software, to determine if it can be used reliably in a clinical environment.

METHODS

The standing lateral x-rays of 30 subjects were randomly selected from the database of two medical institutions. The normal population had standard radiographs on which the various pertinent landmarks were marked by one operator prior to digitization, whereas the scoliotic population had digital radiographs that obviated the need for prior marking of landmarks. Four individuals measured all variables on the 30 x-rays on two occasions, with a 15-day interval between the two sessions. Statistical analysis was done with intraclass correlation coefficients (ICCs).

RESULTS

The ICC measured within observers was between 0.93 and 0.99, whereas the ICC between observers varied between 0.92 and 0.99. The variations observed were similar for normal and scoliotic subjects, and prior marking of the x-rays had no significant influence.

CONCLUSION

We conclude that the variability of measurements with this method is lower than with similar radiologic measures done manually and that the use of this software can be recommended for future clinical and research studies of spinopelvic sagittal balance.

Accuracy assessment of human trunk surface 3D reconstructions from an optical digitising system

The lack of reliable techniques to follow up scoliotic deformity from the external asymmetry of the trunk leads to a general use of X-rays and indices of spinal deformity. Young adolescents with idiopathic scoliosis need intensive follow-ups for many years and, consequently, they are repeatedly exposed to ionising radiation, which is hazardous to their long-term health. Furthermore, treatments attempt to improve both spinal and surface deformities, but internal indices do not describe the external asymmetry. The purpose of this study was to assess a commercial, optical 3D digitising system for the 3D reconstruction of the entire trunk for clinical assessment of external asymmetry. The resulting surface is a textured, high-density polygonal mesh. The accuracy assessment was based on repeated reconstructions of a manikin with markers fixed on it. The average normal distance between the reconstructed surfaces and the reference data (markers measured with CMM) was 1.1 +/- 0.9 mm.

[Postural control in idiopathic scoliosis: comparison between healthy and scoliotic subjects]

PURPOSE OF THE STUDY

To assess the effects of idiopathic scoliosis on undisturbed postural control in young female teenagers.

MATERIAL AND METHODS

The centre of pressure (CP) displacements, measured through a force platform, were decomposed into two elementary components in order to differentiate the net postural performance, as revealed by the horizontal motions of the centre of gravity (CGh) and the level of muscular activity expressed by the vertical difference CP-CGv. The CG horizontal displacements were estimated from those of the CP with a low pass filter taking into account the subjects' anthropometry. Fifteen healthy teenagers and fifteen teenagers with idiopathic scoliosis were asked to stand still upright for three successive trials of 64s, rest periods of similar duration being allowed between each trial.

RESULTS

The results indicate differences in the postural control between the two populations: scoliotic CG motions are more important than those of healthy subjects. In the medio-lateral direction, the CGh motions can be primarily explained by the concomitant increase observed at the CP-CGv level. To be more precise, one should have in mind that the CP-CGv amplitudes determine the horizontal acceleration communicated to CGh. On the other hand, despite significative increases of the CP-CGv in the antero-posterior direction, the CGh motions appear to be unaffected by these initial conditions. This feature suggests that appropriate control mechanisms can intervene in this antero-posterior direction for the scoliotic group whereas a similar organization cannot be observed in the medio-lateral direction.

DISCUSSION

The differences observed in the postural performance for the healthy and scoliotic groups can be explained in various ways. The capacity of the scoliotic group to counteract huge CP-CGv motions may be linked to the location of the postural muscles involved in this control: the triceps surae for the antero-posterior direction, and the abductor-adductor hip muscles for the medio-lateral one. Only the action of the latter group can be indeed perturbed by the specific constraints occurring at the hip level and resulting from the scoliosis. On the other hand, the general increase of the CP-CGv motions, by expressing an overwhelming muscular activity in the whole set of postural muscles, does suggest that the drive of the descending motor commands is largely modulated and is likely the consequence of a central disfunctionning.

Biomechanical modelling of growth modulation following rib shortening or lengthening in adolescent idiopathic scoliosis

A biomechanical model was developed to evaluate the long-term correction resulting from rib shortening or lengthening in adolescent idiopathic scoliosis (AIS). A finite element model of the trunk, personalised to the geometry of a scoliotic patient, was used to simulate rib surgery. Stress relaxation of ligaments following surgery was integrated into the model, as well as longitudinal growth of vertebral bodies and ribs and its modulation due to mechanical stresses. Simulations were performed in an iterative fashion over 24 months. A concave side rib shortening, inducing load patterns on the vertebral end-plates that could act against the scoliosis progression, was tested. A fractional factorial experimental design of 16 runs documented the effects of six modelling parameters. Wedging of the apical vertebra in the frontal plane decreased from 5.2 degrees initially to a mean value of 3.8 degrees after 24 months. The wedging decrease in the thoracic apical region was reflected by changes in the spine curvature, with a Cobb angle decrease from 46 degrees to 44 degrees immediately after the surgery and to a mean of 41 degrees after 24 months. However, both rib hump and vertebral axial rotation increased, on average, by 4 degrees at the curve apex. The most significant parameters were the growth sensitivity to stress in ribs and vertebrae and the rate of stress relaxation of intercostal ligaments. The results confirmed the potential of long-term correction of spinal curvature resulting from the rib shortening on the concavity. This modelling approach could be used for further design of less invasive surgery, taking into account residual growth, for scoliosis correction.

