finite element method

Keratoconus (KC) is a non-inflammatory and degenerative disease of the cornea. It is manifested by the formation of cone-shaped regions accompanying severe eyesight issues. Implantation of intrastromal corneal ring segments (ICRS)‎ is a popular treatment to improve visual acuity. Controversies exist over restoring functionality of different ICRSs. In this study, numerical models were used to quantify the mechanical and optical effects of different ICRSs on a reference cornea with central cone. Finite element (FE) simulations were used to simulate the implantation of two classes of ICRS sets common in clinical settings: a) single segment arcs of 360º (1 × 360), 350º (1 × 350), 320º (1 × 320), and b) symmetric double-segment arcs of 160º each (2 × 160), 150º each (2 × 150), 120º each (2 × 120), and 90º each (2 × 90). Results showed that implantation of symmetric double-segment arcs caused the symmetric displacement and stress distribution contours on both anterior and posterior corneal surfaces. This study shows the potential impact of a detailed mechanical analysis of ICRS placement and represents a first step toward the development of an evidence-based nomogram for the different implantation techniques and the optimization of the surgical intervention based on patient-specific modeling.

Microneedles are a type of micron-sized needle that is the third most widely used delivery system after oral and injectable drug delivery, used in a variety of fields including drug release and rejuvenation. Optimizing the geometry of microneedles to reduce pain and inflammation has been important in recent years. Due to the high cost of microneedle fabrication, numerical simulation of microneedle penetration into the skin can be useful to evaluate the microneedle strength as well as its effect on the skin during penetration. In this study, first a new simulation method in Abaqus software with explicit method using cohesive elements to investigate the penetration of microneedles in the skin of the human forearm was presented. The skin was considered as Ogden and bilayer hyperelastic models. The microneedle was considered as a rigid body and a constant velocity of 0.6 mm/s was applied to it .The microneedle with bulk with bio-inspired titles was examined and its important parameters such as height, sharpness and bulk angle were evaluated. Finally, some proposed models of microneedles with longitudinal grooves are presented to increase the concentration of stress on the skin and prevent friction. A comparison of the designed microneedle with the barbless microneedle shows that the barbed microneedle concentrates more than twice as much stress on the skin, but reduces the penetration force by as much as 15%, making it easier to penetrate the skin. The results show that the reduction longitudinal grooves increase the tension created in the skin by about 10%, but have little effect on the penetrating force on the skin.

Regarding the application of testing and analysis of bone fractures in both medical and engineering fields, finding proper specimens for measuring fracture properties is important. In this study, the experimental and numerical fracture analyses of bovine cortical bone were performed for 4 anatomical regions using arc-shaped specimens. The tensile fracture tests for arc-shaped specimens were performed at ambient temperature. In practice, the stress intensity factor was calculated using standard analytical formula for arc-shaped specimens and also the related finite element (FE) models. In order to validate the FE models, the stress and strain analyses results were compared with the results obtained from digital image correlation (DIC) method. The very good agreement between these results was indicative of the accuracy of FE analyses. There were also good correlations between the initiation and propagation of crack from both experimental and FE results and the measured fracture toughness values were in good agreement with those reported in the literature. The results of this study showed that the analytical stress intensity expressions can give accurate results for the arc-shaped specimens excised from posterior and anterior regions. However, for the medial and lateral regions only the FE models can provide the required accuracy.

Intervertebral disc (IVD) provides flexibility and shock absorption for the spine in the load transmission procedure. Disc degeneration may occur as a result of aging and inappropriate types of loading. Assessing biomechanical parameters of intact IVD in comparison to the degenerated disc with different grades of degeneration can facilitate the detection procedure and planning for suitable therapeutic treatment. In the present study, a real three-dimensional model of cercival IVD (C_5-C_6) with adjacent vertebrae is constructed by using computed tomography (CT-scan) images. In order to accurately define mechanical properties, the disc and the vertebrae are modelled as poroviscoelastic and poroelastic materials, respectively. A porous medium approach is adopted to consider the considerable water content of both media alongside the solid matrix. For the solid phase of the IVD, the related viscoelastic parameters are extracted from an experimental test on a sheep lumbar intervertebral disc and stress vs. time data are fitted to the generalized Maxwell model with two Maxwell arms. By employing the finite element method, time-dependent response of the intact IVD and three different levels of the degenerated IVD (mild, moderate, and severe) are studied in a relaxation test. Results indicate that during relaxation procedure, intradiscal fluid velocity decreases as a result of disc degeneration. This may oppositely affect the flexibility of IVD in the load bearing. It is also observed that stress relaxation of the severe degenerated IVD almost increases up to 16% relative to the intact IVD. Assessing the amount of disc bulging under load application shows enhancement for the degenerated disc compared to the intact disc.

