Sébastien Laporte

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

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

3D reconstruction of the pelvis from bi-planar radiography

3D personalized models are more and more requested for clinical and biomechanical studies. Techniques based on bi-planar X-rays present the advantage of a low radiation dose for the patient. However, up to now, such techniques have shown limited accuracy in the case of pelvis reconstruction. This study proposes and validates a method providing accurate 3D personalized model of the pelvis from bi-planar X-rays. The algorithm is based on the fast computation of an initial solution followed by local deformations based on 2D anatomical points and contours that are digitized in both radiographs. Results were close to CT-scan reconstructions (mean difference 1.6 mm and differences under 4.3 mm for 95% of the points). Moreover, 3D morphometry of the pelvis could be obtained with an accuracy of 5%. This technique provides 3D patient specific model with a low radiation dose.

3D reconstruction of the proximal femur with low-dose digital stereoradiography.

Objective: Accurate three-dimensional (3D) geometry of the proximal femur may be helpful for fracture risk evaluation, as well as for planning and assisting surgical procedures. The purpose of this study was to apply and validate a stereoradiographic 3D reconstruction method on the proximal femur from radiographic contours identified on bi-planar radiographs. Materials and Methods: Twenty-five excised non-pathologic proximal femurs were investigated using a low-dose digital radiographic device. Three-dimensional personalized models were reconstructed using the Non-Stereo Corresponding Contours (NSCC) algorithm. Three-dimensional CT-scan reconstructions were defined as geometric references for the comparison protocol, in order to assess the accuracy and reproducibility of the personalized 3D stereoradiographic reconstructions. In addition, the reliability of a set of 3D parameters obtained from stereoradiographic models was evaluated. Results: This study demonstrated the validity of the NSCC method when applied to the proximal femur, with good results for accuracy (mean error = 0.7 mm) and reproducibility (Wilcoxon test: p > 0.28). Moreover, a precision study for the set of 3D parameters yielded a coefficient of variation lower than 5%. Conclusions: Once this approach has been validated in vivo, it should find multiple applications in therapeutic fields (e.g., for surgical planning, computer assisted surgery, etc.), as well as in diagnostic contexts (e.g., equilibrium studies or osteoporosis fracture risk assessment).

Three-Dimensional Analysis of the Cervical Spine Kinematics: Effect of Age and Gender in Healthy Subjects

Study Design. A three-dimensional (3D) analysis of the cervical spine kinematics in vivo about a large asymptomatic database in order to evaluate the impact of age and gender on the necks performances. Objective. To investigate the effect of age and gender on kinematical parameters of the cervical spine, specifically quantitative parameters concerning coupled movements and proprioception, using the infra-red POLARIS measurement system. Summary of Background Data. Cervical spine kinematics has been investigated in vivo by numerous authors using various devices. However, few is known about the influence of gender and age on the 3D cervical biomechanics, specifically regarding coupled movements and proprioceptive abilities. Methods. A total of 140 asymptomatic volunteers (70 men and 70 women) aged 20 to 93 years old were enrolled. The noninvasive infrared system Polaris was used to quantify the 3D range of motion (ROM) of cervical spine and to evaluate proprioceptive abilities. For validating the protocol in terms of reproducibility, 12 volunteers were tested 3 times by 2 independent operators. Results. The standard error of measurement for the maximal ROM in the 3 space planes was 5%. Gender had no significant influence on the 3D cervical ROM, except for the “70–79 years old” group. Age had a significant influence on all main movements showing 0.55° to 0.79° magnitude decrease per decade. Age and gender had no significant influence on coupled movements. “Head-to-Target” proprioception was significantly affected by the age only in the horizontal plane. Conclusion. A data base for cervical ROM, pattern of motion, and proprioceptive capability was established in population of 140 healthy subjects of various age and gender. Significant age-related decrease in ROM and proprioceptive abilities were observed in this study. Coupled movements did not vary with gender or age; however, their role in the cervical performance increased with age since main movements were limited.