Effect of Body Morphology on Standing Balance in Adolescent Idiopathic Scoliosis

OBJECTIVES

The objective of this study was to determine the effect of body somatotype on standing balance in girls with adolescent idiopathic scoliosis (AIS) who are under observation but not wearing a body brace.

DESIGN

In all, 74 girls participated in this study to form the able-bodied (n = 36) and the AIS (n = 38) groups, having an average age of 13 yrs. Quiet standing balance was tested using a force platform. Afterward, subjects in each group were divided according to their dominant body somatotype, namely endomorphs (fatness), mesomorphs (muscular), or endomorphic ectomorphs (lean).

RESULTS

The center of pressure measured in the anteroposterior position was closer to the heels for the AIS ectomorphic group by approximately 14 mm (P = 0.00497). Only the AIS mesomorphic group displayed a statistically significant 12-mm shift to the right in their center of pressure (P = 0.01211) compared with the able-bodied girls of the same morphotype. In the endomorphic group, the sway area was statistically higher for the scoliotic subjects (P = 0.00839). The distances traveled by the AIS subjects were all statistically longer for all three body morphologic somatotypes.

CONCLUSION

Different postural responses seem to be dependent on body somatotypes. The endomorphic AIS girls had a larger sway area than their able-bodied counterparts while maintaining a similar center of pressure position. The AIS ectomorphic girls had a tendency to lean further back than a comparable able-bodied group. This could be emphasizing a hypokyphotic trunk attitude and increasing the risk of spinal deformity progression. The AIS mesomorphic subjects characterized by a large muscular and bony structure had a tendency to position their center of mass more to their right, indicating less postural adaptability and a stiffer trunk.

Biomechanical modelling of orthotic treatment of the scoliotic spine including a detailed representation of the brace-torso interface

As part of the development of new modelling tools for the simulation and design of brace treatment of scoliosis, a finite element model of a brace and its interface with the torso was proposed. The model was adapted to represent one scoliotic adolescent girl treated with a Boston brace. The 3D geometry was acquired using multiview radiographs. The model included the osseo-ligamentous structures, thoracic and abdominal soft tissues, brace foam and shell, and brace-torso interface. The simulations consisted of brace opening to include the patient's trunk followed by brace closing. To validate the model, the resulting geometry was compared with the real in-brace geometry, and the resulting contact reaction forces at the brace-torso interface were compared with the equivalent forces calculated from pressure measurements made on the in-brace patient. Differences between coronal equivalent and reaction forces were less than 7N. However, sagittal reaction forces (47N) were computed on the abdomen, whereas negligible equivalent forces were measured. The simulated geometry presented partially reduced coronal Cobb angles (1-4 degrees), over-corrected sagittal Cobb angles and maximum deformation plane (5 degrees), completely corrected coronal shift, and sagittal shift and rib humps that were not corrected. This study demonstrated the feasibility of a new approach that represents the load transfer from the brace to the spine more realistically than does the direct application of forces.

Personalized biomechanical simulations of orthotic treatment in idiopathic scoliosis

OBJECTIVES

To analyse patient-specific bracing biomechanics in the treatment of scoliosis.

DESIGN

Two complementary computer tools have been developed to quantify the brace action on scoliotic spine from pressure measurements, and to simulate its effect on patient-adapted finite element model.

BACKGROUND

Brace pad forces and brace effect on spine deformities have been reported. However, the brace mechanisms still need to be better understood to obtain more effective treatments.

METHODS

The 3D geometry of the spine and rib cage of three scoliotic adolescents treated by the Boston brace was obtained using a multiview radiographic reconstruction technique. A personalized biomechanical model was constructed for each patient. Pressures generated by the brace on the thorax were measured using pressure sensors. For each zone with a threshold pressure higher than 30 mmHg, a total equivalent force was calculated and applied to the corresponding model nodes.

RESULTS

The pressure were generally scattered on the overall torso, with the highest pressures measured on five distinct regions: right thoracic, left lumbar, abdominal, right and left sides of the pelvis. The equivalent forces were of 18-73 N. Differences between simulated deformed shapes and real in-brace geometry of the patients were less than 6 and 9.8 mm for the vertebral positions in the coronal and sagittal planes, and 7.7 degrees for the Cobb angles.

CONCLUSION

The results supported the feasibility of such approach to analyse patient-specific bracing biomechanics, which may be useful in the design of more effective braces.