With the increase in lifespan there are many concerns related to ability of the hard tissues such as teeth to meet the physical demands over an extended period of function. The dentin has a special microstructural feature that governs its mechanical behavior, e.g., fracture mechanics: cylindrical tubules that are called dentin tubules. These tubules are gradually occluded in the elderly. The present study is aimed to investigate the effects of microstructure and its aging-related changes of the considered fiber-reinforced composite dentin on the fracture behavior and crack propagation trajectory, utilizing linear elastic fracture mechanics and finite element method. Obtained results indicate that the crack propagation path depends on geometrical microstructure of the dentin as well as respective mechanical properties and arrangement of dentin tubules. Also our results delineate that occlusion of dentinal tubule due to the aging plays a significant role at crack propagation trajectory and behaves as a barrier to crack growth.

Syrinx growth in Syringomyelia desease causes progressive neurological disorders. Thus, the examination of effective factors in syrinx development is so important for controlling this desease. One of clinical assumptions related to the reason of syrinx development, considers the propagation of pressure wave shock in subarachnoid-space fluid as the main reason for fluid motion in syrinx and syrinx development and increasing damage to spinal cord. Modeling and analysis have been performed to test the theory in this research using finite element method. So a 3d model was created including syrinx, spinal cord, cerebrospinal-fluid in subarachnoid-space, dura mater and stenosis. Pressure puls stimulation was applied to the superior surface of the subarachnoid-space fluid model simulating arterial puls of skull. Cerebrospinal-fluid has been assumed as a Newtonian fluid with laminar flow. The solid phase has been considered to be linear elastic. The fluid-solid interface was analized using ADINA software and fluid flow characteristics were extracted including velocity and pressure field as well as tissue stresses. Results show that pressure wave propagation in subarachnoid-space fluid causes the induction of motion in syrinx fluid, and stress concentration is created in spinal tissue due to the fluid cessation in syrinx and increasing local pressure, however these stress values are lower than spinal tissue strength and pressure wave propagation in this situation cannot be the main reason of syrinx development.

Understanding the mechanism of artificial disc degeneration using animal models is useful to study the regenerative techniques in hope of finding potential therapeutic strategies. For any type of potential therapeutic techniques, first we need to have the degenerated model. Disc degeneration can be mimicked in animal studies using needle puncture. However, the detailed mechanical response of the artificial degenerated disc using needle puncture under physiological diurnal activities has not been analyzed well. Hence, reverse finite element analyses combined with in-vitro experiments were used in this study to find the mechanical properties of intact (N=8) and injured discs using needle puncture (N=8). Afterward, specimen-specific FE models for 16 discs were simulated during physiological diurnal activity. The results showed that the variation of axial displacement, intradiscal pressure, and total fluid exchange in intact discs were significantly higher than the injured ones after 24h. But the maximum axial stress within disc was significantly higher in injured group. The achieved results are correlated with previous human cadaver data for natural disc degeneration. Therefore, it is concluded that the G-16 needle puncture injury is a simple and cost-effective methodology which can be used to mimic the degeneration mechanism in animal models.

