Effect of pre-stress on dynamic finite element analysis of the temporomandibular joint.

The pre-stress of the temporomandibular joint (TMJ) at the intercuspal position (ICP) was often neglected, which would cause errors in the finite element analysis. The purpose of this study was to investigate the effect of pre-stress on dynamic finite element analysis of the TMJs. One healthy female adult was recruited for medical imaging and motion data acquisition of the reference position (RP) to the ICP and the clicking teeth. The three-dimensional maxillofacial model including the maxilla, mandible, articular cartilages, discs, and discal attachments was reconstructed. Motion from the RP to the ICP was simulated to obtain pre-stress at the ICP. Two groups of the clicking teeth were simulated: (1) the group without pre-stress (GWoP); (2) the group with pre-stress (GwP). Significant differences were found between the two groups at the initial moment of movement, during the open-mouth phase, and during the collision phase between the upper and lower teeth. The maximum difference in the discal contact stress between both groups was even more than double. The relaxation of the TMJ at the beginning of the mouth opening was simulated in the GwP. In addition, an increase in the TMJ stress during teeth tapping was simulated in the GwP. These were not reflected in the GWoP. If pre-stress at the ICP was not considered, part of the true results would be lost. It is necessary to consider pre-stress in the dynamic finite element analysis of the TMJ.

A 3D dynamic finite element analysis of biomechanical behaviour of maxilla and fixative appliances following advancement Le Fort I surgery applied in different lengths.

OBJECTIVES: Dynamic analysis of chewing impact on the stability of rigid fixation techniques following Le Fort I osteotomy has not been investigated in the previous literature. The aim of the present study was to evaluate segmental displacement and von Mises (VM) stress values on the fixation devices following different amounts of Le Fort I advancements under dynamic loading conditions. MATERIALS AND METHODS: The 3D finite element models simulating 3, 5 and 8 mm advancement of maxilla at the Le Fort I level were generated using CBCT scan data. The models included two anterior L plates and two posterior I plates fixations bilaterally. Dynamic finite element analysis was performed to evaluate their biomechanical behavior against chewing cornflakes bio. Von Mises stresses and displacement values on three points were calculated. RESULTS: Calculations were made in a time of 38, 40 and 40.5 ms for 3, 5 and 8 mm advancement models, respectively. As the advancement increased, stress values on the plates and displacement values in the D1 (intersection of the apex of the canine tooth with the osteotomy line), D2 (the most prominent point of zygomatic buttress on the osteotomy line), and D3 (intersection of the midline of the second molar tooth with the osteotomy line) points increased. The lowest stress and displacement values were found in the 3 mm advancement model. As advancement increased, the highest values were found in the I plates. The stress levels on the plates and screws remained within safe limits. CONCLUSIONS: The von Mises stresses and displacement values tend to increase in according with the amount of advancement. More stress is transferred to posterior I plates and screws under dynamic forces.

Dynamic Finite Element Model Based on Timoshenko Beam Theory for Simulating High-Speed Nonlinear Helical Springs.

Helical springs with nonlinear geometric parameters nowadays have shown great advantages over classical linear springs, especially due to their superior performance in diminishing dynamic responses in high-speed situations. However, existing studies are mostly available for springs with linear properties, and the sole FE spring models using solid elements occupy significant computational resources. This study presents an FE spring model based on Timoshenko beam theory, which allows for high-speed dynamic simulations of nonlinear springs using a beehive valve spring sample. The dynamic results are also compared with the results of the FE model using solid elements and the results of the engine head test and indicate that the proposed FE model can accurately predict dynamic spring forces and the phenomenon of coil clash when simulating the beehive spring at engine speeds of both 5600 and 8000 RPM. The results also indicate that rapid coil impact brings significant spike forces. It should also be noted that the FE spring model using beam elements displays sufficient accuracy in predicting the dynamic responses of nonlinear springs while occupying much fewer computational resources than the FE model using solid elements.

Bioprinting-Associated Shear Stress and Hydrostatic Pressure Affect the Angiogenic Potential of Human Umbilical Vein Endothelial Cells.

