- [Show abstract] [Hide abstract] ABSTRACT: Lower limb deformation in children with osteogenesis imperfecta (OI) impairs ambulation and may lead to fracture. Corrective surgery is based on empirical assessment criteria. The objective was to develop a reconstruction method of the tibia for OI patients that could be used as input of a comprehensive finite element model to assess fracture risks. Data were obtained from three children with OI and tibia deformities. Four pQCT scans were registered to biplanar radiographs, and a template mesh was deformed to fit the bone outline. Cortical bone thickness was computed. Sensitivity of the model to missing slices of pQCT was assessed by calculating maximal von Mises stress for a vertical hopping load case. Sensitivity of the model to ±5 % of cortical thickness measurements was assessed by calculating loads at fracture. Difference between the mesh contour and bone outline on the radiographs was below 1 mm. Removal of one pQCT slice increased maximal von Mises stress by up to 10 %. Simulated ±5 % variation of cortical bone thickness leads to variations of up to 4.1 % on predicted fracture loads. Using clinically available tibia imaging from children with OI, the developed reconstruction method allowed the building of patient-specific finite element models.
- [Show abstract] [Hide abstract] ABSTRACT: In the field of biomechanics, the offer of models which are more and more realistic requires to integrate a physiological response, in particular, the controlled muscle bracing and the reflexes. The following work aims to suggest a unique methodology which couples together a sensory and motor loop with a finite element model. Our method is applied to the study of the oscillation of the elbow in the case of a biceps brachial stretch reflex. The results obtained are promising in the purpose of the development of reactive human body models.
- [Show abstract] [Hide abstract] ABSTRACT: Background: Sacroiliac joint (SIJ) is a known chronic pain-generator. The last resort of treatment is the arthrodesis. Different implants allow fixation of the joint, but to date there is no tool to analyze their influence on the SIJ biomechanics under physiological loads. The objective was to develop a computational model to biomechanically analyze different parameters of the stable SIJ fixation instrumentation. Methods: A comprehensive finite element model (FEM) of the pelvis was built with detailed SIJ representation. Bone and sacroiliac joint ligament material properties were calibrated against experimentally acquired load-displacement data of the SIJ. Model evaluation was performed with experimental load-displacement measurements of instrumented cadaveric SIJ. Then six fixation scenarios with one or two implants on one side with two different trajectories (proximal, distal) were simulated and assessed with the FEM under vertical compression loads. Results: The simulated S1 endplate displacement reduction achieved with the fixation devices was within 3% of the experimentally measured data. Under compression loads, the uninstrumented sacrum exhibited mainly a rotation motion (nutation) of 1.38° and 2.80° respectively at 600 N and 1000 N, with a combined relative translation (0.3 mm). The instrumentation with one screw reduced the local displacement within the SIJ by up to 62.5% for the proximal trajectory vs. 15.6% for the distal trajectory. Adding a second implant had no significant additional effect. Conclusion: A comprehensive finite element model was developed to assess the biomechanics of SIJ fixation. SIJ devices enable to reduce the motion, mainly rotational, between the sacrum and ilium. Positioning the implant farther from the SIJ instantaneous rotation center was an important factor to reduce the intra-articular displacement. Clinical relevance: Knowledge provided by this biomechanical study enables improvement of SIJ fixation through optimal implant trajectory.
- [Show abstract] [Hide abstract] ABSTRACT: Study design: Experimental study of pedicle screw fixation in human cadaveric vertebrae. Objective: To experimentally characterize pedicle screw fixation under non-axial loading and to analyze the effect of the surgeons' screw and placement choices on the fixation risk of failure. Summary of background data: Pedicle screw fixation performance is traditionally characterized withaxial pullout tests, which do not fully represent the various tridimensional loads sustained by the screws during correction maneuvers of severe spinal deformities. Previous studies have analyzed the biomechanics of non-axial loads on pedicle screws but their effects on the screw loosening mechanisms are still notwell understood. Methods: A design of experiment (DOE) approach was used to evaluate two screw thread designs (single-lead and dual-lead threads), two insertion trajectories in the transverse and sagittal planes and two loading directions (lateral and cranial). Pedicle screws were inserted in both pedicles of twelve cadaveric lumbar vertebrae for a total of 24 tests. Four sinewave loading cycles (0N to 400N) were applied, orthogonally to the screw axis, at the screw head. The resulting forces, displacements, and rotations of the screws were recorded. Results: In comparison to the other cycles, the first loading cycle revealed important permanent deformation of the bone (mean permanent displacement of the screw head of 0.79 mm), which gradually accumulated over the following cycles to 1.75 mm on average (plowing effect). The cranial loading direction caused significantly lower (p < 0.05) bone deformation than lateral loading. The dual-lead screw had a significantly higher (p < 0.05) initial stiffness than the single-lead thread screw. Conclusions: Non-axial loads induce screw plowing which lead to bone compacting and subsequent screw loosening or even bone failure thus reducing the pedicle screw fixation strength. Lateral loads induce greater bone deformation and risks of failure than cranial loads. Level of evidence: N/A.
