Biomedical subjects

W Skalli

Publications and source records attributed to W Skalli.

At least 19 recordsLinked to original sources

PubMed2006

Apparent Young's modulus of human radius using inverse finite-element method.

The ability to assess the elastic and failure properties of cortical bone at the radial diaphysis has a clinical importance. A new generation of quantitative ultrasound (QUS) devices and peripheral quantitative computed tomography (p-QCT) has been developed to assess non-invasively bone material and structural properties at the distal radius. This anatomical site is characterized by a thin cortical thickness that complicates traditional mechanical testing methods on specimens. Until now, mechanical properties of cortical bone at distal radius (e.g., elastic modulus, yield stress and strain) remain rarely studied probably due to experimental difficulties. The present study introduces an inverse finite-element method strategy to measure the elastic modulus and yield properties of human cortical specimens of the radial diaphysis. Twenty millimeter-thick portions of diaphysis were cut from 40 human radii (ages 45-90) for biomechanical test. Subsequently the same portion was modeled in order to obtain a specimen-specific three dimensional finite-element model (3D-FEM). Longitudinal elastic constants at the apparent level and stress characterizations were performed by coupling mechanical parameters with isotropic linear-elastic simulations. The results indicated that the mean apparent Young's modulus for radial cortical bone was 16 GPa (SD 1.8) and the yield stress was 153 MPa (SD 33). Breaking load was 12,946 N (SD 3644), cortical thickness 2.9 mm (SD 0.6), structural effective strain at the yield (epsilon(y)=0.0097) and failure (epsilon(u)=0.0154) load were also calculated. The 3D-FEM strategy described here may help to investigate bone mechanical properties when some difficulties arise from machining mechanical sample.

Aged↗

PubMed2006

Total disc arthroplasty: consequences for sagittal balance and lumbar spine movement.

This in vivo biomechanical study was undertaken to analyze the consequences for sagittal balance and lumbar spine movement in three different lumbar disc prostheses. A total of 105 patients underwent total disc replacement in three different centers. The Maverick prosthesis was used in 46 patients, the SB Charité device was used in 49 patients and the Prodisc device was utilized in 10 patients. The analysis was computer assisted, using Spineview and Matlab softwares. The intra and inter-observer reliability and measurement uncertainty was performed. The analysis of lateral X-ray films in flexion-extension allowed to measure the prosthesis positioning, the range of motion (ROM), the localization of the mean center of rotation (MCR), the vertebral translation and the disc height, for each prosthesis device. The sagittal balance was analyzed on a full spine film. The parameters studied were described by Duval-Beaupère. The results were compared to the data found in literature, and compared to 18 asymptomatic volunteers, and 61 asymptomatic subjects, concerning the sagittal balance. The prostheses allowed an improvement of the ROM of less than 2 degrees. The ROM of L5-S1 prostheses ranged from 11.6 to 15.6% of the total lumbar motion during flexion-extension. At L4-L5 level, the ROM decreased when there was an arthrodesis associated at the L5-S1 level. There was no difference of ROM between the three prostheses devices. The MCR was linked to the ROM, but did not depend on the prosthesis offcentering. The disc height improved for any prosthesis, and decreased in flexion or in extension, when the prosthesis was offcentered. An increase of translation indicated a minor increase of the ROM at L4-L5 level after Maverick or SB Charité implantation. The L5-S1 arthrodesis was linked with an increase of the pelvic tilt. The lumbar lordosis curvature increased between L4 and S1, even more when a prosthesis was placed at the L3-L4 level. Total disc arthroplasty is useful in the surgical management of discogenic spinal pathology. The three prostheses studied allowed to retorate the disc height, the ROM, without disrupting the sagittal balance, but induced modification of the lumbar curvature.

Arthroplasty, Replacement↗

PubMed2006

Volumetric quantitative computed tomography of the proximal femur: relationships linking geometric and densitometric variables to bone strength. Role for compact bone.

