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PubMed2002

Factors influencing stresses in the lumbar spine after the insertion of intervertebral cages: finite element analysis.

Intervertebral cages in the lumbar spine have been an advancement in spinal fusion to relieve low back pain. Even though initial stability is accepted as a requirement for fusion, there are other factors. The load transfer and its effect on the tissues adjacent to the cage may also play an essential role, which is not easily detectable with experimental tests. In this study the effects of an intervertebral cage insertion on a lumbar functional spinal unit were investigated using finite element analyses. The influences of cage material, cancellous bone density and spinal loading for the stresses in a functional spinal unit were evaluated. Three-dimensional (3D) finite element models of L2-L3 were developed for this purpose. An anterior approach for a monobloc, box-shaped cage was modelled. Models with cage were compared to the corresponding intact ones. The results showed that inserting a cage increased the maximum von Mises stress and changed the load transfer in the adjacent structures. Varying the cage material or the loading conditions had a much smaller influence than varying the cancellous bone density. The denser the cancellous bone, the more the stress was concentrated underneath the cage, while the remaining regions were unloaded. This study showed that the density of the underlying cancellous bone is a more important factor for the biomechanical behaviour of a motion segment stabilized with a cage, and its eventual clinical success, than the cage material or the applied load. Inserting an intervertebral cage markedly changed the load transfer. The altered stress distribution may trigger bone remodelling and explain damage of the underlying vertebrae.

Adult↗

PubMed2005

Finite element analysis of the effect of meniscal tears and meniscectomies on human knee biomechanics.

BACKGROUND: Many authors have suggested that the high levels of shear and tensile stresses that appear in the articular cartilage after meniscectomy are partly responsible for cartilage pathologies, such as osteoarthrosis. METHODS: In this paper, we investigate the effect of meniscal tears and meniscectomies on the human knee joint. Solid models of the tibia, femur, menisci and cartilage were generated from MRI images. A three-dimensional finite element model was developed that included the femur, tibia, cartilage layers, menisci and ligaments. The femur and tibia were considered to be rigid, the articular cartilage and menisci to be linearly elastic, isotropic and homogeneous and the ligaments were modelled as hyperelastic. Three different situations were compared: a healthy tibio-femoral joint, a tibio-femoral joint with tears in one meniscus and a tibio-femoral joint after meniscectomy. FINDINGS: The minimal principal stresses corresponding to a compressive load at 0 degrees flexion were obtained for the posterior zone of the medial meniscus and the corresponding region of the articular cartilage. Under an axial femoral compressive load, the maximal contact stress in the articular cartilage after meniscectomy was about twice that of a healthy joint. INTERPRETATION: This fact could partially explain the cartilage damage and degeneration that have been observed after meniscectomy.

Adult↗

PubMed2001

Parametric finite element analysis of vertebral bodies affected by tumors.

The vertebral column is the most frequent site of metastatic involvement of the skeleton. Due to the proximity to the spinal cord, from 5% to 10% of all cancer patients develop neurologic manifestations. As a consequence, fracture risk prediction has significant clinical importance. In this study, we model the metastatically involved vertebra so as to parametrically investigate the effects of tumor size, material properties and compressive loading rate on vertebral strength. A two-dimensional axisymmetric finite element model of a spinal motion segment consisting of the first lumbar vertebral body (no posterior elements) and adjacent intervertebral disc was developed to allow the inclusion of a centrally located tumor in the vertebral body. After evaluating elastic, mixed, and poroelastic formulations, we concluded that the poroelastic representation was most suitable for modeling the metastatically involved vertebra's response to compressive load. Maximum principal strains were used to localize regions of potential vertebral trabecular bone failure. Radial and axial vertebral body displacements were used as relative indicators of spinal canal encroachment and endplate failure. Increased tumor size and loading rate, and reduced trabecular bone density all elevated axial and radial displacements and maximum tensile strains. The results of this parametric study suggest that vertebral tumor size and bone density contribute significantly to a patients risk for vertebral fracture and should be incorporated in clinical assessment paradigms.

Biomechanical Phenomena↗

PubMed2005

What affects esophageal injury during radiofrequency ablation of the left atrium? An engineering study based on finite-element analysis.