Thoracic Pedicle Screw Insertion Using a Transpedicular Drill Guide

Insertion of thoracic pedicle screws can lead to major complications. This study reports the use of a transpedicular drill guide (TDG) for safe pedicle screw insertion in the thoracic spine. The conventional anatomic technique and the TDG were both used to drill pilot holes into the pedicles of four anatomic models of the thoracic spine. Ninety-nine percent of the 96 pilot holes drilled with the TDG were within 2 mm from the pedicle wall compared with 79% for the anatomic technique (P < 0.001). The TDG reduced the proportion and the extent of medial perforations. The TDG was easy to use and was superior to the conventional anatomic technique. It could be combined with fluoroscopy and pedicle palpation in certain clinical applications, especially for training surgeons, but only after confirming its accuracy in a cadaveric study.

Biomechanical simulations of the spine deformation process in adolescent idiopathic scoliosis from different pathogenesis hypotheses

It is generally recognized that progressive adolescent idiopathic scoliosis (AIS) evolves within a self-sustaining biomechanical process involving asymmetrical growth modulation of vertebrae due to altered spinal load distribution. A biomechanical finite element model of normal thoracic and lumbar spine integrating vertebral growth was used to simulate the progression of spinal deformities over 24 months. Five pathogenesis hypotheses of AIS were represented, using an initial geometrical eccentricity (gravity line imbalance of 3 mm or 2 degrees rotation) at the thoracic apex to trigger the self-sustaining deformation process. For each simulation, regional (thoracic Cobb angle, kyphosis) and local scoliotic descriptors (axial rotation and wedging of the thoracic apical vertebra) were evaluated at each growth cycle. The simulated AIS pathogeneses resulted in the development of different scoliotic deformities. Imbalance of 3 mm in the frontal plane, combined or not with the sagittal plane, resulted in the closest representation of typical scoliotic deformities, with the thoracic Cobb angle progressing up to 39 degrees (26 degrees when a sagittal offset was added). The apical vertebral rotation increased by 7 degrees towards the convexity of the curve, while the apical wedging increased to 8.5 degrees (7.3 degrees with the sagittal eccentricity) and this deformity evolved towards the vertebral frontal plane. A sole eccentricity in the sagittal plane generated a non-significant frontal plane deformity. Simulations involving an initial rotational shift (2 degrees ) in the transverse plane globally produced relatively small and non-typical scoliotic deformations. Overall, the thoracic segment predominantly was sensitive to imbalances in the frontal plane, although unidirectional geometrical eccentricities in different planes produced three-dimensional deformities at the regional and vertebral levels, and their deformities did not cumulate when combined. These results support the hypothesis of a prime lesion involving the precarious balance in the frontal plane, which could concomitantly be associated with a hypokyphotic component. They also suggest that coupling mechanisms are involved in the deformation process.

Patient-specific mechanical properties of a flexible multi-body model of the scoliotic spine

The flexibility of the scoliotic spine is an important biomechanical parameter to take into account in the planning of surgical instrumentation. The objective of the paper was to develop a method to characterise in vivo the mechanical properties of the scoliotic spine using a flexible multi-body model. Vertebrae were represented as rigid bodies, and intervertebral elements were defined at every level using a spherical joint and three torsion springs. The initial mechanical properties of motion segments were defined from in vitro experimental data reported in the literature. They were adjusted using an optimisation algorithm to reduce the discrepancy between the simulated and the measured Ferguson angles in lateral bending of three spine segments (major or compensatory left thoracic, right thoracic and left lumbar scoliosis curves). The flexural rigidity of the spine segments was defined in three categories (flexible, nominal, rigid) according to the estimated mechanical factors (alpha). This approach was applied with ten scoliotic patients undergoing spinal correction. Personalisation of the model resulted in an increase of the initial flexural rigidity for seven of the ten lumbar segments (1.38 < or = alpha < or = 10.0) and four of the ten right thoracic segments (1.74 < or = alpha < or = 5.18). The adjustment of the mechanical parameters based on the lateral bending tests improved the model's ability to predict the spine shape change described by the Ferguson angles by up to 50%. The largest differences after personalisation were for the left lumbar segments in left bending (4 degrees +/- 3 degrees). The in vivo identification of the mechanical properties of the scoliotic spine will improve the ability of biomechanical models adequately to predict the surgical correction, which should help clinicians in the planning of surgical instrumentation manoeuvres.

Assessment of the 3-d reconstruction and high-resolution geometrical modeling of the human skeletal trunk from 2-D radiographic images

This paper presents an in vivo validation of a method for the three-dimensional (3-D) high-resolution modeling of the human spine, rib cage, and pelvis for the study of spinal deformities. The method uses an adaptation of a standard close-range photogrammetry method called direct linear transformation to reconstruct the 3-D coordinates of anatomical landmarks from three radiographic images of the subject's trunk. It then deforms in 3-D 1-mm-resolution anatomical primitives (reference bones) obtained by serial computed tomography-scan reconstruction of a dry specimen. The free-form deformation is calculated using dual kriging equations. In vivo validation of this method on 40 scoliotic vertebrae gives an overall accuracy of 3.3 +/- 3.8 mm, making it an adequate tool for clinical studies and mechanical analysis purposes.