INTRODUCTION: In assessing cervical fractures of the proximal femur, this in vitro quantitative computed tomography (QCT) study had three objectives: to compare QCT to dual-energy X-ray absorptiometry (DXA) for predicting the failure load of the proximal femur, to compare the contributions of density and geometry to bone failure load, and to compare the contributions of cortical and trabecular bone to bone failure load. A novel three-dimensional (3D) analysis tool [medical image analysis framework (MIAF-Femur)] was used to analyze QCT scans. METHODS: The proximal ends of 28 excised femurs were studied (1) using QCT to separately measure bone mineral density (BMD) and geometric variables of trabecular and cortical bone, (2) using mechanical tests to failure in a stance configuration, and (3) using DXA to measure BMD. The variables were described with mean, standard deviation, and range. Correlation matrix and multivariate linear models were computed. RESULTS: Among correlations, cortical thicknesses of the femoral neck were significantly correlated with femoral failure load, especially of the inferoanterior quadrant (r2=0.41; p<0.001), as was cortical volume at the "extended neck" (r2=0.41; p<0.001). Femoral failure load variance was best explained by a combination of QCT variables. Combining densitometric and geometric variables measured by QCT explained 76% of femoral failure load variance compared with 69% with the DXA model. Geometric variables (measured by QCT) explained 43% of femoral failure load variance compared with 72% for densitometric variables (measured by QCT). A model including only trabecular variables explained 52% of femoral failure load variance compared with 59% for a model including only cortical variables. CONCLUSION: The QCT-MIAF reported here provides analysis of both geometric and densitometric variables characterizing cortical and trabecular bone. Confirmation of our results in an independent sample would suggest that QCT may better explain failure load variance for cervical fracture than the gold standard DXA-provided BMD.

Absorptiometry, Photon↗

PubMed2006

Biomechanics of the deltoideus.

The objective of this study was to determine the direction of the migration engendered by the middle deltoideus on the upper end of the humerus. Eleven patients suffering from shoulder pathology underwent an MRI examination (3 mm thick slices). From these MRI slices, 3D reconstructions were obtained for each patient by using a manual data capture system (SliceOmatic((R))). From this geometry, a mechanical model of the deltoideus was produced, taking into account the contacts between the latter and the following anatomical parts: supraspinatus, infraspinatus and humeral head. For the 11 shoulders, we have obtained a deltoideus showing a global resultant oriented upwards. There was, however, a component oriented downwards (at the level of the humeral head), its intensity being 40-80% less than the component oriented upwards (at the level of the deltoideus V). It is important to note that this study is valid only in the initial degrees of lateral elevation. The deltoideus is an elevator muscle of the humeral head in the glenoid, presenting nevertheless a component oriented downwards. The deltoideus would, therefore, intervene to recenter the shoulder during an abduction movement.

Adult↗

PubMed2006

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.

Aged↗

PubMed2006

Finite element model of the human neck during omni-directional impacts. Part II: relation between cervical curvature and risk of injury.

A detailed 3D FE model of the human neck was used to assess a possible relationship between risk of injury and cervical spine curvature for various impacts. A FE model was previously developed, representing the head and neck of a 50th percentile human with a normal lordotic curvature. The model behaviour was omni-directionally validated for various impacts using published results. For the present study, the model was deformed in order to obtain a straight and a kyphotic curvature, and for each geometry, rear-end, frontal, lateral and oblique impact were simulated. Although results showed similar kinematic patterns, significant differences were found in the distribution and peak values of ligament elongations, forces and moments along the cervical spine for the three configurations. It was concluded that the variability observed on the curvature of the human cervical spine may have a significant influence both on the behaviour and on the risk of injury of the neck during impact.

Cervical Vertebrae↗

PubMed2005

Validation of new clinical quantitative analysis software applicable in spine orthopaedic studies.