Recent studies on intraoperative radiofrequency ablation of atrial fibrillation have reported some cases of injury to the esophagus. The aim of this study was to perform computer simulations using a theoretical model in order to investigate the effect of different factors on the temperature distributions in the esophagus during ablation. A three-dimensional model was built to include an active electrode, atrial tissue, epicardial fat layer and a fragment of esophagus, aorta and lung, all linked by connective tissue. The finite-element method was used to calculate the temperature distribution during a procedure of constant-temperature ablation. The lesion geometry was assessed using a 50 degrees C isotherm. Our results show that the electrical power directly applied to the esophagus is insignificant and hence the esophageal injury is exclusively due to thermal conduction from the atrium. The esophageal lesion is mainly influenced by the thickness of connective tissue. Both the programmed target temperature of the electrode and the duration of the ablation also have a significant effect on the lesion in the esophagus. In contrast, the epicardial fat layer (0.9 mm thickness) did not show a significant influence. In conclusion, this theoretical model allows us to study the effect of different factors on the thermal injury in the esophagus during intraoperative radiofrequency ablation of atrial tissue.

Catheter Ablation↗

PubMed2004

Finite element analysis of a femoral retrograde intramedullary nail subject to gait loading.

Intramedullary nails are routinely used in the treatment of fractures of the femur. While their effectiveness has been demonstrated clinically, a number of complications, including bone refracture and implant failure, persist. This paper presents novel three-dimensional finite element (FE) models, at four stages of gait, of: (i) a realistic femur analogue known as third generation composite bone, and (ii) a system consisting of an intramedullary nail implanted in the femur of (i). A comparison of experimentally measured strains on the surface of the femur with those predicted by the FE model revealed good agreement. The models were then used to identify implant/bone load sharing patterns, and areas of stress concentration in both the intramedullary nail and the bone, when statically locked by one or two screws at either end. The results of this study can be used to guide future implant design and surgical procedure.

Bone Nails↗

PubMed2005

Finite-element analysis of the adhesion-cytoskeleton-nucleus mechanotransduction pathway during endothelial cell rounding: axisymmetric model.

Endothelial cells possess a mechanical network connecting adhesions on the basal surface, the cytoskeleton, and the nucleus. Transmission of force at adhesions via this pathway can deform the nucleus, ultimately resulting in an alteration of gene expression and other cellular changes (mechanotransduction). Previously, we measured cell adhesion area and apparent nuclear stretch during endothelial cell rounding. Here, we reconstruct the stress map of the nucleus from the observed strains using finite-element modeling. To simulate the disruption of adhesions, we prescribe displacement boundary conditions at the basal surface of the axisymmetric model cell. We consider different scenarios of the cytoskeletal arrangement, and represent the cytoskeleton as either discrete fibers or as an effective homogeneous layer When the nucleus is in the initial (spread) state, cytoskeletal tension holds the nucleus in an elongated, ellipsoidal configuration. Loss of cytoskeletal tension during cell rounding is represented by reactive forces acting on the nucleus in the model. In our simulations of cell rounding, we found that, for both representations of the cytoskeleton, the loss of cytoskeletal tension contributed more to the observed nuclear deformation than passive properties. Since the simulations make no assumption about the heterogeneity of the nucleus, the stress components both within and on the surface of the nucleus were calculated. The nuclear stress map showed that the nucleus experiences stress on the order of magnitude that can be significant for the function of DNA molecules and chromatin fibers. This study of endothelial cell mechanobiology suggests the possibility that mechanotransduction could result, in part, from nuclear deformation, and may be relevant to angiogenesis, wound healing, and endothelial barrier dysfunction.

Animals↗

PubMed2005

Finite element analysis of undermining of pressure ulcer with a simple cylinder model.

With pressure sores, surface damage indicates that subcutaneous fat tissue necrosis has occurred. We hypothesized that formation of necrosis under a pressure sore changes the stress distribution, which in turn affects further extension of the necrosis. In the present study, two-dimensional finite element models were used to perform analysis under different undermining size conditions. Greater stress concentration was observed in the larger undermining model. This may be the reason that, in clinical situations, a large area of undermining necrosis is sometimes observed under the skin of sores with a small area of damage.

Biomechanical Phenomena↗

PubMedDate not supplied

Finite element analysis studies of a metal-ceramic crown on a first premolar tooth.

PURPOSE: This study examined the stresses developed during loading in a first premolar metal-ceramic crown made of different metal cores, and used them to anticipate the locations and form of the most likely failure modes. The maximum principal stresses in the porcelain are indicators of fracture, and the von Mises stresses in the metal core are indicators of the location of yielding. MATERIALS AND METHODS: Two-dimensional axisymmetric models with different core metals were analyzed using finite element analyses. An axial load of 600 N was applied vertically downward, over a circular area around the crown's fissure. RESULTS: The peak maximum principal tensile stress in the porcelain existed on the surface of the crown, partially outside the cusp, with the greatest peak in the gold-porcelain system (15.8 MPa). An inverse relationship between the peak maximum principal tensile stress of each system and the elastic modulus of each core material was found. According to evaluation of the critical flaw size for each system, even a crack completely through the thickness of the porcelain was not critical. The maximum von Mises stress existed in the metal coping, on the radial edge at the axial/occlusal line angle, with the highest maximum in the nickel-chromium system (143.9 MPa). There existed a proportional relationship between the maximum von Mises stress in each metal and their respective elastic moduli. All maximums were well below the yield strength of the metal alloys used. CONCLUSION: A greater understanding of the influence of an axial load on the resulting stresses has been achieved, showing that the phenomena of fracture and yielding are unlikely for the crown experiencing this axial load.