Biomechanical modeling of posterior instrumentation of the scoliotic spine

Scoliosis is a three-dimensional deformation of the spine that can be treated by vertebral fusion using surgical instrumentation. However, the optimal configuration of instrumentation remains controversial. Simulating the surgical maneuvers with personalized biomechanical models may provide an analytical tool to determine instrumentation configuration during the pre-operative planning. Finite element models used in surgical simulations display convergence difficulties as a result of discontinuities and stiffness differences between elements. A kinetic model using flexible mechanisms has been developed to address this problem, and this study presents its use in the simulation of Cotrel-Dubousset Horizon surgical maneuvers. The model of the spine is composed of rigid bodies corresponding to the thoracic and lumbar vertebrae, and flexible elements representing the intervertebral structures. The model was personalized to the geometry of three scoliotic patients (with a thoracic Cobb angle of 45 degrees, 49 degrees and 39 degrees ). Binary joints and kinematic constraints were used to represent the rod-implant-vertebra joints. The correction procedure was simulated using three steps: (1) Translation of hooks and screws on the first rod; (2) 90 degrees rod rotation; (3) Hooks and screws look-up on the rod. After the simulation, slight differences of 0-6 degrees were found for the thoracic spine scoliosis and the kyphosis, and of 1-8 degrees for the axial rotation of the apical vertebra and for the orientation of the plane of maximum deformity, compared to the real post-operative shape of the patient. Reaction loads at the vertebra-implant link were mostly below 1000 N, while reaction loads at the boundary conditions (representing the overall action of the surgeon) were in the range 7-470 N and maximum torque applied to the rod was 1.8 Nm. This kinetic modeling approach using flexible mechanisms provided a realistic representation of the surgical maneuvers. It may offer a tool to predict spinal geometry correction and assist in the pre-operative planning of surgical instrumentation of the scoliotic spine.

Simulation of progressive deformities in adolescent idiopathic scoliosis using a biomechanical model integrating vertebral growth modulation

While the etiology and pathogenesis of adolescent idiopathic scoliosis are still not well understood, it is generally recognized that it progresses within a biomechanical process involving asymmetrical loading of the spine and vertebral growth modulation. This study intends to develop a finite element model incorporating vertebral growth and growth modulation in order to represent the progression of scoliotic deformities. The biomechanical model was based on experimental and clinical observations, and was formulated with variables integrating a biomechanical stimulus of growth modulation along directions perpendicular (x) and parallel (y, z) to the growth plates, a sensitivity factor beta to that stimulus and time. It was integrated into a finite element model of the thoracic and lumbar spine, which was personalized to the geometry of a female subject without spinal deformity. An imbalance of 2 mm in the right direction at the 8th thoracic vertebra was imposed and two simulations were performed: one with only growth modulation perpendicular to growth plates (Sim1), and the other one with additional components in the transverse plane (Sim2). Semi-quantitative characterization of the scoliotic deformities at each growth cycle was made using regional scoliotic descriptors (thoracic Cobb angle and kyphosis) and local scoliotic descriptors (wedging angle and axial rotation of the thoracic apical vertebra). In all simulations, spinal profiles corresponded to clinically observable configurations. The Cobb angle increased non-linearly from 0.3 degree to 34 degrees (Sim1) and 20 degrees (Sim2) from the first to last growth cycle, adequately reproducing the amplifying thoracic scoliotic curve. The sagittal thoracic profile (kyphosis) remained quite constant. Similarly to clinical and experimental observations, vertebral wedging angle of the thoracic apex progressed from 2.6 degrees to 10.7 degrees (Sim1) and 7.8 degrees (Sim2) with curve progression. Concomitantly, vertebral rotation of the thoracic apex increased of 10 degrees (Sim1) and 6 degrees (Sim2) clockwise, adequately reproducing the evolution of axial rotation reported in several studies. Similar trends but of lesser magnitude (Sim2) suggests that growth modulation parallel to growth plates tend to counteract the growth modulation effects in longitudinal direction. Overall, the developed model adequately represents the self-sustaining progression of vertebral and spinal scoliotic deformities. This study demonstrates the feasibility of the modeling approach, and compared to other biomechanical studies of scoliosis it achieves a more complete representation of the scoliotic spine.