The objective of this study was to evaluate an X-ray films analysis software, i.e. to estimate the reliability and validity of clinical measurements by means of this software. The authors first performed tests of precision and reproducibility of measures. The precision for dynamic modules was estimated at +/-2 degrees for the lumbar analysis and +/-3 degrees for the cervical one. Mean reproducibility coefficients calculated for postural modules are about 4 degrees for the angular parameters and 3 mm for the linear ones. We also evaluated clinical applicability of the software through its validity. Reference values calculated on a population of healthy subjects showed agreement with the literature. Then, when analysing postural X-ray films of severe scoliotic patients, we found that inter-observer reproducibility coefficients show a lower reliability of measurements; the main cause seems to be the low visibility of anatomic landmarks due to the quality of X-ray films and to the degree of deformity. This study allowed to better estimate the reliability and the usefulness of this tool, allowing for multicentric studies and exchanges.

Cervical Vertebrae↗

PubMed2005

[Evaluation of the intervertebral disc spaces with a low dose radiographic system].

PURPOSE: EOS is a new low dose radiographic system based on micro-pattern gaseous detector technology. The purpose was to compare the value of EOS and MRI in the evaluation of degenerative intervertebral disc disease. METHODS AND MATERIALS: Thirty one fresh lumbar discs were studied with EOS, MRI, and discography. Finally, the discs were dissected for macroscopy. They were graded according to their degenerative level using each imaging modality. Osteophytes, herniations, calcifications, loss of disc height, discographic tears or ligamentous ossification were recorded as anomalies. Morphometric data were measured from the cadaveric sections and low-dose radiographs. RESULTS: Thirty nine percent of the discs were narrowed, with a high correlation between EOS and MRI. Anterior and posterior disc margins were respectively depicted in 22% and 64% of cases with EOS vs 84% and 97% with MRI. MRI showed a disc herniation in 9 cases. The correlation between EOS and MRI was poor for the analysis of anterior and posterior disc margins, and disc herniation. Modic 2 abnormalities were present in 19% of disc levels at MRI. Osteophytes were visible in 65% of disc levels with EOS. The density of the disc space was homogeneous in 65% of cases. In the remaining cases, intradiscal calcifications were detected with EOS and confirmed at macroscopy. CONCLUSION: The EOS low dose images allow a good characterisation of disc abnormalities. Especially, intervertebral disc height, density variations and finer bony details (osteophytes) were detected with high confidence. EOS is a promising new low dose radiographic method for spine imaging.

Aged↗

PubMed2005

Finite element simulation of spinal deformities correction by in situ contouring technique.

Biomechanical models have been proposed in order to simulate the surgical correction of spinal deformities. With these models, different surgical correction techniques have been examined: distraction and rod rotation. The purpose of this study was to simulate another surgical correction technique: the in situ contouring technique. In this way, a comprehensive three-dimensional Finite Element (FE) model with patient-specific geometry and patient-specific mechanical properties was used. The simulation of the surgery took into account elasto-plastic behavior of the rod and multiple moments loading and unloading representing the surgical maneuvers. The simulations of two clinical cases of hyperkyphosis and scoliosis were coherent with the surgeon's experience. Moreover, the results of simulation were compared to post-operative 3D measurements. The mean differences were under 5 degrees for vertebral rotations and 5 mm for spinal lines. These simulations open the way for future predictive tools for surgical planning.

Adult↗

PubMed2004

Three-dimensional X-ray absorptiometry (3D-XA): a method for reconstruction of human bones using a dual X-ray absorptiometry device.