Bicuspid↗

PubMed2003

Finite element analysis of the influence of implant inclination, loading position, and load direction on stress distribution.

The selection of the appropriate alignment of an implant and the position of implantation are vital for its longterm success. Excessive load is generated around inclined implants, causing microcracks in the bone, which result in implant loosening and eventual failure. This study was designed to analyze the stress distribution caused by varying the degree of inclination of an implant body and varying the loading position and direction, using the finite-element method of stress analysis. Buccal and lingual two-dimensional simulation models of a cylinder implant, embedded in the first molar edentulous cross-section of the mandible, were prepared, and the stress distribution and maximum principal stresses were recorded. Regardless of the point and direction of loading, compressive stresses were relatively greater when the implant was inclined. This tendency became more pronounced when a 45 degrees loading direction and eccentric loading were tested. For the inclined model, with a 45 degrees loading direction, the compressive stress was observed on the cortical bone adjacent to the direction of inclination, while tensile stress was observed on the opposite side.

Computer Simulation↗

PubMed2005

Finite element analysis of the temporomandibular joint during lateral excursions of the mandible.

One of the most significant characteristics of the temporomandibular joint (TMJ) is that it is in fact composed of two joints. Several finite element simulations of the TMJ have been developed but none of them analysed the different responses of its two sides during nonsymmetrical movement. In this paper, a lateral excursion of the mandible was introduced and the biomechanical behaviour of both sides was studied. A three-dimensional finite element model of the joint comprising the bone components, both articular discs, and the temporomandibular ligaments was used. A fibre-reinforced porohyperelastic model was introduced to simulate the behaviour of the articular discs, taking into account the orientation of the fibres in each zone of these cartilage components. The mandible movement during its lateral excursion was introduced as the loading condition in the analysis. As a consequence of the movement asymmetry, the discs were subjected to different load distributions. It was observed that the maximal shear stresses were located in the lateral zone of both discs and that the lateral attachment of the ipsilateral condyle-disc complex suffered a large distortion, due to the compression of this disc against the inferior surface of the temporal bone. These results may be related with possible consequences of a common disorder called bruxism. Although it would be necessary to perform an exhaustive analysis of this disorder, including the contact forces between the teeth during grinding, it could be suggested that a continuous lateral movement of the jaw may lead to perforations of both discs in their lateral part and may damage the lateral attachments of the disc to the condyle.

Bruxism↗

PubMed2002

[Finite element analysis of DAI animal model caused by nonimpact with half bound head in cats].

This study sought to make a biomechanical analysis of the diffuse axonal injury(DAI) animal model caused by nonimpact with half bound head in cats. A three-dimensional finite element model of cat's head was established. The head of an anesthetized cat was scanned in 2 mm section. The nods and element meshes were signed out according to the geometry of every section. The geometric data were put into the computer and the element mesh body of cat's head was established in vizi CAD system. The maximum stress, minimum stress and von Mises stress were calculated by Super SAP (93ed) finite elemental software when the force was loaded on the right or left side of model in zero section. The analysis showed that the maximum stress appeared in the anterior and posterior loaded point and extended to cranial base in the cranial shell. There was high stress in the brain surface also. Because of cerebellar tentorium, cerebral falx, petrosal bone and sellar process, the stress did not decrease equivalently while approaching the deep brain, but it was distributed in cerebral-cerebellar peduncles, brain stem, corpus callosum and basal ganglia area at high values. The results suggest that the stress caused by rotational force is widespreadly and unequivalently distributed in brain tissue, which is mainly effected by the cerebellar tentorium, cerebral falx and the irregular geometric forms of cranial bone.

Animals↗

PubMed2003

Finite element analysis of the effect of the bucco-lingual position of artificial posterior teeth under occlusal force on the denture supporting bone of the edentulous patient.