Rib cage surgery for the treatment of scoliosis: a biomechanical study of correction mechanisms

Rib shortening or lengthening are surgical options that are used to address the cosmetic rib cage deformity in scoliosis, but can also alter the equilibrium of forces acting on the spine, thus possibly counteracting in a mechanical way the scoliotic process and correcting the spinal deformities. Although rib surgeries have been successful in animal models, they have not gained wide clinical acceptance for mechanical correction of scoliosis due to the lack of understanding of the complex mechanisms of action involved during and after the operation. The objective of this study was to assess the biomechanical action of different surgical approaches on the rib cage for the treatment of scoliosis using a patient-specific finite element model of the spine and rib cage. Several unilateral and bilateral rib shortening/lengthening procedures were tested at different locations on the ribs (convex/concave side of the thoracic curvature; at the costo-transverse/costo-chondral joint; 20 and 40 mm adjustments). A biomechanical analysis was performed to assess the resulting geometry and load patterns in ribs, costo-vertebral articulations and vertebrae. Only slight immediate geometric variations were obtained. However, concave side rib shortening and convex side rib lengthening induced important loads on vertebral endplates that may lead to possible scoliotic spine correction depending on the remaining growth potential. Convex side rib shortening and concave side rib lengthening produced mostly cosmetic rib cage correction, but generated inappropriate loads on the vertebral endplates that could aggravate vertebral wedging. This study supports the concept of using concave side rib shortening or convex side rib lengthening as useful means to induce correction of the spinal scoliotic deformity during growth, though the effects of growth modulation from induced loads must be addressed in more detail to prove the usefulness of rib shortening/lengthening techniques.

Electromyogram and kinematic analysis of lateral bending in idiopathic scoliosis patients

In adolescent idiopathic scoliosis (AIS), surgical planning currently relies on spinal flexibility evaluation using lateral bending radiographs. The aim was to evaluate the feasibility of non-invasive dynamic analysis of trunk kinematics and muscle activity in patients with AIS before surgical correction. During various lateral trunk bending tasks, erector spinae (18 sites) and abdominal (four sites) muscle activity was sampled using surface electrodes in ten AIS patients and in ten controls. Simultaneously, the spatial displacements of infrared emitting diodes located on the trunk were sampled. Parameters considered were the heterolateral-to-homolateral root-mean-square EMG ratios R at each site and total lateral bending and thoracic and lumbar curvature angle courses. Main alterations concerned apical muscle activity during left bending tasks. ANOVA results showed a significant effect of side (p = 2.1 x 10(-9)), EMG recording site (p = 1.9 x 10(-16)), pathology (p = 3.9 x 10(-16)) and task (p = 2.2 x 10(-11)) on R ratios. The R ratio at T10 and L1 for a simple lateral bending task during left bending averaged 4.8 (SD 4.3) and 3.0 (SD 3.1) in AIS patients, and 2.3 (SD 2.8) and 1.3 (SD 0.4) in controls (p = 6.4 x 10(-4) and 2.5 x 10(-3), LSD post hoc). This preliminary study allowed the development of a functional, non-invasive, non-irradiating dynamic tool for pre-operative evaluation in AIS.

Knee flexors/extensors in gait of elderly and young able-bodied men (II)

In this study the fundamental tasks of muscle activity at the knee during gait in elderly and young able-bodied subjects were identified using principal component analysis (PCA). Role discrepancies between the older and younger subjects for the actions executed by the knee flexors and extensors during the gait cycle were also investigated. The t-test for independent groups was applied to determine significant differences between spatio-temporal and peak muscle moment parameters (P<0.05). PCA as a multivariate classification and curve structure detection method was applied to the sagittal knee muscle moment curves of twenty elderly (72 +/-5.5 years) and twenty young (25 +/- 8.1 years) subjects. The first three principal components (PCs) which accounted for 80% (older) and 93% (younger) of the information were retained for further analysis. Providing stable locomotion was recognised as a major task of the knee in the older subjects, while for the younger subjects the knee contributed to both balance and propulsion. Supporting the body during single limb support should be considered the only common task at the knee level in elderly and young subjects' gait. The lack of muscle power for propulsion might be the reason for not identifying the knee extensor muscle roles in the first three major tasks during elderly gait. Functional asymmetry can be considered the result of a different ordering of the functional roles of the muscles acting at the knee level in elderly and young subjects.

Main functional roles of knee flexors/extensors in able-bodied gait using principal component analysis (I)

This study was undertaken to demonstrate how principal component analysis (PCA) can be used to detect the main functional structure of actions taken by knee flexors/extensors during able-bodied gait. PCA was applied as a classification and curve structure detection method for knee sagittal muscle moment developed during walking of 20 young, healthy male subjects. Over 90% of the information provided by the first three principal components (PCs) was chosen for further biomechanical interpretation. PCA was able to identify the three main functional contributions of knee sagittal muscle moment during able-bodied gait, namely control balance, foot clearance/limb preparation and shock absorption.