Three-dimensional accurate evaluation of the geometry of the proximal femur may be helpful for hip fracture risk evaluation. The purpose of this study was to apply and validate a stereo-radiographic 3D reconstruction method of the proximal femur, using contours identification from biplanar DXA images. Twenty-five excised human proximal femurs were investigated using a standard DXA unit. Three-dimensional personalized models were reconstructed using a dedicated non-stereo corresponding contours (NSCC) algorithm. Three-dimensional CT-scan reconstructions obtained on a clinical CT-scan unit were defined as geometric references for the comparison protocol, in order to assess accuracy and reproducibility of the 3D stereo-radiographic reconstructions. The precision of a set of 3D geometric parameters (femoral-neck axis length, mid-neck cross-section area, neck-shaft angle), obtained from stereo-radiographic models was also evaluated. This study shows that the NSCC method may be applied to obtain 3D reconstruction from biplanar DXA acquisitions. Applied to the proximal femur, this method showed good accuracy as compared with high-resolution personalized CT-scan models (mean error = 0.8 mm). Moreover, precision study for the set of 3D parameters yielded coefficients of variation lower than 5%. This is the first study providing 3D geometric parameters from standard 2D DXA images using the NSCC method. It has good accuracy and reproducibility in the present study on cadaveric femurs. In vivo prospective studies are needed to evaluate its discriminating potential on hip fracture risk prediction.

Absorptiometry, Photon↗

PubMed2004

Validation of the relative 3D orientation of vertebrae reconstructed by bi-planar radiography.

The three dimensional (3D) reconstruction of the spine can be obtained by stereoradiographic techniques. To be safely used on a routine clinics basis, stereoradiography must provide both accurate vertebral shape and coherent position. Although the accuracy of the reconstructed morphology of the vertebrae is well documented, only few authors studied the accuracy of the vertebral orientation. Therefore, this paper focuses on the evaluation of the orientation accuracy of the reconstructed vertebrae (obtained by non-stereo corresponding point technique) considering either a 178 point vertebral model or a 6 point vertebral model (previously proposed in the literature). Five dried vertebrae were fixed on holders containing four markers each. The 3D reconstruction of both vertebrae and markers were obtained by stereoradiographic techniques. Using least square method matching from one position to another, the relative orientation was computed for the vertebral models (6 or 178 points) and the four markers. These vertebral and holder orientations were compared (considering the holder's one as reference). The repeatability of these relative orientations (vertebrae and holders) was also evaluated. The mean (RMS) orientation error of 178 point vertebral model was 0.6 degrees (0.8 degrees ), for lateral rotation, 0.7 degrees (1.0 degrees ) for sagittal rotation and 1.4 degrees (1.9 degrees ) for axial rotation. The intra-observer repeatability was 0.5 degrees (0.7 degrees ) for lateral rotation, 0.7 degrees (0.8 degrees ) for sagittal rotation and 0.9 degrees (1.2 degrees ) for axial rotation. The orientation was found more accurate and precise when using the 178 point vertebral model than when using the basic 6 point vertebral model. The relative orientation (in post-operative follow-up with respect to the pre-operative examination) of the vertebrae of one scoliotic patient was performed as an example of clinical application. The stereoradiographic method is a reliable 3D quantitative tool to assess the spine deformity, that can be used in clinics for the follow-up of scoliotic patients.

Adult↗

PubMed2004

A proprioception based regulation model to estimate the trunk muscle forces.

Evaluation of loads acting on the spine requires the knowledge of the muscular forces acting on it, but muscles redundancy necessitates developing a muscle forces attribution strategy. Optimisation, EMG, or hybrid models allow evaluating muscle force patterns, yielding a unique muscular arrangement or/and requiring EMG data collection. This paper presents a regulation model of the trunk muscles based on a proprioception hypothesis, which searches to avoid the spinal joint overloading. The model is also compared to other existing models for evaluation. Compared to an optimisation model, the proposed alternative muscle pattern yielded a significant spine postero-anterior shear decrease. Compared to a model based on combination of optimisation criteria, present model better fits muscle activation observed using EMG (38% improvement). Such results suggest that the proposed model, based on regulation of all spinal components, may be more relevant from a physiologic point of view.

Abdomen↗

PubMed2004

3D finite element simulation of Cotrel-Dubousset correction.