Explicit calibration method and specific device designed for stereoradiography

The three-dimensional geometry of the human spine is noteworthy information that can be obtained by stereoradiographic methods. These methods are based on the identification of anatomical structures in several views which are obtained by rotation of a patient standing on a turntable. Calibration algorithms for computer vision or photogrammetry are well documented, but they generally yield calibration devices which are cumbersome for the use in clinical stereoradiography. This paper presents a calibration method adapted to a two-view stereoradiography calibration (frontal and lateral incidences) and based on a simplified geometric modeling of the radiological environment. The a priori knowledge yields four calibration equations related to the vertical and horizontal planes of both views, leading to a specific calibration procedure and device. Moreover this device is attached to the stereoradiographic system (directly integrated on the turntable) in order to facilitate clinical applications. A validation was performed on 26 dried lumbar vertebrae in order to evaluate clinical situation. The mean accuracy of the stereoradiographic reconstruction was 1.2mm.

Three-dimensional reconstruction of the lower limb from biplanar calibrated radiographs

Three-dimensional (3D) reconstruction of lower limbs is essential for surgical planning and clinical outcome evaluation. 3D reconstruction from biplanar calibrated radiographs may be an alternative to irradiation issues of CT-scan. A previous study proposed a two-step reconstruction method based on parametric models and statistical inferences leading to a fast Initial Solution (IS) followed by manual adjustments. This study aims to improve the IS using a new 3D database, a novel parametric model of the tibia and a different regression approach. The IS was evaluated in terms of shape accuracy on 9 lower limbs and reproducibility of clinical measurements on 22 lower limbs. Reconstruction time was also evaluated. Comparison to the previous method showed an improvement of the IS in terms of shape accuracy (1.3 vs. 1.6 and 2 mm respectively for both femur and tibia) and reproducibility of clinical measurements (i.e. 3.1° vs. 8.3° for neck-shaft-angle; 4.2° and 5° vs. 5° and 6° for tibial and femoral torsion respectively). The proposed approach constitutes a considerable step towards an automatic 3D reconstruction of lower limb.

Parametric and subject-specific finite element modelling of the lower cervical spine. Influence of geometrical parameters on the motion patterns

Morphometrical and postural features of the cervical spine are supposed to significantly influence its biomechanical behaviour. However, the effects of these geometrical parameters are quite difficult to evaluate. An original numerical method is proposed in order to automatically generate parametric and subject-specific meshes of the lower cervical spine. Sixteen finite element models have been built from cadaver specimens using low dose biplanar X-rays. All the generated meshes fulfilled the quality criteria. A preliminary evaluation was performed on the C5-C6 functional units using a database of previous experimental tests. The principal and coupled motions were simulated. The responses of the numerical models were within the experimental standard deviation corridors in most cases. Rotation-moment relationships were then compared to assess the influence of geometry on the mechanical response. Geometry was found to play a significant role in the motion patterns.

Quantitative morphometric study of thoracic spine. A preliminary parameters statistical analysis

