Finite element modelling and simulations in dentistry: a bibliography 1990-2003

The paper gives a bibliographical review of the finite element modelling and simulations in dentistry from the theoretical as well as practical points of view. The bibliography lists references to papers, conference proceedings and theses/dissertations that were published between 1990 and 2003. At the end of this paper, more than 700 references are given dealing with subjects such as: dental materials; oral and maxillofacial mechanics and surgery; orthodontics, tooth movement, orthodontic appliances; root canals, filling and therapy; dental restorations and other topics.     

Finite element modelling and simulations in cardiovascular mechanics and cardiology: a bibliography 1993-2004

The paper gives a bibliographical review of the finite element modelling and simulations in cardiovascular mechanics and cardiology from the theoretical as well as practical points of views. The bibliography lists references to papers, conference proceedings and theses/dissertations that were published between 1993 and 2004. At the end of this paper, more than 890 references are given dealing with subjects as: Cardiovascular soft tissue modelling; material properties; mechanisms of cardiovascular components; blood flow; artificial components; cardiac diseases examination; surgery; and other topics.     

Finite element simulations of a hip joint with femoroacetabular impingement

In this study, a three-dimensional finite element (FE) model based on the specific anatomy of a patient presenting a femoroacetabular impingement of the ‘cam’-type is developed. The FE meshes of the structures of interest are obtained from arthrographic magnetic resonance images. All soft tissues are considered linear elastic and isotropic, and the bones were assumed rigid. A compression of the femur on the acetabular cavity as well as flexural movements and internal rotations are applied. Stresses and contact pressures are evaluated in this patient-specific model in order to better interpret the mechanism of aggression of the femoral and acetabular cartilages. The corresponding results are presented and discussed. The values obtained for the contact pressures are similar to those reported by other models based on idealised geometries. An FE analysis of a non-cam hip is also performed for comparison with the pathological case.     

Finite element simulations of acetylcholine diffusion in neuromuscular junctions

A robust infrastructure for solving time-dependent diffusion using the finite element package FEtk has been developed to simulate synaptic transmission in a neuromuscular junction with realistic postsynaptic folds. Simplified rectilinear synapse models serve as benchmarks in initial numerical studies of how variations in geometry and kinetics relate to endplate currents associated with fast-twitch, slow-twitch, and dystrophic muscles. The flexibility and scalability of FEtk affords increasingly realistic and complex models that can be formed in concert with expanding experimental understanding from electron microscopy. Ultimately, such models may provide useful insight on the functional implications of controlled changes in processes, suggesting therapies for neuromuscular diseases.     

Finite element modeling of trabecular bone damage

This paper presents a finite element-based, computational model for analysis of structural damage to trabecular bone tissues. A modulus reduction method was formulated from elasto-plasticity theory, and was used to account for site-specific trabecular bone tissue damage. Trabecular bone tissue damage is illustrated using a large-scale, anatomically accurate, two-dimensional, microstructural finite element model of a human thoracic vertebral body. Four models with varying specifications for damage accumulation were subjected to compressive loading and unloading cycles. The numerical results and experimental validation demonstrated that the modulus reduction method reproduced the non-linear mechanical behaviour of vertebal trabecular bone. The iterative computational approach presented provides a methodology to study trabecular bone damage, and should provide researchers with a computational approach to study bone fracture and repair and to predict vertebral fragility.     

Paralytic poliomyelitis in Russian Federation in 1998-2005

From 1998 through 2005 3,294 cases of acute flaccid paralysis (AFP) including 93 cases with clinical picture of poliomyelitis were registered in Russian Federation. From the latter cases 91 were classified as vaccine-associated paralytic poliomyelitis (VAPP): 66 were VAPP cases in oral poliomyelitis vaccine (OPV) recipients and 25--VAPP cases in contacts. VAPP rate was 1 case per 1.6 million of distributed OPV doses, 1 case per 2.2 million doses for OPV recipients, and 1 case per 186,000 doses for recipients of 1st OPV dose in children aged < 1 year. Majority of VAPP cases in recipients occurred after 1st dose (89.4%) and in contacts--in non-vaccinated children (76%). Mean interval between OPV administration and onset of VAPP in recipients was 21 days. Children aged < 1 year were predominant among VAPP cases (92.4% among recipient VAPP cases, and 80% among contact VAPP cases). Majority of the patients had unfavorable health status including defects of immunity. Most of poliovirus strains isolated from VAPP cases belonged to type 3 (52.9%) whereas to type 2 and 1--29.8% and 17.4% of strains respectively. All VAPP cases were associated with vaccine-derived polioviruses. A highly diverged poliovirus type 1 (2.65% of nucleotide substitutions in VP1 region) was isolated from patient with contact VAPP. Formation of poliovirus-neutralizing serum antibodies in children with VAPP including persons with immunodeficiency reflects the ability of the organism to produce specific antiviral immune response.     