The Cotrel-Dubousset (CD) scoliosis surgery was simulated for 10 patients with idiopathic scoliosis using a 3D finite element model (FEM) of the patient's entire spine. The geometry of the FEM was extracted from a 3D stereo-radiographic reconstruction, and mechanical properties were personalized using lateral bending films. Finally, each step of the CD correction was simulated and results were compared with the post-operative 3D stereo-radiographic reconstruction. The whole procedure was applied for 10 patients, and quantitative comparison was performed between post-operative spine configuration and predicted configuration. For all patients, mean differences between post-operative measurements and predicted values of vertebral rotation were estimated at 5 degrees (max: 13 degrees) and those for linear position at 6 mm (max: 12 mm). Furthermore, intermediate steps of surgery simulation were consistent with the literature. Then, for one scoliotic patient, the model was used to investigate three alternative surgical strategies. It was found that a one-level change in the instrumentation limit may have a significant effect on spine alignment and correction.

Biomechanical Phenomena↗

PubMed2004

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).

Aged↗

PubMed2003

Variability of the spine and pelvis location with respect to the gravity line: a three-dimensional stereoradiographic study using a force platform.

Analyzing standing posture requires a precise measure of the orientation of the various body segments with respect to the gravitational vector. We studied the posture variability of 34 healthy upright standing subjects. Using a force platform combined with a powerful stereoradiographic technique, we acquired the spine and pelvis three-dimensional (3D) geometry and located it with respect to the gravity line. For our data set, the mean 3D distance between the geometrical center of each vertebral body and the gravity line was 28 mm with a standard deviation of 5.6 mm. The vertebrae location variability, defined as plus or minus twice the mean standard deviation, was +/-40 mm in the sagittal plane and +/-25 mm in the frontal plane. The line connecting the middle of the external acoustic meatus (center of both acoustic meati: CAM) to the middle of the bi-coxo-femoral axis (hip axis: HA) was almost vertical. Its mean distance to the gravity line was 30 mm. Our data show a left lateralization, with respect to the gravity line, of the "Head-Spine-Pelvis" segments. The mean distance was 7.6 mm (SD 1.6 mm). This might be due to uneven partitioning of the body mass on each side of the sagittal plane.

Adult↗

PubMed2003

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.

Aged↗

PubMed2003

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.

Algorithms↗

PubMed2003

[Validation of a tool to measure pelvic and spinal parameters of sagittal balance].

PURPOSE OF THE STUDY: Parameters determining sagittal balance are essential for optimal analysis and treatment of many spinal disorders. The purpose of this work was to validate a software designed to measure the principal parameters involved in sagittal balance of the spine. MATERIAL AND METHODS: Six parameters (lumbar lordosis, thoracic kyphosis, sagittal tilt at T9, pelvic index, pelvic tilt, slope of the sacrum) were measured on lateral views of the spine from 100 healthy volunteers free of any spinal disease. Two measurement techniques were used: manual measurement and automatic computerized measurement with this software. We hypothesized that manual measures could be accepted as reference values because they are currently the most widely used and because the computerized measurements were obtained using digitalized images that may have modified interpretation. The software was therefore validated by comparing the angles measured manually with the computer output. Inter- and intraobserver coefficients of variation were calculated for the two measurement techniques. One operator performed both series of measurements (manual and computerized). Two other operators preformed two series of independent measures using one of the measurement techniques. Finally, two new operators performed a complete series of measurements using both measurement techniques. Comparisons were performed with the t test for paired variables with calculation of the coefficients of correlation. Intraclass coefficients of correlation were determined for inter- and intra-observer variability. RESULTS: There was an excellent correlation between the manual measurements and the computerized measurements with intra-class coefficients of correlation varying from 0.82 to 0.96. Inter- and intra-observer variabilities were comparable for the two measurement techniques used to determine thoracic kyphosis, lumbar lordosis, pelvic index, pelvic tilt, and slope of the sacrum. Inter- and intra-observer variability was lower when the sagittal tilt was measured with the software specially designed. CONCLUSION: This comparison between two techniques for measuring pelvic and spinal parameters of sagittal balance of the spine demonstrated a good correlation between manual and the computerized measurements obtained with the software to be evaluated. Computer-assisted measurements not only provided a saving in time but also minimized inter- and intra-observer variability for the estimation of certain parameters.

Adult↗