- [Show abstract] [Hide abstract] ABSTRACT: Scoliosis is a 3D deformation of the spine and rib cage. For severe cases, surgery with spine instrumentation is required to restore a balanced spine curvature. This surgical procedure may represent a neurological risk for the patient, especially during corrective maneuvers. This study aimed to computationally simulate the surgical instrumentation maneuvers on a patient-specific biomechanical model of the spine and spinal cord to assess and predict potential damage to the spinal cord and spinal nerves. A detailed finite element model (FEM) of the spine and spinal cord of a healthy subject was used as reference geometry. The FEM was personalized to the geometry of the patient using a 3D biplanar radiographic reconstruction technique and 3D dual kriging. Step by step surgical instrumentation maneuvers were simulated in order to assess the neurological risk associated to each maneuver. The surgical simulation methodology implemented was divided into two parts. First, a global multi-body simulation was used to extract the 3D displacement of six vertebral landmarks, which were then introduced as boundary conditions into the personalized FEM in order to reproduce the surgical procedure. The results of the FEM simulation for two cases were compared to published values on spinal cord neurological functional threshold. The efficiency of the reported method was checked considering one patient with neurological complications detected during surgery and one control patient. This comparison study showed that the patient-specific hybrid model reproduced successfully the biomechanics of neurological injury during scoliosis correction maneuvers.
- [Show abstract] [Hide abstract] ABSTRACT: The clinical advantages of reducing spondylolisthesis over fusion in situ have several intuitive reasons such as restore the spinal column into a more anatomic relationship and alignment. However, there is only little evidence in the literature supporting the theoretical advantages of reduction, and its effect on spinopelvic alignment remains poorly defined. In this study, a comprehensive finite element model was developed to analyze the biomechanics of the spine after spinal fusion at L5-S1 in both types of high-grade spondylolisthesis (balanced and unbalanced pelvis). The relevant clinical indices (i.e. spondylolisthesis grade and Dubousset lumbosacral angle), the displacement of L4-L5, pressure within the annulus and nucleus, and stress at L4-L5 were evaluated and compared. The model can well predict the changes of the important clinical indices during the surgery. For a balanced pelvis, the reduction has a minimal effect on the biomechanical conditions at the adjacent level during postsurgical activities. In the unbalanced case, reduction induced larger deformation in the lumbosacral region and a higher stress concentration at adjacent level. Whether such a stress concentration can lead to long-term disc degeneration is not known. The results provide additional information for the clinician considering reduction of high-grade spondylolisthesis.
- [Show abstract] [Hide abstract] ABSTRACT: Some cardiopulmonary resuscitation (CPR) monitoring devices were released in recent years. Some of them are motion sensors. There are no guidelines were to position future or present sensors during CPR. We evaluate the possible influence of the location of motion sensors by a high-speed camera during a CPR on a manikin. We performed a motion analysis by a high-speed camera during chest compression (CC) on a manikin to quantify chest inhomogeneous displacements and rescuer motion. Midline chest was found to have an inhomogeneous depth during CC (19mm for the upper sternum, 27mm for the middle of the sternum, and 47mm for the xiphoid). Rescuer anatomy has a complex motion. The direct application of the sensor under the hand performing CC seems to be the more accurate solution if the device allows it. Copyright © 2015. Published by Elsevier Inc.