Bioprinting-associated shear stress and hydrostatic pressure can negatively affect the functionality of dispensed cells. We hypothesized that these mechanical stimuli can potentially affect the angiogenic potential of human umbilical vein endothelial cells (HUVECs). A numerical simulation model was used to calculate the shear stress during microvalve-based droplet ejection. The impact of different levels of applied pressure and the resulting shear stress levels on the angiogenic potential of HUVECs was investigated after up to 14 days of cultivation. In vitro results showed that bioprinting-associated stress not only has short-term but also long-term effects. The short-term viability results indicate a 20% loss in post-printing cell viability in samples printed under the harshest conditions compared to those with the lowest shear stress level. Further, it was revealed that even in two-dimensional culture, HUVECs were able to form a capillary-like network organization regardless of bioprinting pressure. In three-dimensional culture experiments; however, the HUVECs printed at 3 bar were not able to form tubular structures due to their exposure to high shear stress levels. In conclusion, this study provides new insights into how the bioprinting process should be conducted to control printing-associated shear stress and hydrostatic pressure to preserve the functionality and angiogenetic potential of HUVEC.

Prediction of the natural frequencies of different degrees of degenerated human lumbar segments L2-L3 using dynamic finite element analysis.

BACKGROUND AND OBJECTIVE: Chronic exposure to resonant environment may cause more serious injuries to human lumbar spine than other vibrations. On the condition that the natural frequency of human lumbar spine is known, excitation frequency from an external vibration source can be optimized to keep away from the natural frequency and thus avoid lumbar resonance. Therefore, this study aimed to present an approach to predict the natural frequency of the human lumbar spine. METHODS: Four poroelastic finite element models of human L2-L3 spinal motion segments with different degrees of degeneration were established. Dynamic finite element analyses of these models during 1 h of vibration were then conducted. The mechanical parameters of these models under vibrations at different excitation frequencies were predicted. The excitation frequencies that resulted in the greatest changes in the lumbar mechanical parameters were identified as the natural frequencies of the established L2-L3 spinal motion segments. RESULTS: Simulation results showed that the natural frequencies of the healthy and mildly degenerated L2-L3 spinal motion segments, moderately degenerated L2-L3 spinal motion segments, and seriously degenerated L2-L3 spinal motion segments were in the range of 5-7, 3-5, and 1-3 Hz, respectively. CONCLUSIONS: The predicted results indicated that the natural frequencies of the human L2-L3 spinal motion segments gradually decreased with the severity of degeneration. These phenomena may be related to changes in the lumbar structures and materials because of degeneration. This study provided a feasible method to predict the lumbar natural frequencies for different populations, which may be helpful in optimizing external vibration sources to avoid lumbar resonance.

Finite element modeling of the periodontal ligament under a realistic kinetic loading of the jaw system.

PURPOSE: The stresses and deformations in the periodontal ligament (PDL) under the realistic kinetic loading of the jaw system, i.e., chewing, are difficult to be determined numerically as the mechanical properties of the PDL is variably present in different finite element (FE) models. This study was aimed to conduct a dynamic finite element (FE) simulation to investigate the role of the PDL (PDL) material models in the induced stresses and deformations using a simplified patient-specific FE model of a human jaw system. METHODS: To do that, a realistic kinetic loading of chewing was applied to the incisor point, contralateral, and ipsilateral condyles, through the experimentally proven trajectory approach. Three different material models, including the elasto-plastic, hyperelastic, and viscoelastic, were assigned to the PDL, and the resulted stresses of the tooth FE model were computed and compared. RESULTS: The results revealed the highest von Mises stress of 620.14 kPa and the lowest deformation of 0.16 mm in the PDL when using the hyperelastic model. The concentration of the stress in the elastoplastic and viscoelastic models was in the mid-root and apex of the PDL, while for the hyperelastic model, it was concentrated in the cervical margin. The highest deformation in the PDL regardless of the employed material model was located in the caudal direction of the tooth. The viscoelastic PDL absorbed the transmitted energy from the dentine and led to lower stress in the cancellous bone compared to the elastoplastic and hyperelastic material models. CONCLUSION: These results have implications not only for understanding the stresses and deformations in the PDL under chewing but also for providing comprehensive information for the medical and biomechanical experts in regard of the role of the material models being used to address the mechanical behavior of the PDL in other components of the tooth.