Anaesthetic management of an adolescent for scoliosis surgery with a Fontan circulation

Advances in the treatment of congenital heart disease have led to a new group of adolescents or adults patients with cardiac anomalies. The anaesthetic management of these patients can be challenging especially when they are scheduled for major noncardiac surgery inducing haemodynamic instability. We report the case of a 14-year-old boy scheduled for posterior spinal fusion for idiopathic scoliosis who underwent a Fontan operation in infancy for pulmonary atresia with right ventricle hypoplasia. The preoperative investigations and the anaesthetic management are described.

Evolution of 3D deformities in adolescents with progressive idiopathic scoliosis

The objective of this study was to conduct an intrasubject longitudinal study quantifying the evolution of two- and three-dimensional geometrical scoliotic descriptors. The evolution of regional and local scoliotic descriptors was analyzed between two scoliotic visits on a cohort of 28 adolescents with progressive idiopathic scoliosis. Mean age at the first visit was 12.7 +/- 1.7 years old and averaged time interval between two assessments reached 22.8 +/- 10.8 months. Scoliotic descriptors were obtained from three-dimensionally reconstructed spines. The initial thoracic Cobb angle was on average 35.3 degrees +/- 8.4 degrees (range, 14 degrees-54 degrees). The evolution of spinal curvatures and vertebral deformities was assessed statistically in terms of descriptor absolute variations, and of descriptor variations normalized with respect to time and to the increase in Cobb angle. At the thoracic level, vertebral wedging increased with curve severity in a relatively consistent pattern for most scoliotic patients and axial rotation mainly increased towards curve convexity with scoliosis severity. No consistent evolution was associated with the angular orientation of the maximum wedging. Thoracic kyphosis changes (increase and decrease) were observed in important proportions. Results of this study challenge the existence of a typical scoliotic evolution pattern and suggest that the scoliotic evolution is quite variable and patient-specific.

A new X-ray calibration/reconstruction system for 3D clinical assessment of spinal deformities

The main objective of this study was to develop a 3D X-ray reconstruction system of the spine and rib cage for an accurate clinical assessment of spinal deformities. The proposed system uses an explicit calibration technique and a new calibration object composed of: (1) a set of radiopaque markers embedded in a jacket worn by the patient during the X-ray exposures; (2) six control markers to define a reference vertical plane. Computer simulations were performed to evaluate the accuracy of the 3D reconstruction procedure when different kind of displacements were applied on a reference model. Clinical indices computed from the 3D X-ray reconstruction of the spine for 24 scoliotic subjects were compared to those obtained with the DLT method. The results of the evaluation study showed that the new system allows the patient to adopt a normal attitude without any constraint, compensating for its displacement between exposures.

Simulations of rib cage surgery for the management of scoliotic deformities

Costoplasties are surgical options to treat rib cage deformities. The main concern of rib resections is often for the cosmetic improvement of the back shape of the patient. Other experimental and clinical studies have shown that a costoplasty can also produce mechanical correction of the spine. Based on the assumption that surgery on the rib cage can alter the equilibrium of forces acting on the spine, this study aims to investigate the biomechanical role of the ribs during the surgical treatment of scoliosis using a finite element model of the spine and rib cage. The model was generated from patient-specific geometric data. Concave side rib shortening and convex side rib lengthening have been simulated and evaluated. Slight post-operative immediate geometrical correction of the spine was found in any of the simulations. However, both kinds of simulation induced similar loads on the vertebral endplates. Resulting torques in the frontal plane tended to correct the scoliotic spine in the frontal plane acting against vertebral wedging. Important torques were also found in the sagittal plane, increasing the physiological kyphosis, and derotational torques promoted the improvement of the transverse plane deformation. This biomechanical analysis showed that appropriate rib surgery may counteract the progression of the spine deformity depending on the remaining growth potential. These findings support the concept of early interventions on the rib cage that may be a new approach of treatment to prevent curve progression in small to moderate idiopathic scoliotic deformities.

Spinal mobility and EMG activity in idiopathic scoliosis through dynamic lateral bending tests

Lateral bending test is a common evaluation of AIS patients prior to their surgical correction. Traditionally this evaluation is made by the assessment of the curve's flexibility from side-bending radiographs. As a complement to this static test, dynamic bending was experimented while simultaneously quantifying muscular and kinematic behavior of the spine. The biggest contribution to total EMG output was 36% from lumbar muscles in healthy and 35% from abdominal muscles in scoliotic subjects. Continuous measuring of kinematics and muscle activation patterns throughout lateral bending could be an evaluation tool for distinguishing pathological from normal behavior.

Self-calibration of biplanar radiographs for a retrospective comparative study of the 3D correction of adolescent idiopathic scoliosis

A novel technique for the 3D reconstruction of the spine from X-ray images is presented. The algorithm is based on the self-calibration of biplanar radiographs. It allows the 3D reconstruction of spines from old uncalibrated preoperative and postoperative radiographs. The reliability of the new self-calibration technique was investigated by validating its results against those of the Direct Linear Transform (DLT) on real images. An accuracy experiment was also performed using a dry spine specimen under controlled conditions. The results indicate that self-calibration is a viable technique, accurate enough to extract meaningful 3D clinical data for retrospective studies.

