Yoann Lafon

Intraoperative Three-Dimensional Correction During Rod Rotation Technique

Study Design. A numerical study was conducted by simulating the Cotrel–Dubousset (CD) surgery. Objective. To quantify intraoperative correction during CD surgery. Summary of Background Data. Very few methods have been reported in literature to analyze the effect of intraoperative surgical gestures, and none considers the three-dimensional correction of the entire spine during the main surgical gestures. Intraoperative frontal radiographs limit analysis to two-dimensional correction, and movement tracking devices focus the kinematics study of specific vertebrae in the instrumented area only. Methods. This study included 20 patients, mean age 15 years, with severe idiopathic scoliosis treated by CD surgery. A patient-specific finite-element model (T1–L5 and pelvis), based on preoperative stereo-radiography and flexibility test radiographs, was constructed for each patient. An automated algorithm simulated all the main steps of the CD surgery. For each step, vertebral kinematics was exported to compute the evolution of various clinical parameters. Coherence of the simulations was evaluated by comparing the virtual postoperative spinal configuration with postoperative in vivo data. Results. The CD surgery affected the vertebral levels inside but also outside the fused spinal area, in a three-dimensional complex kinematics. Every intraoperative maneuver contributes to scoliosis correction. The second rod insertion, focused on the apical vertebra, leading to a global modification of all the curves. Conclusions. The automated patient-specific simulation of CD surgery may improve our understanding of surgical biomechanics. Therefore, it could increase the relevance of preoperative surgery planning.

In vivo distribution of spinal intervertebral stiffness based on clinical flexibility tests.

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

Intraoperative three dimensional correction during in situ contouring surgery by using a numerical model.

Study Design. A numerical study was conducted by simulating in situ contouring (ISC) surgery. Objective. To quantify intraoperative correction during ISC surgery. Summary of Background Data. Surgical techniques correcting scoliosis, like the ISC one, lead to a complex 3-dimensional correction of the spine. Using motion analysis devices to analyze the effect of intraoperative surgical maneuvers was tedious and limited the study to the kinematics of exposed vertebrae. An alternative method consisted in simulating the surgical gestures. However, proposed models were based on rigid instrumentations, and focused attention on specific gestures of the rod-rotation and the distraction techniques through operator-dependent simulations. Methods. This study included 10 patients with severe idiopathic scoliosis treated by ISC surgery. From a patient-specific finite-element model (T1–L5 and pelvis), all main steps of the ISC surgery were automatically simulated. A specific algorithm was developed to determine the sequences of bending maneuvers according to the rod shapes chosen by the surgeon. The accuracy of the automated surgery simulation was assessed regarding the virtual postoperative spinal configuration and postoperative clinical data. For each maneuver, vertebral kinematics was computed as well as the evolution of various clinical parameters. Results. The bending maneuvers of both the first and the second rods provided complementary effects inside, but also outside the fused spinal area. These main maneuvers combined the intraoperative spinal corrections induced by maneuvers specific to the rod-rotation surgery. Conclusion. The automated patient-specific simulation of ISC surgery may improve the understanding of the main mechanisms involved in the scoliosis surgical correction.

Combination of a model-deformation method and a positional MRI to quantify the effects of posture on the anatomical structures of the trunk

Understanding the postural effects on organs and skeleton could be crucial for several applications. This paper reports on a methodology to quantify the three-dimensional effects of postures on deformable anatomical structures. A positional MRI scanner was used to image the full trunk in four postures: supine, standing, seated and forward-flexed. The MRI stacks were processed with a custom toolbox, implemented using open source software. The semi-automated segmentation was based on the deformation of generic models of the pelvis, sternum, femoral heads, spine, liver, kidneys, spleen, skin, thoracic and abdominal cavities. The toolbox was designed to be easily extended by additional image filters, deformation schemes, or new generic models. Results obtained on one subject demonstrate that the method can be used to quantify the effects of postures on skeleton and organs. The spinal curvature, the pelvic parameters and the volume of the thoracic cavity were affected by the four postures. The volumes of the kidneys, spleen, liver and abdominal object were mostly unaffected. The movement of organs was coherent with the effect of gravity. The deformation of organs between postures was expressed using geometrical transformations. Investigations should be pursued on a larger population to confirm the patterns observed on the first subject.

Effect of the muscle activation level distribution on normal stress field: a numerical study.

Deformable approaches to model muscles with contractile capability are being investigated more and more. The effects on the strain field of the muscle architecture (Blemker et al. 2005) and of the ...

Multi-objective optimisation for musculoskeletal modelling: application to a planar elbow model.

