Credibility assessment of patient-specific biomechanical models to investigate proximal junctional failure in clinical cases with adult spine deformity using ASME V&V40 standard.

Computational models are increasingly used to assess spine biomechanics and support surgical planning. However, varying levels of model verification and validation, along with characterization of uncertainty effects limit the level of confidence in their predictive potential. The objective was to assess the credibility of an adult spine deformity instrumentation model for proximal junction failure (PJF) analysis using the ASME V&V40:2018 framework. To assess model applicability, the surgery, erected posture, and flexion movement of actual clinical cases were simulated. The loads corresponding to PJF indicators for a group of asymptomatic patients and a group of PJF patients were compared. Model consistency was demonstrated by finding PJF indicators significantly higher for the simulated PJF vs. asymptomatic patients. A detailed sensitivity analysis and uncertainty quantification were performed to further establish the model credibility.

In vivo dynamic compression has less detrimental effect than static compression on newly formed bone of a rat caudal vertebra.

Fusionless devices are currently designed to treat spinal deformities such as scoliosis by the application of a controlled mechanical loading. Growth modulation by dynamic compression was shown to preserve soft tissues. The objective of this in vivo study was to characterize the effect of static vs. dynamic loading on the bone formed during growth modulation. Controlled compression was applied during 15 days on the 7(th) caudal vertebra (Cd7) of rats during growth spurt. The load was sustained in the "static" group and sinusoidally oscillating in the "dynamic" group. The effect of surgery and of the device was investigated using control and sham (operated on but no load applied) groups. A high resolution CT-scan of Cd7 was acquired at days 2, 8 and 15 of compression. Growth rates, histomorphometric parameters and mineral density of the newly formed bone were quantified and compared. Static and dynamic loadings significantly reduced the growth rate by 20% compared to the sham group. Dynamic loading preserved newly formed bone histomorphometry and mineral density whereas static loading induced thicker (+31%) and more mineralized (+12%) trabeculae. A significant sham effect was observed. Growth modulation by dynamic compression constitutes a promising way to develop new treatment for skeletal deformities.

Biomechanical modeling of the lateral decubitus posture during corrective scoliosis surgery.

BACKGROUND: Patient prone positioning in scoliosis surgeries modifies the spinal curves prior to instrumentation. However, the biomechanical effects of the lateral decubitus posture, used in anterior approaches and minimally invasive techniques, have not yet been investigated. The objectives were to develop and validate a finite element model simulating the spinal changes resulting from this positioning. METHODS: The 3D pre-op reconstructed geometries of six adolescent patients with idiopathic scoliosis were used to develop personalized finite element models of the spine, which integrated a three-step method simulating the lateral posture. Clinical indices were measured on pre- and intra-operative radiographs to validate the finite element model. FINDINGS: The major Cobb angle and apical vertebral translation were reduced by 44% and 37% respectively between the pre- and intra-op postures. Using appropriately oriented gravity forces and boundary conditions, the finite element model simulations represented adequately these changes, with average differences of 4 degrees for the major Cobb angle and 4mm for the apical vertebral translation with the radiographic values. INTERPRETATION: Lateral decubitus positioning significantly reduces the spinal deformities prior to instrumentation, as demonstrated by the finite element model. This study is a first step in the development of a modeling tool for the optimal adjustments of intra-operative positioning, which remains to be further investigated with complementary clinical studies.

Quantification of intervertebral efforts during walking: comparison between a healthy and a scoliotic subject.

The accurate quantification of internal efforts in the human body is still a challenge in biomechanics. The aim of this study is to quantify the intervertebral efforts along the spine during walking, in order to compare the dynamical behaviours between a healthy and a scoliotic subject. Practically, one healthy subject, one scoliotic patient before an instrumentation surgery (Cobb 41 degrees ) and after this instrumentation (Cobb 7.5 degrees ) walked on a treadmill at 4 km/h. The acquisition system included optokinetic sensors, recording the 3D-joint coordinates, a treadmill equipped with strain gauges, measuring the external forces independently applied to both feet, and bi-planar radiographs, enabling the 3D reconstruction of the spine from C7 to L5, using a free form interpolation technique. The intervertebral efforts were computed using an inverse dynamical model of the human body in 3D. As results, significant differences of the spine kinematics were recorded which lead to different internal effort behaviour in magnitude, shift, coordination and pattern when normalized to the subject mass. Particularly, the normalized antero-posterior intervertebral torques are less uniform for the scoliotic patient (from min -2.5 to max 1.9 Nm/kg) than the healthy subject (from -1.5 to 1.5 Nm/kg). This disequilibrium in the left-right balance of the scoliotic patient is a bit rectified after surgery (from -1.3 to 1.1 Nm/kg).

Preoperative planning simulator for spinal deformity surgeries.