Correlation study between indices describing the scoliotic spine

This study intended to investigate the correlations between different geometrical indices in order to assess their usefulness in the characterization of scoliotic deformities. Analytical evaluation of indices was obtained from 3D reconstruction of vertebrae. Scoliotic indices included the Cobb angle, the Cobb angle in the plane of maximum curvature, the angular orientation of the plane of maximum curvature, the kyphosis and the maximal axial rotation. This analysis was applied to the thoracic curve of 100 scoliotic adolescents. The correlations between these five indices were separately investigated for the RT and RTLL curve types. A statistical correlation was found between the angular orientation of the plane of maximum curvature and the kyphosis. The results indicate a relative independence between most indices. Hence, an evaluation of several complementary indices is required to provide a more complete description of 3D scoliotic deformities.

Biomechanical modelling of spinal growth modulation for the study of adolescent scoliotic deformities: a feasibility study

A model of growth modulation was formulated with variables integrating a biomechanical stimulus of growth modulation, a sensitivity factor to the stimulus and time. It was integrated into a finite element model of the thoracic and lumbar spine using an iterative procedure. A simulation on the personalized geometry of a mild scoliotic patient allowed qualitative investigation of scoliotic deformities over 12 cycles (months) in response to a load variation due to an eccentricity of the patient's gravity line in the frontal plane. Resulting frontal, sagittal and transverse spinal views correspond to clinically observable scoliotic configurations. The simulation adequately reproduces a progressing thoracic scoliotic curve while the slight increasing kyphosis represents a possible condition although a thoracic hypokyphosis is frequently reported in the literature. At the thoracic apex, increased wedging as well as axial rotation evolving towards curve convexity are in agreement with clinical and experimental observations reported with curve progression. This study demonstrates the feasibility of the approach and, compared to other biomechanical models, it achieves a more complete representation of the scoliotic spine by incorporating vertebral growth modulation.

Relation between patient positioning, trunk flexibility and surgical correction of the scoliotic spine

The purpose of this work is to investigate the relations between the correction of idiopathic scoliosis obtained by surgical instrumentation and the positioning of the patient on the surgical table as well as the curve reduction following the bending test. A retrospective study of preoperative standing, supine and lateral bending films as well as postoperative standing films was made using multiple regressions in order to identify the most significant parameters and define linear statistical models to predict the surgical correction. Postoperative thoracic Cobb angle is significantly associated to left and right bending Cobb angles and the post-op lumbar Cobb is associated to the supine and the left bending Cobb angles. This preliminary study suggests that such parameters should be considered in the pre-operative planning of surgery as well as in biomechanical models to obtain more adequate predictive values of the surgical outcome and to better understand the biomechanics of scoliosis surgical correction.

Validation of the NSCP technique on scoliotic vertebrae

The purpose of the present study is to validate a quite recent stereoradiographic 3D reconstruction technique, called Non Stereo Corresponding Points (NSCP), on scoliotic patients. The validation of the NSCP technique on scoliotic patients was performed on 59 scoliotic vertebrae from 14 patients, by comparison to the CT scan. The results of this study show mean errors of 1.5 mm. These results should still be optimized for the geometrical modelling. Nevertheless, this technique may already be used as a diagnosis tool in clinics.

Feasibility study of patient-specific surgical templates for the fixation of pedicle screws

Surgery for scoliosis, as well as other posterior spinal surgeries, frequently uses pedicle screws to fix an instrumentation on the spine. Misplacement of a screw can lead to intra- and post-operative complications. The objective of this study is to design patient-specific surgical templates to guide the drilling operation. From the CT-scan of a vertebra, the optimal drilling direction and limit angles are computed from an inverse projection of the pedicle limits. The first template design uses a surface-to-surface registration method and was constructed in a CAD system by subtracting the vertebra from a rectangular prism and a cylinder with the optimal orientation. This template and the vertebra were built using rapid prototyping. The second design uses a point-to-surface registration method and has 6 adjustable screws to adjust the orientation and length of the drilling support device. A mechanism was designed to hold it in place on the spinal process. A virtual prototype was build with CATIA software. During the operation, the surgeon places either template on patient's vertebra until a perfect match is obtained before drilling. The second design seems better than the first one because it can be reused on different vertebra and is less sensible to registration errors. The next step is to build the second design and make experimental and simulations tests to evaluate the benefits of this template during a scoliosis operation.