Establishment and Finite Element Analysis of a Three-dimensional Dynamic Model of Upper Cervical Spine Instability.

OBJECTIVES: To establish a dynamic three-dimensional (3D) model of upper cervical spine instability and to analyze its biomechanical characteristics. METHODS: A 3D geometrical model was established after CT scanning of the upper cervical spine specimen. The ligament of the specimen was fatigued to establish the upper cervical spine-instability model. A 100-N preloaded stress was applied to the upper surface of the occipital bone, and then a 1.5-Nm moment was applied in the occipital-sagittal direction to simulate upper cervical spine flexion and extension. Subsequently, the 3D dynamic model was established based on trajectory data that were measured using a motion-capture system. The stress on the main ligament and the relative motion angle of the joint were analyzed. RESULTS: The shape of the model grid was regular and the total number of its units was 627 000. After finite-element analysis was conducted, results of the ligament stress and relative movement angle were obtained. After the upper cervical spine instability, the pressure of the alar ligament during the upper cervical spine extension was increased from 2.85 to 8.12 MPa. The pressure of the flavum ligament was increased during the upper-cervical spine flexion, from 0.90 to 1.21 MPa. The pressure of the odontoid ligament was reduced during the upper cervical spine flexion and extension, from 10.46 to 6.67 MPa and 25.66 to 16.35 MPa, respectively. The pressure of the anterior longitudinal ligament and cruciate ligament was increased to a certain degree during upper cervical spine flexion and extension. The pressure of the anterior longitudinal ligament was increased during flexion and extension, from 7.70 to 10.10 MPa and 10.45 to 13.75 MPa, respectively. The pressure of the cruciate ligament was increased during flexion and extension, from 2.29 to 4.34 MPa and 2.32 to 4.40 MPa, respectively. In addition, after upper cervical spine instability, the articular-surface relative-movement angle of the atlanto-occipital joint and atlanto-axial joint had also changed. During upper cervical spine flexion, the angle of the atlanto-occipital joint was increased from 3.49° to 5.51°, and the angle of the atlanto-axial joint was increased from 8.84° to 13.70°. During upper cervical spine extension, the angle of the atlanto-occipital joint was increased from 11.16° to 12.96°, and the angle of the atlanto-axial joint was increased from 14.20° to 17.20°. Therefore, the movement angle of the atlanto-axial joint was most obvious after induction of instability. CONCLUSION: The 3D dynamic finite-element model of the upper cervical spine can be used to analyze and summarize the relationship between the change of ligament stress and the degree of instability in cervical instability. Frequent or prolonged flexion activities are more likely to lead to instability of the upper cervical spine.

Comparative study of stress characteristics in surrounding bone during insertion of dental implants of three different thread designs: A three-dimensional dynamic finite element study.

The objective of this study is to evaluate the stress distribution characteristics around three different dental implant designs during insertion into bone, using dynamic finite element stress analysis. Dental implant placement was simulated using finite element models. Three implants with different thread and body designs (Model 1: root form implant with three different thread shapes; Model 2: tapered implant with a double-lead thread; and Model 3: conical tapered implant with a constant buttress thread) were assigned to insert into prepared bone cavity models until completely placed. Stress and strain distributions were descriptively analyzed. The von Mises stresses within the surrounding bone were measured. At the first 4-mm depth of implant insertion, maximum stress within cortical bone for Model 3 (175 MPa) was less than the other models (180 MPa each). Stress values and concentration area were increasing whereas insertion depth increased. At full implant insertion depth, maximum stress level in Model 1 (35 MPa) within the cancellous bone was slightly greater than in Models 2 (30 MPa) and 3 (25 MPa), respectively. Generally, for all simulations, the highest stress value and the location of the stress concentration area were mostly in cortical bone. However, the stress distribution patterns during the insertion process were different between the models depending on the different designs geometry that contacted the surrounding bone. Different implant designs affect different stress generation patterns during implant insertion. A range of stress magnitude, generated in the surrounding bone, may influence bone healing around dental implants and final implant stability.