SummaryStudy Design: A quantitative morphometric study of the thoracic spine from T1 to T12 was conducted on 50 dried spines.Objective: To quantify anatomic descriptive parameters concerning thoracic vertebrae, and to investigate statistical correlation between some of them, in order to 1) refine knowledge of some dimensions and orientations for clinical application 2) aid for implant’s designing, and, 3) improve existing geometric and mechanical models or 3D reconstruction techniques concerning thoracic spine.Summary of Background Data: A few three-dimensional quantitative studies were performed, either on a limited number of parameters, or on a limited series of specimens. No investigation of statistical correlation between parameters was previously performed concerning thoracic vertebrae.Methods: 373 thoracic vertebrae from 50 dried spines were considered. Three spatial coordinates of 140 points spread on the surface for each vertebrae (130 for T10, 120 for T11 and T12) were measured using an electromagnetic system (FASTRAK©, estimated accuracy: ±0.4 mm). Descriptive parameters were then calculated using Excel® and statistical correlation was investigated using xlSTAT®.Results: The mean values and standard deviations of 81 linear, angular and surfacic parameters were obtained for the thoracic vertebrae on each level. High correlation was found for some parameters, such as interarticularis facet position, pedicles height and width, with the dimensions of the vertebral bodies that can be measured on X-rays.Conclusion: A detailed and accurate quantitative description of thoracic vertebrae is provided. For the first time, statistical correlation is demonstrated between some descriptive parameters which are visible on X-rays, and others which are not.RésuméTravail réalisé: Une étude morphométrique quantitative de la colonne thoracique, de T1 à T12, a été réalisée sur 50 colonnes vertébrales sèches.Objectifs: Quantifier les paramètres anatomiques décrivant la vertèbre thoracique et analyser les corrélations statistiques existant entre eux, dans le but 1) d’approfondir les connaissances sur certaines dimensions et orientations pour des applications cliniques, 2) d’aider à la conception d’implants, et, 3) d’améliorer les modèles géométriques et mécaniques existants ainsi que les techniques de reconstruction 3D de la colonne thoracique.Résumé de la bibliographie: Peu d’études quantitatives tridimensionnelles concernent des vertèbres thoraciques, et celles-ci ne s’intéressent qu’à un nombre limité de paramètres ou ne sont réalisées que sur des séries limitées de spécimens. Aucune recherche sur les corrélations statistiques entre paramètres n’a été encore réalisée pour les vertèbres thoraciques.Méthodes: 373 vertèbres thoraciques provenant de 50 colonnes ont été considérées. Les coordonnées spatiales de 140 points dispersés sur la surface de chaque vertèbre (130 pour T10 et 120 pour T11 et T12) ont été relevées grâce à un système de mesure électromagnétique (FASTRAK©, précision estimée: ± 0,4 mm). Les paramètres descriptifs ont été alors calculés avec EXCEL® et les corrélations statistiques ont été étudiées avec xlSTAT®.Résultats: les moyennes ainsi que les écarts types ont été obtenus pour 81 paramètres linéaires, surfaciques ou angulaires, pour chacun des niveaux vertébraux thoraciques. De fortes corrélations ont été découvertes entre certains paramétres, tels que la localisation des facettes articulaires ou la largeur et la hauteur des pédicules, et les dimensions du corps vertébral, qui sont accessibles sur des radiographies.Conclusion: Une description fine et détaillée de la vertèbre thoracique est fournie. Pour la première fois, une corrélation statistique est démontrée entre quelques paramètres descriptifs, visibles à la radiographie, et d’autres qui ne le sont pas.

Links between mechanical behavior of cancellous bone and its microstructural properties under dynamic loading

Previous studies show that in vivo assessment of fracture risk can be achieved by identifying the relationships between microarchitecture description from clinical imaging and mechanical properties. This study demonstrates that results obtained at low strain rates can be extrapolated to loadings with an order of magnitude similar to trauma such as car crashes. Cancellous bovine bone specimens were compressed under dynamic loadings (with and without confinement) and the mechanical response properties were identified, such as Young׳s modulus, ultimate stress, ultimate strain, and ultimate strain energy. Specimens were previously scanned with pQCT, and architectural and structural microstructure properties were identified, such as parameters of geometry, topology, connectivity and anisotropy. The usefulness of micro-architecture description studied was in agreement with statistics laws. Finally, the differences between dynamic confined and non-confined tests were assessed by the bone marrow influence and the cancellous bone response to different boundary conditions. Results indicate that architectural parameters, such as the bone volume fraction (BV/TV), are as strong determinants of mechanical response parameters as ultimate stress at high strain rates (p-value<0.001). This study reveals that cancellous bone response at high strain rates, under different boundary conditions, can be predicted from the architectural parameters, and that these relations with mechanical properties can be used to make fracture risk prediction at a determined magnitude.

Viscoelastic properties of the human sternocleidomastoideus muscle of aged women in relaxation.

Improving the numerical models of the head and neck complex requires understanding the mechanical properties of the muscles; however, most of the data in existing literature have been obtained from studies on animal muscles. Muscle is hyper-elastic, but also viscoelastic. The hyper-elastic behaviour of the human sternocleidomastoideus muscle has been previously studied. The aim of this study is to propose a characterization of the viscoelastic properties of the same human muscle in relaxation. Ten muscles were tested in vitro. The viscoelastic behaviour was modelled with a generalized Maxwells model studied at the first and second order, using an inverse approach with a subject-specific, finite-element model of each muscle. Based on these models, relaxation times τ (first order: 103s; second order: 18s and 395s) and ratio moduli γ (first order: 0.33; second order: 0.20 and 0.19) were identified. The first-order model provided a good estimate of the relaxation curve (R(2): 0.82), but the second-order model was more representative of the experimental response (R(2): 0.97). Our results provide evidence that the viscoelastic behaviour of the human sternocleidomastoideus muscle can be described using a second-order Maxwells model and that - combined with the previously identified hyper-elastic properties - the response of the muscle in tension and relaxation is fully characterized.