Investigation of muscle recruitment patterns in scoliosis using a biomechanical finite element model

The objective of this project is to study the characteristics of trunk muscle recruitment strategies experimentally observed for scoliotic subjects using a finite element model of the trunk. The personalized biomechanical model includes elements representing the osseo-ligamentous structures of the spine, rib cage and pelvis. It also integrates the principal agonistic muscles necessary for trunk movement and a neural control model based on the Equilibrium Point hypothesis (lambda model of Feldman). Muscle recruitment patterns of normal and scoliotic subjects obtained from the simulation of lateral bending movements were qualitatively compared. The generation process of motor control variables was studied by analysing the relationships between central commands and spine segment mobility. Differences in recruitment patterns between normal and scoliotic subjects were observed, especially for paraspinal fascicles crossing the thoracic curve segment. The generation of central commands for normal subjects was strongly correlated with the amplitude of bending, but this relation was weaker for scoliotic subjects and this difference was worst at the apex vertebra. These results show that neuromuscular disorders could occur at a local level. The proposed approach should provide a simulation tool to study the multifactorial origin of scoliosis, and to investigate the implication of muscles and central commands in spinal dysfunctions.

Personalized biomechanical modeling of Boston brace treatment in idiopathic scoliosis

The aim of this study was to describe how the Boston brace modify the scoliotic curvatures using a finite element (FE) model and experimental measurements. The experimental protocol, applied on 12 scoliotic girls, was composed of the pressure measurement at the brace-torso interface followed by two radiographic acquisitions of the patient's torso with and without brace. A 3D FE model of the trunk was built for each unbraced patient. The brace treatment was represented by two different modeling approaches: 1) using equivalent forces calculated from the measured pressures; 2) by an explicit personalized FE model of the brace (hexahedral elements) and its interface with the torso (contact elements). In the first model, measured brace forces less than 40N and up to 113N induced respectively less than 21% and up to 87% of real correction. Thoracic forces induced the main correction, affecting partially both lumbar and thoracic curves, in agreement with the literature. In the second model, the brace closing reduced the curves up to 35% of real correction. Contact reaction forces (16-79N) were similar to real brace forces (11-72N). The results suggested that other mechanisms than brace pads contribute to the equilibrium of the patients. Postural control by the muscular system remains a problem to address in a future study. The second model represented more realistically the load transfer from the brace to the spine than external forces application. With such model, it is expected to predict the effect of a brace before its design and manufacturing, and also to improve its design.

Study of patient positioning on a dynamic frame for scoliosis surgery

The goal of this clinical trial was to measure patient geometry on a dynamic positioning frame in various prone positions. Fourteen subjects (2 males and 12 females) were recruited from the scoliosis clinic at Ste-Justine Hospital on a volunteer basis. The subjects were AIS patients who were potential candidates for surgery. The Cobb angle, averaged 50 degrees (32 degrees-64 degrees). The mean age was 14.1 years (11-17). A Polaris system (Northern Digital inc, Canada) with 10 passive reflective markers was used to measure various indices of the patient's trunk geometry. Acquisitions were made while the unanaesthetized patient was in five different prone positions: I similar to the standard positioning on a Relton-Hall frame; II addition of a force applied to the ribcage at the apex of the curve; III application of a force at the apex of the curve in the lumbar region; IV, the shoulder pads were elevated to increase the patient's kyphosis; V adjustment of each pad and the application of thoracic and lumbar forces to obtain an optimal correction. The measurements of trunk geometry at each position were compared using position I as a base. A paired student t-test determined a significant difference between positions. When comparing position I to position II there was a significant difference and correction of the rib hump. There was also a significant change in shoulder angle that resulted in over correction. Position III had a significantly negative change in the rib hump. During position IV, there was a measurable increase in kyphosis. During the optimal correction, position V, a significant increase in spine length was observed as well as a significant correction in rib hump and shoulder angle. Patient trunk geometry can be improved by the application of different forces on a dynamic positioning frame. Caution is necessary as over correction and unintended negative effects were observed. The optimal patient position has not yet been found and future studies are directed at determining this.

3D biplanar statistical reconstruction of scoliotic vertebrae

A new 3D reconstruction method of scoliotic vertebrae of a spine, using two calibrated conventional radiographic images (postero-anterior and lateral), and a global prior knowledge on the geometrical structure of each vertebra is presented. This geometrical knowledge is efficiently captured by a statistical deformable template integrating a set of admissible deformations, expressed by the first modes of variation in the Karhunen-Loeve expansion of the pathological deformations observed on a representative scoliotic vertebra population. The proposed reconstruction method consists in fitting the projections of this deformable template with the preliminary segmented contours of the corresponding vertebra on the two radiographic views. The 3D reconstruction problem is stated as the minimization of a cost function for each vertebra and solved with a gradient descent technique. The reconstruction of the spine is then made vertebra by vertebra. The proposed method allows also to efficiently obtain an accurate 3D reconstruction of each scoliotic vertebra and, consequently, it allows also to get an accurate knowledge of the 3D structure of the whole scoliotic spine. This reconstruction method is in final phase of validation.