STUDY DESIGN: Proof of concept of a spine surgery simulator (S3) for the assessment of scoliosis instrumentation configuration strategies. OBJECTIVE: To develop and assess a surgeon-friendly spine surgery simulator that predicts the correction of a scoliotic spine as a function of the patient characteristics and instrumentation variables. SUMMARY OF BACKGROUND DATA: There is currently no clinical tool sufficiently user-friendly, reliable and refined for the preoperative planning and prediction of correction using different instrumentation configurations. METHODS: A kinetic model using flexible mechanisms has been developed to represent patient-specific spine geometry and flexibility, and to simulate individual substeps of correction with an instrumentation system. The surgeon-friendly simulator interface allows interactive specification of the instrumentation components, surgical correction maneuvers and display of simulation results. RESULTS: The simulations of spinal instrumentation procedures of 10 scoliotic cases agreed well with postoperative results and the expected behavior of the instrumented spine (average Cobb angle differences of 3.5 degrees to 4.6 degrees in the frontal plane and of 3.6 degrees to 4.7 degrees in the sagittal plane). Forces generated at the implant-vertebra link were mostly below reported pull-out values, with more important values at the extremities of the instrumentation. CONCLUSION: The spine surgery simulator S3 has proven its technical feasibility and clinical relevance to assist in the preoperative planning of instrumentation strategies for the correction of scoliotic deformities.

Biomechanical modeling of anterior spine instrumentation in AIS.

This study is part of a larger project regarding the development of a Spine Surgery Simulator (S3), which has shown good results for posterior instrumentation surgeries. The aim was to develop a biomechanical model for the anterior instrumentation of the scoliotic spine. A biomechanical model using flexible mechanism was developed and surgical manoeuvres (instrumentation, rod installation and compression) were reproduced. Validation of the model was done by comparing the results for the instrumented part of the spine to the post-operative data (analytical Cobb angles in the frontal and sagittal planes, plane of maximum deformity, etc.). To date, surgeries of four patients operated by thoracotomy were reproduced. Preliminary results show that anterior instrumentation of the scoliotic spine can be adequately modelled using pre-operative geometric data and using mechanical properties from literature. Once validated with a larger sample of cases, the anterior instrumentation model could be implemented into S3 and used by orthopaedic surgeons to test various instrumentation strategies in virtual reality before performing the actual surgery.

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.

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.

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.

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.

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.

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.

Biomechanical simulation of Colorado instrumentation of the scoliotic spine: a preliminary study.

Few biomechanical models of the scoliotic spine were developed to simulate the Cotrel-Dubousset instrumentation, although none was dedicated to the Colorado system. The objective of this study is to adapt and assess an existing biomechanical model to simulate the effect of the Colorado instrumentation of the scoliotic spine as a function of pre-operative geometry and surgical planning. Fifteen scoliotic patients operated with a Colorado system were analysed using a knowledge extraction technique to simplify surgical procedure and to establish the biomechanical model (boundary conditions, simulation procedures,...). Preliminary results on one patient show that the Colorado surgical technique can be adequately modelled using the preoperative geometric data and limited simulation strategy parameters.

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.

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.

Spinal surgery procedure discretization.

In the last decades, scientists developed analytic models of spinal surgery to assess surgical choices and instrumentation parameters. They noted the difficulty to represent the boundary conditions on their deterministic models and recognize the lack of knowledge in surgical procedures. This paper presents a formalization technique applied to spinal surgery to improve the formulation of biomechanical models. This technique consisted into two steps: knowledge extraction and knowledge representation. The protocol was established with an expert surgeon using Colorado2 instrumentation. Surgeon's knowledge acquisition has permitted to define eleven detailed independent data cards for the different steps of surgery like hook or screw insertion, rod installation, etc... The behaviour of the concerned elements on its neighbouring entity were specified using three matrices. The link between surgery and modelling becomes easier and permits to better define the boundary conditions on each entity during the simulation.

Preoperative and early postoperative three-dimensional changes of the rib cage after posterior instrumentation in adolescent idiopathic scoliosis.

Rib cage deformity is an important component of scoliosis, but few authors have reported the three-dimensional (3-D) effect of surgical procedures with posterior instrumentation systems on the shape of the rib cage. The objective of this prospective clinical study was to measure the short-term 3-D changes in the shape of the rib cage at the apex of the curve after corrective surgery of adolescent idiopathic scoliosis by a posterior approach using a multi rod, hook and screw system. The 3-D shape of the spine and rib cage was modelled pre- and postoperatively using a 3-D reconstruction technique based on multi-planar radiography in a group of 29 adolescents with idiopathic scoliosis. Geometrical indices describing the scoliotic deformity of the rib cage were computed from these models and were compared pre- and postoperatively using Student's t-tests. The frontal spinal curve correction averaged 53% in the frontal plane, while no significant change was noted in the sagittal plane. Significant changes were noted in the shape of the rib cage: rib hump at the apex and at the adjacent lower level were improved (36% and 38%), and small but significant differences were detected in rib frontal orientation in the concavity of the curves at the apex and adjacent lower rib levels. Multi rod, hook and screw instrumentation systems, such as Cotrel-Dubousset instrumentation, are effective in producing significant improvements in the 3-D shape of the rib cage, but these changes are less important than those observed at the spine level.