Investigating the mechanical stimuli on stem cells under in vitro and in vivo conditions is a very important topic to achieve an ability to control the cellular responses like growth, proliferation and differentiation. Many investigations carried out about biomechanical factors involved in this phenomenon and nowadays, it is proved that some factors like as cell morphology, subcellular elements configuration, scaffold architecture, substrate stiffness and mechanical stimulation via substrate displacement or fluid flow, have got an important effect on cellular responses. In this study, we have tried to evaluate the responses of a stem cell to the stiffness and thickness of the substrate by the means of finite element method. For this purpose, we have used collagen-based scaffolds as the artificial ECM and a cell culture in a bioreactor with fluid flow was simulated.by use of fluid-structure interaction method and solving the equations in two-way coupling scheme, the results show that the increase in thickness and stiffness of the substrate will result in 15 percent change in cell-substrate stresses, respectively. Also, it was seen that the change of substrate stiffness only in the range of 0.1-100 KPa could affect the cell response to an external stimulation.These results, along with other similar investigations, could be used as an instructor by the researchers to optimize the stem cell’s microenvironment in vitro, and finally get the most out of their stem cell related Investigations.

Body cells, including mesenchymal stem cells are subject to a lot of mechanical forces. The type and magnitude of these forces are different in different physiological and pathological conditions. They cause a wide variety of cell responses and are able to change metabolisms and functions of the cell. Analysis of stem cell response to mechanical stimulation is very important in recognizing healthy and diseased condition of tissues and cells. Differentiation potential of mesenchymal stem cells to specialized cells makes them important cell sources in tissue engineering. In this study, atomic force microscopy and finite element method and used mechanical effects on a stem cellaresimulated which includes cell behavior due to strain andstress distributions in internal components of the cell. In this study, the ADINA software used to simulate mechanical behavior of the cell components (cell membrane, cytoplasm and nucleus) under a compressiveload. Results indicate mechanical response of stem cells in the body through which they can differentiate into bone cells and cartilage under compressive loads in the physiological range. This study has some considerable innovations as compared with the similar studies in the literature which is because of the kind of cells has been used (adipose-derived stem cells) as well as and also using precise material models for cell components based on the data extracted from laboratory tests for mechanical properties of the cell. Furthermore, this study can be considered as an important initial step for future studies on different patho-cells and analyzing their responses to mechanical loading using a similar method of this study to find new diagnostic methods. Also, it can be used to deepen pathological studies of the cells and the tissues.

Computationally fast biomechanical models are required to present the actual behavior of soft tissue in real-time simulation. These models are applied in medical diagnosis, surgical planning and training. One of the challenges in the surgical simulation is soft tissue cutting that requires topology changes and elements remeshing in real-time.This paper present a new algorithm for soft tissue cutting using its geometric analysis. This method creates a less number of degrees of freedom and shows a stable simulation that leads in less tissue damage as compared to other methods. According to the simulation results, the proposed algorithm has a relatively high speed. In addition, a mapping method has been proposed that relates physical and visual model and consequently shows a more realistic surgery. In order to achieve a physics based, accurate and reliable force model, Finite Element Method is used. Finally, the proposed algorithm is simulated for three-dimensional soft tissue tumor and evaluated using the SOFA-Framework.

Orthodontic specialists interest in study of tooth movement mechanic, such as the relationship between applied force and the rate of tooth movement in orthodontic treatment. It is because of the complexity and variety of factors that can affect orthodontic treatment. The friction force at the contact surfaces with an undetermined magnitude, makes the orthodontic treatment unpredictable. In this study, friction coefficient and forces were investigated in new designed bracket that had beveled edge which has been modeled based on standard bracket. Torque, tip and angulations angles of the brackets slot are designed. Arch wires were modeled by two rectangular and circular cross-sections and the effect of geometry on the stress distribution and the friction force was investigated using Finite Element Method (FEM). The results have showed that the stress concentration generated in the bracket which has been the most curvature, decreased compared to the standard bracket at the contact wire and bracket braces. In addition, results have showed that friction in the beveled edge bracket was significantly decline compared to the standard bracket and also are less than the type with minor curvature. Results of investigation of friction between the two types of round and square wire, have revealed that the round wire has lower friction and confirmed previous studies. Finally, due to the reduced friction in the brackets which have been the most curvature, this type of design is appropriate to decrease friction force.