- [Show abstract] [Hide abstract] ABSTRACT: This work was conducted to study biomechanical properties and macroscopic analysis of petrous fracture by lateral impact. Seven embalmed intact human cadaver heads were tested to failure using an electrohydraulic testing device. Dynamic loading was done at 2 m/s on temporal region with maximal deflection to 12 mm. Anthropometric and pathological data were determined by pretest and posttest computed tomography images, macroscopic evaluation, and anatomical dissection. Biomechanical data were obtained. Results indicated the head to have nonlinear structural response. The overall mean values of failure forces, deflections, stiffness, occipital, and frontal peak acceleration were 7.1 kN (±1.1), 9.1 mm (±1.8), 1.3 kN/mm (±0.4), 90.5 g (±22.5), and 65.4 g (±16), respectively. The seven lateral impacts caused fractures, temporal fractures in six cases. We observed very strong homogeneity for the biomechanical and pathological results between different trials in our study and between data from various experiments and our study. No statistical correlation was found between anthropometric, biomechanical, and pathological data. These data will assist in the development and validation of finite element models of head injury.
- [Show abstract] [Hide abstract] ABSTRACT: Recently, a T2*-weighted template and probabilistic atlas of the white and gray matter (WM, GM) of the spinal cord (SC) have been reported. Such template can be used as tissue-priors for automated WM/GM segmentation but can also provide a common reference and normalized space for group studies. Here, a new template has been created (AMU40), and accuracy of automatic template-based WM/GM segmentation was quantified. The feasibility of tensor-based morphometry (TBM) for studying voxel-wise morphological differences of SC between young and elderly healthy volunteers was also investigated.
- [Show abstract] [Hide abstract] ABSTRACT: Cervical myelopathy diagnosis and management in clinical practice still lacks of objective markers of potential surgery outcome. Therefore, we applied a multimodal MRI protocol, combining DTI (known to be more predictive of surgical outcome than the sole presence of T2 hyperintensity) and inhomogeneous magnetization transfer (ihMT, myelin-specific technique) to 2 patients before and 3 months after decompressive surgery. We observed both metrics evolution after surgery and neurological function evolution to see whether this multimodal protocol could help in understanding the evolutive pattern of the disease after surgery. Longitudinal follow-up until 1year post-surgery will help in answering the raised question.
- [Show abstract] [Hide abstract] ABSTRACT: The geometric fidelity of the inner organs on finite-element model (FEM)s of the human body and the choice to use discontinuous mesh engender the appearance of empty spaces that do not reflect the real-life situation of human body cavities. The aim of this study is to assess the influence of these empty spaces on the behavior of a simplified FEM build with three different structures in interaction which properties are relevant with the abdominal cavity. This FEM is made up of a large sphere (peritoneum) containing two hemispheres (liver and spleen). The space between peritoneum and inner organs was defined with two different approaches and assessed under impact conditions. The first is a Meshfree Space approach, e.g. consider the space as a perfect gas. The second approach, Meshed Space, entailed adding volumetric elements in the empty space. From each approach, one optimal configuration was identified regarding the recorded force vs. compression, the mobility of inner organs and the space incompressibility. This space has a considerable influence on the behavior of the FEM and mainly on the applied loadings of inner organs (difference reaching 70% according to the configuration). For the first approach, the incompressible gas is designated because it guarantees space incompressibility (vf/vi=1) and inner organs loading with the lowest delay (for high impact velocity: Peak force = 89N, compression 47%). For the second approach, the discontinuous volumetric mesh is preferred because it promotes space incompressibility (vf/vi=0.94) and acceptable force reaction (for high impact velocity: Peak force = 97N, compression 49%). The current study shows the importance of this space on the human FEMs cavities behavior and proposes 2 configurations able to be used in a future study including detailed FEM.
- [Show abstract] [Hide abstract] ABSTRACT: Mesenteric avulsion, corresponding to a tearing of intestine's root, generally results from high deceleration in road accidents. The biomechanical analysis of bowel and mesenteric injuries is a major challenge for injury prevention, particularly because seat belt restraint may paradoxically increase their risk of occurrence. The aim of this study was to identify the biomechanical behavior of mesentery and small bowel (MSB) tissue samples under dynamical loading conditions. A dedicated test bench was designed in order to perform tensile tests on fresh MSB porcine specimens, with quasi-static (1 mm/s) and dynamic (100 mm/s) loading conditions. The mechanical behavior of MSB specimens was investigated and compared to isolated mesenteric and isolated small bowel specimens. The results show a high sensitivity of MSB stiffness (1.0 ± 0.2 and 1.3 ± 0.3 N/mm at 1 and 100 mm/s, p = 0.001) and ultimate force (22 ± 5 and 35 ± 8 N at 1 and 100 mm/s, p = 0.001) to the loading rate but not for the displacement at failure. This leads to postulate on a failure criteria based on strain level regardless of the strain rate. These experimental results could be further used to develop refined finite element models and to further investigate on injury mechanisms associated to seat belt restraints, as well as to evaluate and improve protective devices.