A three-dimensional radiographic comparison of Cotrel-Dubousset and Colorado instrumentations for the correction of idiopathic scoliosis.

STUDY DESIGN: A prospective clinical study comparing two instrumentation systems for the correction of idiopathic scoliosis. OBJECTIVES: To measure the short-term three-dimensional changes in the shape of the spine after corrective surgery and compare the Cotrel-Dubousset instrumentation to the more recent Colorado instrumentation to determine whether one system provides better three-dimensional correction. SUMMARY OF BACKGROUND DATA: Adequate three-dimensional correction of scoliotic deformities has been reported with the Cortrel-Dubousset instrumentation system. During the past decade, a new generation of more versatile and user-friendly spinal implants has appeared, but there are no reports available to indicate whether similar or better correction can be obtained with these newer systems. METHODS: The three-dimensional geometry of the thoracic and lumbar spine was documented in the standing position using a three-dimensional reconstruction technique based on multiplanar radiography in 67 adolescents with idiopathic scoliosis undergoing correction by a posterior approach. Changes in spinal shape were measured 3 days before and 1 month after the surgery in 31 patients with Cotrel-Dubousset instrumentation and 36 patients with Colorado instrumentation. RESULTS: In both groups, adequate three-dimensional correction of the scoliotic deformities was documented for thoracic and lumbar curves, with significant changes in the frontal plane, in the plane of maximum curvature, and in its orientation. When comparing both groups, better correction was obtained in the frontal plane with the Colorado instrumentation (65% vs. 48% with Cotrel-Dubousset), a finding that may be explained by the significantly greater proportion of pedicle screws used in this group. CONCLUSION: Both instrumentation techniques achieve an effective and comparable three-dimensional correction of the scoliotic deformities.

Intra-operative tracking of the trunk during surgical correction of scoliosis: a feasibility study.

OBJECTIVE: The purpose of this study was to evaluate the feasibility of a technique for intra-operative tracking of the trunk during scoliosis surgery. MATERIALS AND METHODS: Eleven magnetic sensors placed on specific anatomical landmarks are used to compute 11 geometric indices of the trunk. This technique was assessed on a cohort of 40 subjects (19 normal, 21 scoliotic) using an experimental set-up simulating the position of patients during scoliosis surgery. RESULTS: The indices varied less than 2 mm and 1 degrees when breathing (except for chest AP diameter), and less than 1 mm and 1 degrees after transient displacement from the initial positioning of the subjects. No significant changes were observed for most of the indices between two acquisition sessions. Comparison between normal and scoliotic subjects demonstrated that the trunk geometry is more influenced by the positioning of each subject on the operating table than by the magnitude of the spinal deformity. CONCLUSION: The proposed technique will allow intra-operative tracking of the trunk and enable the surgeon to optimize the correction of both spinal and trunk deformities. The technique can also be used to evaluate the adequacy of patient positioning on the operating table, and to obtain a complete follow-up of patients in pre-, intra-, and post-surgical conditions.

[Perioperative radiographic reconstruction of the scoliotic vertebral column]. / Reconstruction radiographique peropératoire de la colonne vertébrale scoliotique.

We have developed a new per-operative three dimensional (3D) reconstruction technique to evaluate the 3D correction of a scoliotic spine induced by surgery using Cotrel-Dubousset instrumentation. A small object with 15 embedded markers was used to calibrate the radiographic system. During surgery, the calibration object was sterilized and fixed to the patient just before the acquisition of two pairs of posterior-anterior and sagittal radiographs; one pair before the rotation maneuver of the rod and one pair after the maneuver. The markers were digitized on each radiograph and their relative 3D positions were measured to establish the relation between the 3D positions of the anatomical structures and their 2D positions on the radiographs. This relation was used to calculate the 3D position of six anatomical landmarks per vertebra (the centers of the superior and inferior vertebral body endplates and the proximal and distal bodies of both pedicles) from the identification of these landmarks on each radiograph. We made a 3D representation of the thoracic and lumbar spine of three patients with idiopathic scoliosis undergoing corrective surgery by the posterior approach. Clinical indices (Cobb angle, axial rotation and the plane of maximum curvature) computed from the 3D reconstruction of the spine obtained before and after the rotation maneuver of the rod were compared to evaluate the 3D correction performed during the surgery. The new proposed approach allows the surgeon to evaluate the per-operative shape of the spine. This approach is simpler, faster and less risky for the patient than the previous method which employed an electromagnetic digitizer to measure the 3D coordinates of anatomical landmarks located on the posterior part of the spine. Furthermore, the 3D representation of the spine visualized from different points of view gives the surgeon an accurate evaluation of the 3D correction during the surgical procedure.