Finite element simulations of a focal knee resurfacing implant applied to localized cartilage defects in a sheep model

Articular resurfacing metal implants have recently been tested in animal models to treat full thickness localized articular cartilage defects, showing promising results. However, the mechanical behavior of cartilage surrounding the metal implant has not been studied yet as it is technically challenging to measure in vivo contact areas, pressures, stresses and deformations from the metal implant. Therefore, we implemented a detailed numerical finite element model by approximating one of the condyles of the sheep tibiofemoral joint and created a defect of specific size to accommodate the implant. Using this model, the mechanical behavior of the surrounding of metal implant was studied. The model showed that the metal implant plays a significant role in the force transmission. Two types of profiles were investigated for metal implant. An implant with a double-curved profile, i.e., a profile fully congruent with the articular surfaces in the knee, gives lower contact pressures and stresses at the rim of the defect than the implant with unicurved spherical profile. The implant should be placed at a certain distance into the cartilage to avoid damage to opposing biological surface. Too deep positions, however, lead to high shear stresses in the cartilage edges around the implant. Mechanical sealing was achieved with a wedge shape of the implant, also useful for biochemical sealing of cartilage edges at the defect.     

Finite element modeling of human brain response to football helmet impacts

The football helmet is used to help mitigate the occurrence of impact-related traumatic (TBI) and minor traumatic brain injuries (mTBI) in the game of American football. While the current helmet design methodology may be adequate for reducing linear acceleration of the head and minimizing TBI, it however has had less effect in minimizing mTBI. The objectives of this study are (a) to develop and validate a coupled finite element (FE) model of a football helmet and the human body, and (b) to assess responses of different regions of the brain to two different impact conditions – frontal oblique and crown impact conditions. The FE helmet model was validated using experimental results of drop tests. Subsequently, the integrated helmet–human body FE model was used to assess the responses of different regions of the brain to impact loads. Strain-rate, strain, and stress measures in the corpus callosum, midbrain, and brain stem were assessed. Results show that maximum strain-rates of 27 and 19 s^?1 are observed in the brain-stem and mid-brain, respectively. This could potentially lead to axonal injuries and neuronal cell death during crown impact conditions. The developed experimental-numerical framework can be used in the study of other helmet-related impact conditions.     

Finite element modeling for strain rate dependency of fracture resistance in compact bone

Crack growths in compact bones driven by various strain rate levels were studied using finite element modeling. The energy resistance curves in bovine femur cortical bones were characterized, whereas the orthotropic viscoelasticity in bone materials was accounted for to assess the effect of strain rate on the energy resistance curve. The models were also used to justify the anticipated plane strain response as a result of rather thick specimens used in experiments. Similarities were found between the experimental and model results when crack resistance ability exhibited in bones with slow loading rates, while unstable crack growth existed in bones with rapid loading rates. The critical energy release rates slightly decreased with the increase in strain rates. The hybrid experimental and computational method introduced in this study could be beneficial for application in fracture study in which standard experiments cannot be validly performed.     

Finite element modeling of the first ray of the foot: a tool for the design of interventions

Disorders of the first ray of the foot (defined as the hard and soft tissues of the first metatarsal, the sesamoids, and the phalanges of the great toe) are common, and therapeutic interventions to address these problems range from alterations in footwear to orthopedic surgery. Experimental verification of these procedures is often lacking, and thus, a computational modeling approach could provide a means to explore different interventional strategies. A three-dimensional finite element model of the first ray was developed for this purpose. A hexahedral mesh was constructed from magnetic resonance images of the right foot of a male subject. The soft tissue was assumed to be incompressible and hyperelastic, and the bones were modeled as rigid. Contact with friction between the foot and the floor or footwear was defined, and forces were applied to the base of the first metatarsal. Vertical force was extracted from experimental data, and a posterior force of 0.18 times the vertical force was assumed to represent loading at peak forefoot force in the late-stance phase of walking. The orientation of the model and joint configuration at that instant were obtained by minimizing the difference between model predicted and experimentally measured barefoot plantar pressures. The model were then oriented in a series of postures representative of push-off, and forces and joint moments were decreased to zero simultaneously. The pressure distribution underneath the first ray was obtained for each posture to illustrate changes under three case studies representing hallux limitus, surgical arthrodesis of the first ray, and a footwear intervention. Hallux limitus simulations showed that restriction of metatarsophalangeal joint dorsiflexion was directly related to increase and early occurrence of hallux pressures with severe immobility increasing the hallux pressures by as much as 223%. Modeling arthrodesis illustrated elevated hallux pressures when compared to barefoot and was dependent on fixation angles. One degree change in dorsiflexion and valgus fixation angles introduced approximate changes in peak hallux pressure by 95 and 22 kPa, respectively. Footwear simulations using flat insoles showed that using the given set of materials, reductions of at least 18% and 43% under metatarsal head and hallux, respectively, were possible.     