To improve the quality of the complete denture prosthesis, the bucco-lingual position of the artificial posterior teeth must be determined with consideration of the shape of the maxillary and mandibular residual ridge and the relationship between them. The arrangement of posterior artificial teeth should be considered not only for the denture stability but also for the avoidance of high pressure on the supporting structures. A two-dimensional finite element method program to investigate the statics for the contour of the complete denture and the residual ridge was developed. With this program, the effect of the bucco-lingual position of the artificial posterior teeth under occlusal force on the denture supporting bone could be investigated.

Bite Force↗

PubMed1998

Altered collagen concentration in mitral valve leaflets: biochemical and finite element analysis.

BACKGROUND: Ischemic mitral regurgitation or ventricular wall motion abnormalities will alter the stress distribution in the mitral valve. We hypothesize that in response, the regional collagen concentration will be altered and will significantly impact the stress distribution in the mitral valve. METHODS: Two sheep served as normal (sham) controls. Two other sheep had coronary ligation resulting in abnormal ventricular wall motion. Four sheep underwent ligation to infarct the posteromedial papillary muscle, resulting in ischemic regurgitation. After 4 or 8 weeks, the mitral valves were excised, and the anterior leaflet sections were subjected to an assay for collagen concentration. Next, in a finite element model, to simulate changes in collagen concentration, the tissue stiffness was increased by 20%, and then decreased by 20%. In another model, the thickness of the tissue was increased by 20%, and then combined with decreased tissue stiffness. Physiologic loading pressures were applied, and leaflet stress, chordal stress, and coaptation results were analyzed. RESULTS: The average collagen concentration in the normal sheep leaflets was 59.2% (dry weight), 50.6% in the ischemic controls, and 45.8% in the papillary muscle infarct group. Collagen concentration was greatest at the midline and decreased toward the commissures. Increased tissue stiffness resulted in increased leaflet and chordal stresses, as well as reduced coaptation. Decreased stiffness resulted in the opposite. Increased tissue thickness reduced leaflet and chordal stresses, but also reduced coaptation. The combination of increased tissue thickness and decreased stiffness demonstrated the greatest reduction in leaflet and chordal stress, while maintaining normal leaflet coaptation. CONCLUSIONS: The observed changes may demonstrate an early effort to compensate for increased leaflet stress. Microstructural alterations may demonstrate an early effort to compensate for altered physiologic loading to reduce stress and maintain coaptation. It is crucial in repairing or partially replacing thickened tissue that normal geometry and physiology be restored.

Animals↗

PubMed1990

A two-dimensional, finite element analysis of vasogenic brain edema.

The authors constructed a two-dimensional model of vasogenic brain edema, using the finite element method (FEM). The model incorporates the following physical parameters: cerebrovascular hydraulic conductivity, metabolic water production, tissue hydraulic conductivity and compliance, tissue and plasma osmotic and hydrostatic pressures, cerebrospinal fluid hydrostatic pressure and absorption, intracerebral stress, and shift of brain tissue. These parameters, related by partial differential equations, were approximated numerically in time and space by the FEM. The results of computer simulation of brain edema by the FEM were compared with experimental data obtained in a feline cold injury brain edema model. Computer simulation predicted the changes observed in interstitial pressure, extent of edema, intracerebral stress distribution, and shift of brain tissue. The results indicate that the FEM, applied to a model of vasogenic brain edema, can be used to predict the time course and regional distribution of fluid accumulation and the accompanying regional stress and deformation of brain tissue.

Animals↗

PubMed2007

Comparison of the effects of bilateral posterior dynamic and rigid fixation devices on the loads in the lumbar spine: a finite element analysis.

A bilateral dynamic stabilization device is assumed to alter favorable the movement and load transmission of a spinal segment without the intention of fusion of that segment. Little is known about the effect of a posterior dynamic fixation device on the mechanical behavior of the lumbar spine. Muscle forces were disregarded in the few biomechanical studies published. The aim of this study was to determine how the spinal loads are affected by a bilateral posterior dynamic implant compared to a rigid fixator which does not claim to maintain mobility. A paired monosegmental posterior dynamic implant was inserted at level L3/L4 in a validated finite element model of the lumbar spine. Both a healthy and a slightly degenerated disc were assumed at implant level. Distraction of the bridged segment was also simulated. For comparison, a monosegmental rigid fixation device as well as the effect of implant stiffness on intersegmental rotation were studied. The model was loaded with the upper body weight and muscle forces to simulate the four loading cases standing, 30 degrees flexion, 20 degrees extension, and 10 degrees axial rotation. Intersegmental rotations, intradiscal pressure and facet joint forces were calculated at implant level and at the adjacent level above the implant. Implant forces were also determined. Compared to an intact spine, a dynamic implant reduces intersegmental rotation at implant level, decreases intradiscal pressure in a healthy disc for extension and standing, and decreases facet joint forces at implant level. With a rigid implant, these effects are more pronounced. With a slightly degenerated disc intersegmental rotation at implant level is mildly increased for extension and axial rotation and intradiscal pressure is strongly reduced for extension. After distraction, intradiscal pressure values are markedly reduced only for the rigid implant. At the adjacent level L2/L3, a posterior implant has only a minor effect on intradiscal pressure. However, it increases facet joint forces at this level for axial rotation and extension. Posterior implants are mostly loaded in compression. Forces in the implant are generally higher in a rigid fixator than in a dynamic implant. Distraction strongly increases both axial and shear forces in the implant. A stiffness of the implant greater than 1,000 N/mm has only a minor effect on intersegmental rotation. The mechanical effects of a dynamic implant are similar to those of a rigid fixation device, except after distraction, when intradiscal pressure is considerably lower for rigid than for dynamic implants. Thus, the results of this study demonstrate that a dynamic implant does not necessarily reduce axial spinal loads compared to an un-instrumented spine.