- [Show abstract] [Hide abstract] ABSTRACT: Study Design: Detailed biomechanical analysis of the anchorage performance provided by different pedicle screw design and placement strategies under pullout loading. Objective: To biomechanically characterize the specific effects of surgeon-specific pedicle screw design parameters on anchorage performance using a finite element model (FEM). Summary of Background Data: Pedicle screw fixation is commonly used in the treatment of spinal pathologies. However, there is little consensus on the selection of an optimal screw type, size, and insertion trajectory depending on vertebra dimension and shape. Methods: Different screw diameters and lengths, threads and insertion trajectories were computationally tested using a design of experiment (DOE) approach. A detailed FEM of an L3 vertebra was created including elastoplastic bone properties and contact interactions with the screws. Loads and boundary conditions were applied to the screws to simulate axial pullout tests. Force-displacement responses and internal stresses were analyzed to determine the specific effects of each parameter. Results: The DOE analysis revealed significant effects (P<0.01) for all tested principal parameters along with the interactions between diameter and trajectory. Screw diameter had the greatest impact on anchorage performance. The best insertion trajectory to resist pullout involved placing the screw threads closer to the pedicle walls using the straight-forward insertion technique, which showed the importance of the cortical layer grip. The simulated cylindrical single-lead thread screws presented better biomechanical anchorage than the conical dual-lead thread screws in axial loading conditions. Conclusions: The model made it possible to quantitatively measure the effects of both screw design characteristics and surgical choices, enabling to recommend strategies to improve single pedicle screw performance under axial loading.
- [Show abstract] [Hide abstract] ABSTRACT: The spleen is a frequently injured abdominal organ in road accidents, with an injury frequency close to 30 %. The splenic avulsion exhibit a significant ratio of morbidity. It is clinically described as the complete failure of the pancreatico-splenic ligament (PSL) which is composed of splenic vessels and connective tissues. What are the biomechanical mechanisms involved with spleen avulsion? Is it possible to quantify tolerance levels of PSL structure? The current work combines both experimental and finite element (FE) investigations to determine the splenic avulsion process. Tensile tests on 13 PSL samples were performed up to failure. The experimental results provide reference data for model validation and showed a failure process starting at a peak force of 70 ± 34 N combined with a peak strain of 105 ± 26 %. In an attempt to identify possible vessel ruptures within the PSL, a FE model of the PSL was developed including both vessels and connective tissues. The vessel wall behaviour up to failure was reproduced using an Ogden law and calibrated by inverse analysis according to literature data. The connective tissues function was modelled by a cohesion-loss interface. Once model correlation to experimental results was achieved, numerical simulation revealed that haemorrhage could occur even before the maximum peak is reached. Indeed, the first vessel ruptures were recorded at a strain of 92 % at the upper lobe vein.
- [Show abstract] [Hide abstract] ABSTRACT: Road accidents can lead to abdominal injuries ranging from severe to lethal, that include hemorrhage of organs and their attachment system. A good understanding and prediction of abdominal injuries therefore requires investigation of the mechanical properties of the attachment systems of abdominal organs. In particular, the gastrocolic ligament (GCL) is one major link between the stomach and the transverse colon. This study aims to investigate the mechanical properties of the GCL under very low and high strain rate uniaxial tensile tests until failure. Thirty-five GCL samples were dissected from 7 embalmed cadavers and tested at a rate of 1 mm/s and 1 m/s. Incidence of freezing was also evaluated. The mechanical response of GCL samples showed an approximately bilinear curve. Within the first linear region (less than 5% of ligament strain), the apparent elastic modulus was estimated at 247±144 kPa, while in the second region, it was estimated at 690±282 kPa. The average failure stress (σfail) and failure strain (εfail) were 131.6±50 kPa and 29%±8%, respectively. High strain rate loading also showed high sensitivity to strain rate. The estimated GCL mechanical properties in this study can be implemented in finite element models of the abdomen to further investigate the mechanical contribution of the organ attachment system under traumatic loading conditions.