Orthopedic screws are widely used devices for fixation of bone fractures. Progressive loosening of bone fixation screws, induced by stress shielding and subsequent adaptive bone remodeling, results in bone loss around the screw. A set of two-dimensional finite element models including cortical and cancellous bone with a functionally graded Ti-Hap screw was developed. A dimensionless set of stress-transfer parameters (STP) and strain energy density-transfer parameter (SEDTP) were developed to quantify the screw–bone load sharing. Lower STP and SEDTP values indicate weak stress and strain energy density transfer to bone which is a sign of stress shielding. The results indicated that STP and SEDTP values for FGM screw are higher than those of a fully metal screw. Moreover, reducing elastic modulus of metal fraction and increasing the volume fraction of ceramic decrease the stress shielding. For a partially graded screw (with both homogenous and FGM parts), the longer FGM part is, the greater are STP and SEDTP values. Furthermore, the results showed that decreasing compositional distribution exponent which shows composition change of FGM content from metal fraction toward ceramic fraction, increases the parameters. Results from this study are in admissible agreement with available clinical and experimental study.

Arterial embolism is one of the major killers of the people who have heart diseases. In cerebral arteries, the danger of embolism is that the ruptured particles are carried into the brain, provoking neurological symptoms or a stroke. In this research, for the first time, we have presented a numerical model to study the complete blockage of the human common carotid artery resulted from the physical motion of a blood clot bulk with spherical geometry in it. In the numerical model, a transient flow was assumed in an axisymmetric finite length tube. The incompressible Navier-Stokes equations were used as the governing equations for the fluid and a linear elastic model was utilized for the blood clot bulk. In order to model the contact conditions between the blood clot and arterial wall, an axisymmetric rigid contact model was used. The arbitrary Lagrangian-Eulerian formulation (ALE) was applied to analyze the solid large displacements inside fluid flow. The results indicated that during contact between stenosis and the clot, separation and reattachment regions were occurred on the stenosis extensively which are susceptible to thrombosis onset and growth. By abruption of the clot from the arterial wall during its passage through the stenosis, an extensive recirculation zone occurred downstream of the stenosis and beneath the moving clot bulk. Analysis of the clot motion and deformation have showed that when the clot passed the stenosis completely, the areas near the clot peak had a large tendency to expand which indicated the propensity of these areas to disperse.

Arterial stenosis and the consequent cardiovascular diseases such as atherosclerosis remain the major cause of mortality in the world. In this study, blood flow was analyzed in a three-dimensional model of stenosed carotid artery with asymmetric stenosis utilizing fluid-structure interaction method. The modeling was performed by ANSYS finite element software. To overcome the software inconsistency in FSI mode, a new code was designed in ANSYS multi-physics environment for coupling of solid and fluid domains via incremental boundary iteration method. The results indicated a considerable variation of local blood pressure, velocity and shear stress in stenosed artery, high pressure drop along stenosis, compressive stress and larger flow separation zone in the post-stenotic region as the result of increased eccentricity of stenosis. The results might be applied in evaluation of plaque severity, progression of disease, plaque growth and vulnerable regions of plaque to fracture.

The methods of focusing ultrasonic waves in order to apply hyperthermia cancer therapy have studied and a transducer capable of focusing waves on cancerous tissues with the aid of its piezoelectricelements has introduced. The amount of absorbed energy was computed by solving numerically the acoustic pressure equation using Rayleigh-Summerfield Integral, with the intention to determine the optimum spatial array of piezoelectric elements for energy concentration. In order to control the treatment procedure, the numerical solution of Bio-heat Transfer Equation (BHTE), along with the finite-element simulation of thermal energy distribution in a cervix cancerous tissue is considered.

Several linear and nonlinear finite element models of intact and fixed lumbar spine were analyzed. The intact model was developed based on CT images, and following verification, was employed to simulate the spinal fixation procedure using two different commercial pedicle screw systems. The results including the force-deformation behavior and the stress distribution within the structures were studied in detail. The effects of pedicle morphology, insertion errors and material properties of bone graft on the stress distribution pattern within the vertebrae and implant components were also studied. The results suggest superiority of titanium implants over steel implants, necessity of bone graft insertion, and a higher failure risk for screws due to osteoporosis. It has been recommended that surgeons use thicker screws when dealing with pedicels with larger anterior posterior length and avoid insertion errors to minimize the risk of screw fracture.