Biomechanical Phenomena↗

PubMed2006

Stresses and strains in the medial meniscus of an ACL deficient knee under anterior loading: a finite element analysis with image-based experimental validation.

The menisci are believed to play a stabilizing role in the ACL-deficient knee, and are known to be at risk for degradation in the chronically unstable knee. Much of our understanding of this behavior is based on ex vivo experiments or clinical studies in which we must infer the function of the menisci from external measures of knee motion. More recently, studies using magnetic resonance (MR) imaging have provided more clear visualization of the motion and deformation of the menisci within the tibio-femoral articulation. In this study, we used such images to generate a finite element model of the medial compartment of an ACL-deficient knee to reproduce the meniscal position under anterior loads of 45, 76, and 107 N. Comparisons of the model predictions to boundaries digitized from images acquired in the loaded states demonstrated general agreement, with errors localized to the anterior and posterior regions of the meniscus, areas in which large shear stresses were present. Our model results suggest that further attention is needed to characterize material properties of the peripheral and horn attachments. Although overall translation of the meniscus was predicted well, the changes in curvature and distortion of the meniscus in the posterior region were not captured by the model, suggesting the need for refinement of meniscal tissue properties.

Aged↗

PubMed2000

Finite Element Analysis of Interbody Cages in a Human Lumbar Spine.

An innovative surgical procedure is vertebral stabilization by interbody cages. It is currently being used to separate and stabilize vertebral bodies and to promote bony fusion of the vertebrae onto or through the cages. This surgery, at some spine levels, can be performed through a laparoscope as an outpatient procedure with low morbidity. Because the procedure is new, little structural information is available on the interbody cages. The objective of this study was to evaluate the human lumbar spine stabilized by interbody cages biomechanically. The finite element method was used to compare cage designs by considering stresses in the cage and in the bone as well as relative displacements between the cage and the adjacent bone at the interface. The biomechanical evaluation considered different bone densities and considered axial, torsional, and bending loads on the lumbar spine. Stress analysis predicts local regions of stress concentration that could be damaging to cancellous bone and will likely require a remodeling response for local damage. This study predicts relative micromotion that could cause the bone resorption and fibrous tissue formation on the contact surfaces of the cage. The geometric constraints caused by the use of two cages will reduce the relative motion and therefore be more likely to allow bone ingrowth at the posterocentral contact region. Finite element analysis suggests that cages are a promising method for separation and stabilization of the vertebral bodies.

Journal Article↗

PubMedDate not supplied

Finite-element analysis to determine effect of monolimb flexibility on structural strength and interaction between residual limb and prosthetic socket.

Monolimb refers to a kind of transtibial prostheses having the socket and shank molded into one piece of thermoplastic material. One of its characteristics is that the shank is made of a material that can deform during walking, which can simulate ankle joint motion to some extent. Changes in shank geometry can alter the stress distribution within the monolimb and at the residual limb-socket interface and, respectively, affect the deformability and structural integrity of the prosthesis and comfort perceived by amputees. This paper describes the development of a finite-element model for the study of the structural behavior of monolimbs with different shank designs and the interaction between the limb and socket during walking. The von Mises stress distributions in monolimbs with different shank designs at different walking phases are reported. With the use of distortion energy theory, possible failure was predicted. The effect of the stiffness of the monolimb shanks on the stress distribution at the limb-socket interface was studied. The results show a trend--the peak stress applied to the limb was lowered as the shank stiffness decreased. This information is useful for future monolimb optimization.

Amputation, Surgical↗