- [Show abstract] [Hide abstract] ABSTRACT: Study Design. Measurements of cervical and thoraco-lumbar human spinal cord (SC) geometry based on in vivo MRI and investigation of morphological "invariants".Objective. The current work aims at providing morphological features of the complete in vivo human normal spinal cord and at investigating possible "invariant" parameters that may serve as normative data for individualized study of SC injuries.Summary of Background Data. Few in vivo MR-based studies have described human SC morphology at the cervical level, and similar description of the entire SC only relies on post-mortem studies, which may be prone to atrophy biases. Moreover, large inter-individual variations currently limit the use of morphological metrics as reference for clinical applications or as modeling inputs.Methods. Absolute metrics of SC (transverse and antero-posterior diameters, anterior and posterior horns width, cross-sectional SC area and white matter percentage) were measured using semi-automatic segmentation of high resolution in vivo T2*-weighted transverse images acquired at 3T, at each SC level, on healthy young (N = 15) and older (N = 8) volunteers. Robustness of measurements, effects of subject, age, or sex, as well as comparison to previously published post mortem data were investigated using statistical analyses (Separate analysis of variance, Tukey-HSD, Bland-Altman). Normalized-to-C3 parameters were evaluated as invariants using a leave-one-out analysis. Spinal canal parameters were measured and occupation ratio (OR) border values were determined.Results. Metrics of SC morphology showed large intra- and inter-individual variations, up to 30% and 13% respectively in average. Sex had no influence except on posterior horns width (p<0.01). Age-related differences were observed for anteroposterior diameter and white matter percentage (p<0.05) and all postmortem metrics were significantly lower than in vivo values (p<0.001). In vivo normalized SC area and diameters appeared to be invariants (R>0.74, RMSE<10%) Finally, minimal and maximal OR were 0.2 and 0.6, respectively.Conclusion. This study presented morphological characteristics of the complete in vivo human spinal cord. Significant differences linked to age and postmortem state have been identified. Morphological "invariants" that could be used to accurately calculate the normally expected morphology, were also identified. These observations should benefit to biomechanical and spinal cord pathology studies.
- [Show abstract] [Hide abstract] ABSTRACT: Thoracolumbar spine fracture classifications are mainly based on a post-traumatic observation of fracture patterns, which is not sufficient to provide a full understanding of spinal fracture mechanisms. This study aimed to biomechanically analyze known fracture patterns and to study how they relate to fracture mechanisms. The instigation of each fracture type was computationally simulated to assess the fracture process. A refined finite element model of three vertebrae and intervertebral connective tissues was subjected to 51 different dynamic loading conditions divided into four categories: compression, shear, distraction and torsion. Fracture initiation and propagation were analyzed, and time and energy at fracture initiation were computed. To each fracture pattern described in the clinical literature were associated one or several of the simulated fracture patterns and corresponding loading conditions. When compared to each other, torsion resulted in low-energy fractures, compression and shear resulted in medium energy fractures, and distraction resulted in high-energy fractures. Increased velocity resulted in higher-energy fracture for similar loadings. The use of a finite element model provided quantitative characterization of fracture patterns occurrence complementary to clinical and experimental studies, allowing to fully understand spinal fracture biomechanics.
- [Show abstract] [Hide abstract] ABSTRACT: To date, developing geometrically personalized and detailed solid finite element models of the spine remains a challenge, notably due to multiple articulations and complex geometries. To answer this problem, a methodology based on a free form deformation technique (kriging) was developed to deform a detailed reference finite element mesh of the spine (including discs and ligaments) to the patient-specific geometry of 10 and 82-year old asymptomatic spines. Different kriging configurations were tested: with or without smoothing, and control points on or surrounding the entire mesh. Based on the results, it is recommended to use surrounding control points and smoothing. The mean node to surface distance between the deformed and target geometries was 0.3 mm ± 1.1. Most elements met the mesh quality criteria (95%) after deformation, without interference at the articular facets. The methods novelty lies in the deformation of the entire spine at once, as opposed to deforming each vertebra separately, with surrounding control points and smoothing. This enables the transformation of reference vertebrae and soft tissues to obtain complete and personalized FEMs of the spine with minimal post-processing to optimize the mesh.
Dataset: Supplementary material 2
Dataset: Supplementary material 1