Finite element simulations of the active stress in the imaginal disc of the Drosophila Melanogaster

During the larval stages of development, the imaginal disc of Drosphila Melanogaster is composed by a monolayer of epithelial cells, which undergo a strain actively produced by the cells themselves. The well-organized collective contraction produces a stress field that seemingly has a double morphogenetic role: it orchestrates the cellular organization towards the macroscopic shape emergence while simultaneously providing a local information on the organ size. Here we perform numerical simulations of such a mechanical control on morphogenesis at a continuum level, using a three-dimensional finite model that accounts for the active cell contraction. The numerical model is able to reproduce the (few) known qualitative characteristics of the tensional patterns within the imaginal disc of the fruit fly. The computed stress components slightly deviate from planarity, thus confirming the previous theoretical assumptions of a nonlinear elastic analytical model, and enforcing the hypothesis that the spatial variation of the mechanical stress may act as a size regulating signal that locally scales with the global dimension of the domain.     

Finite element modeling for predicting the contact pressure between a foam mattress and the human body in a supine position

In this paper, we generated finite element (FE) models to predict the contact pressure between a foam mattress and the human body in a supine position. Twenty-year-old males were used for three-dimensional scanning to produce the FE human models, which was composed of skin and muscle tissue. A linear elastic isotropic material model was used for the skin, and the Mooney–Rivlin model was used for the muscle tissue because it can effectively represent the nonlinear behavior of muscle. The contact pressure between the human model and the mattress was predicted by numerical simulation. The human models were validated by comparing the body pressure distribution obtained from the same human subject when he was lying on two different mattress types. The experimental results showed that the slope of the lower part of the mattress caused a decrease in the contact pressure at the heels, and the effect of bone structure was most pronounced in the scapula. After inserting a simple structure to function as the scapula, the contact pressure predicted by the FE human models was consistent with the experimental body pressure distribution for all body parts. These results suggest that the models proposed in this paper will be useful to researchers and designers of products related to the prevention of pressure ulcers.     

Finite element modeling reveals complex strain mechanics in the aponeuroses of contracting skeletal muscle

A finite element model was used to investigate the counter-intuitive experimental observation that some regions of the aponeuroses of a loaded and contracting muscle may shorten rather than undergo an expected lengthening. The model confirms the experimental findings and suggests that pennation angle plays a significant role in determining whether regions of the aponeuroses stretch or shorten. A smaller pennation angles (25°) was accompanied by aponeurosis lengthening whereas a larger pennation angle (47°) was accompanied by mixed strain effects depending upon location along the length of the aponeurosis. This can be explained by the Poisson effect during muscle contraction and a Mohr’s circle analogy. Constant volume constraint requires that fiber cross sectional dimensions increase when a fiber shortens. The opposing influences of these two strains upon the aponeurosis combine in proportion to the pennation angle. Lower pennation angles emphasize the influence of fiber shortening upon the aponeurosis and thus favor aponeurosis compression, whereas higher pennation angles increase the influence of cross sectional changes and therefore favor aponeurosis stretch. The distance separating the aponeuroses was also found to depend upon pennation angle during simulated contractions. Smaller pennation angles favored increased aponeurosis separation larger pennation angles favored decreased separation. These findings caution that measures of the mechanical properties of aponeuroses in intact muscle may be affected by contributions from adjacent muscle fibers and that the influence of muscle fibers on aponeurosis strain will depend upon the fiber pennation angle.     

Seasonal variation of neonatal transient hyperthyrotropinemia in Tehran province, 1998-2005

Seasonal aggregation and the monthly rate of neonatal transient hyperthyrotropinemia (THT) were assessed. From November 1998 to April 2005, neonates of gestational age ≥37 wks, birth weight 2500-4000?g, birth length 45-55 cm, and 1st min Apgar score >3, who had thyrotropin (TSH) ≥20 mU/L in their cord dried-blood specimen, but without congenital hypothyroidism, were enrolled in the study. The recall rate equals the rate of THT occurrence in this study. Of 47,945 neonates, 555 had THT (recall rate: 1.2%). The aggregated seasonal recall rate (recall for further assessment to rule out congenital hypothyroidism) was significantly higher in winter (January, February, and March) than the other seasons (p < .0001). Winter had higher recall rate in each year as compared to other seasons, but the overall rate of recalls decreased in 2001 and 2002. Excluding the first 6 months (due to erratic variations), the remaining 72 months revealed a relatively sinusoidal pattern in monthly recall rates; indeed, there was an initial 11-month high recall rates (1.7%), followed by a 33-month decrease (0.7%), a 19-month increase (1.9%), and a final 9-month decrease (0.8%). The recall rate of each of these time intervals was significantly different from that of the next time interval (p < .0001). The monthly recall rates were best fitted to cubic curve estimation and then autoregressive integrated moving average (ARIMA) (0, 1, 1) models. THT occurs significantly more in winter than in other seasons, and this suggests a possible role for time-varying factor(s) contributing to its seasonal preponderance.     