One of the open issues in musculoskeletal modelling remains the choice of the objective function that is used to solve the muscular redundancy problem. Some authors have recently proposed to introduce joint reaction forces in the objective function, and the question of the weights associated with musculo-tendon forces and joint reaction forces arose. This question typically deals with a multi-objective optimisation problem. The aim of this study is to illustrate, on a planar elbow model, the ensemble of optimal solutions (i.e. Pareto front) and the solution of a global objective method that represent different compromises between musculo-tendon forces, joint compression force, and joint shear force. The solutions of the global objective method, based either on the minimisation of the sum of the squared musculo-tendon forces alone or on the minimisation of the squared joint compression force and shear force together, are in the same range. Minimising either the squared joint compression force or shear force alone leads to extreme force values. The exploration of the compromises between these forces illustrates the existence of major interactions between the muscular and joint structures. Indeed, the joint reaction forces relate to the projection of the sum of the musculo-tendon forces. An illustration of these interactions, due to the projection relation, is that the Pareto front is not a large surface, like in a typical three-objective optimisation, but almost a curve. These interactions, and the possibility to take them into account by a multi-objective optimisation, seem essential for the application of musculoskeletal modelling to joint pathologies.

Kinematics of the Normal Knee during Dynamic Activities: A Synthesis of Data from Intracortical Pins and Biplane Imaging

Few studies have provided in vivo tibiofemoral kinematics of the normal knee during dynamic weight-bearing activities. Indeed, gold standard measurement methods (i.e., intracortical pins and biplane imaging) raise ethical and experimental issues. Moreover, the conventions used for the processing of the kinematics show large inconsistencies. This study aims at synthesising the tibiofemoral kinematics measured with gold standard measurement methods. Published kinematic data were transformed in the standard recommended by the International Society of Biomechanics (ISB), and a clustering method was applied to investigate whether the couplings between the degrees of freedom (DoFs) are consistent among the different activities and measurement methods. The synthesised couplings between the DoFs during knee flexion (from 4° of extension to −61° of flexion) included abduction (up to −10°); internal rotation (up to 15°); and medial (up to 10 mm), anterior (up to 25 mm), and proximal (up to 28 mm) displacements. These synthesised couplings appeared mainly partitioned into two clusters that featured all the dynamic weight-bearing activities and all the measurement methods. Thus, the effect of the dynamic activities on the couplings between the tibiofemoral DoFs appeared to be limited. The synthesised data might be used as a reference of normal in vivo knee kinematics for prosthetic and orthotic design and for knee biomechanical model development and validation.

Soft tissue artefacts: compensation and modelling

Soft tissue artefacts (STA) are one of the main contemporary problems in 3D motion analysis. They are known to strongly affect the joint kinematics assessment and several methods of compensation have been proposed (Leardini et al. 2005). Many of these methods are based on using a cluster of markers, while the bone pose is computed by least squares. In this way, STA deformation is discarded (as a residual error), but it is hardly ever modelled or interpreted. Several methods, well known in the field of computed vision or structural mechanics could help in both computing the bone pose and modelling the STA deformation. Among them, four methods get out of the line for their computational expediency: affine (Sommer et al. 1982), kriging (Trochu 1993), radial basis function (RBF; Orr 1995) and modal decomposition (Clough and Penzien 1993). This study is focused on the evaluation, on simulated gait data, of these four methods in the same time as singular value decomposition (SVD; Soderkvist and Wedin 1993).

A Markerless Method for Personalizing a Digital Human Model from a 3D Body Surface Scan

Digital Human Models (DHM) are used for ergonomic design of products. For instance, vehicle ingress/egress motions are simulated for assessing vehicle accessibility. In order to validate simulations, experiments are often needed implying motion capture and motion reconstruction using a DHM. The first step for motion reconstruction is to create a personalized DHM respecting the anthropometric dimensions of the volunteer performing the task. However creating a personalized DHM from external body shape is not straight forward, because the internal skeleton has to be identified from external body shape. Here we propose a four-step method for generating a personalized DHM which matches a 3D scan. The first step is to clean the scan data and to prepare a DHM and a third body surface template. Then, thanks to the use of the third common body template, the correspondence between the DHM and scan surface points is established, making it possible to calculate the transformation parameters by kriging. From estimated position of joint centers, the internal skeleton is scaled and positioned from a known reference posture to the scan position. The third step is then to attach the surface points to their corresponding skeletal segments. The last step is to check and correct the attached skin points around some joints so as to respect the skin to segment structure specific to a DHM. Compared to the method used in the past by manually adjusting a DHM on calibrated photos of several points of views; the proposed method is operator independent and much less time consuming.