Finite element modeling and parameter identification of the seated human body exposed to vertical vibration

Biomechanics and Modeling in Mechanobiology - Developing a biomechanical model which connected with the actual anatomy of the human body is helpful to understand the human response to vibration. A...     

Finite element modeling of embolic coil deployment: Multifactor characterization of treatment effects on cerebral aneurysm hemodynamics

Endovascular coiling is the most common treatment for cerebral aneurysms. During the treatment, a sequence of embolic coils with different stiffness, shapes, sizes, and lengths is deployed to fill the aneurysmal sac. Although coil packing density has been clinically correlated with treatment success, many studies have also reported success at low packing densities, as well as recurrence at high packing densities. Such reports indicate that other factors may influence treatment success. In this study, we used a novel finite element approach and computational fluid dynamics (CFD) to investigate the effects of packing density, coil shape, aneurysmal neck size, and parent vessel flow rate on aneurysmal hemodynamics. The study examines a testbed of 80 unique CFD simulations of post-treatment flows in idealized basilar tip aneurysm models. Simulated coil deployments were validated against in vitro and in vivo deployments. Among the investigated factors, packing density had the largest effect on intra-aneurysmal velocities. However, multifactor analysis of variance showed that coil shape can also have considerable effects, depending on packing density and neck size. Further, linear regression analysis showed an inverse relationship between mean void diameter in the aneurysm and mean intra-aneurysmal velocities, which underscores the importance of coil distribution and thus coil shape. Our study suggests that while packing density plays a key role in determining post-treatment hemodynamics, other factors such as coil shape, aneurysmal geometry, and parent vessel flow may also be very important.     

Finite element modeling of impulsive excitation and shear wave propagation in an incompressible,transversely isotropic medium

Elastic properties of materials can be measured by observing shear wave propagation following localized, impulsive excitations and relating the propagation velocity to a model of the material. However, characterization of anisotropic materials is difficult because of the number of elasticity constants in the material model and the complex dependence of propagation velocity relative to the excitation axis, material symmetries, and propagation directions. In this study, we develop a model of wave propagation following impulsive excitation in an incompressible, transversely isotropic (TI) material such as muscle. Wave motion is described in terms of three propagation modes identified by their polarization relative to the material symmetry axis and propagation direction. Phase velocities for these propagation modes are expressed in terms of five elasticity constants needed to describe a general TI material, and also in terms of three constants after the application of two constraints that hold in the limit of an incompressible material. Group propagation velocities are derived from the phase velocities to describe the propagation of wave packets away from the excitation region following localized excitation. The theoretical model is compared to the results of finite element (FE) simulations performed using a nearly incompressible material model with the five elasticity constants chosen to preserve the essential properties of the material in the incompressible limit. Propagation velocities calculated from the FE displacement data show complex structure that agrees quantitatively with the theoretical model and demonstrates the possibility of measuring all three elasticity constants needed to characterize an incompressible, TI material.     

Finite element modeling of human skin using an isotropic, nonlinear elastic constitutive model

The collagen network in skin is largely responsible for the nonlinear mechanical stress-strain response of skin. We hypothesize that the force-stretch response of collagen is governed by the entropics of long-chain molecules. We show that a constitutive model derived from the statistical mechanics of long-chain molecules, corresponding to the fibrous collagen network in skin, captures the mechanical response of skin. A connection between the physiologically meaningful parameters of network molecular chain density and free length of collagen fibers and the constitutively significant parameters of initial modulus and limiting stretch is thus established. The relevant constitutive law is shown to have predictive capabilities related to skin histology by replicating in vivo and in vitro experimental results. From finite element simulations, this modeling approach predicts that the collagen network in hypertrophic scars is more dense and the constituent collagen fibers have shorter free lengths than in healthy skin. Additionally, the model is shown to predict that as rat skin ages, collagen network density increases and fiber free length decreases. The importance of knowledge of the in situ stress state for analyzing skin response and validating constitutive laws is also demonstrated.