Numerical Test for Cell Durotaxis Migration on Substrate with Moderate Gradient Stiffness / 医用生物力学

Objective Based on fibroblast cell model and photopolymerized hydrogel substrate with moderate gradient stiffness, to analyze the effect of process and performance parameters on cell migration and provide theoretical guidance for artificial scaffold design and fabrication. Methods A mathematical model of the test system was built and the corresponding numerical program was compiled, including viscoelastic dynamic finite element of the cell model, reaction kinetic equation of focal adhesions, and the strategy to deal with dynamic boundary and multi-scale time. Results The relationship between process parameters and performance parameters was formulated based on experimental data; cell migration speed and traction increased with the substrate stiffness increasing and were accompanied by rapid fluctuation when stiffness gradient was constant, then cell movement gradually stabilized with the extension of observation time. Increasing stiffness gradient moderately obviously promoted cell migration, and cells could maintain a limited speed on substrate with a large stiffness gradient. Smaller photomask opacity gradient resulted in larger substrate stiffness gradient and less time spent for cell to reach the target. These results agreed with the experimental results reported in the literature. Conclusions The experimental result provided an effective digital simulation platform to test the influence of process and performance parameter of photopolymerized hydrogel substrate with moderate gradient stiffness on cell migration.

Dynamic finite element simulation of dental prostheses during chewing using muscle equivalent force and trajectory approaches.

The long-term application of dental prostheses inside the bone has a narrow relation to its biomechanical performance. Chewing is the most complicated function of a dental implant as it implements different forces to the implant at various directions. Therefore, a suitable holistic modelling of the jaw bone, implant, food, muscles, and their forces would be deemed significant to figure out the durability as well as functionality of a dental implant while chewing. So far, two approaches have been proposed to employ the muscle forces into the Finite Element (FE) models, i.e. Muscle Equivalent Force (MEF) and trajectory. This study aimed at propounding a new three-dimensional dynamic FE model based on two muscle forces modelling approaches in order to investigate the stresses and deformations in the dental prosthesis as well as maxillary bone during the time of chewing a cornflakes bio. The results revealed that both contact and the maximum von Mises stress in the implant and bones for trajectory approach considerably exceed those of the MEF. The maximum stresses, moreover, are located around the neck of implant which should be both clinically and structurally strong enough to functionally maintain the bone-implant interface. In addition, a higher displacement due to compressive load is observed for the implant head in trajectory approach. The results suggest the benefits provided by trajectory approach since MEF approach would significantly underestimate the stresses and deformations in both the dental prosthesis and bones.

Effects of implant drilling parameters for pilot and twist drills on temperature rise in bone analog and alveolar bones.

This study concerns the effects of different drilling parameters of pilot drills and twist drills on the temperature rise of alveolar bones during dental implant procedures. The drilling parameters studied here include the feed rate and rotation speed of the drill. The bone temperature distribution was analyzed through experiments and numerical simulations of the drilling process. In this study, a three dimensional (3D) elasto-plastic dynamic finite element model (DFEM) was proposed to investigate the effects of drilling parameters on the bone temperature rise. In addition, the FE model is validated with drilling experiments on artificial human bones and porcine alveolar bones. The results indicate that 3D DFEM can effectively simulate the bone temperature rise during the drilling process. During the drilling process with pilot drills or twist drills, the maximum bone temperature occurred in the region of the cancellous bones close to the cortical bones. The feed rate was one of the important factors affecting the time when the maximum bone temperature occurred. Our results also demonstrate that the elevation of bone temperature was reduced as the feed rate increased and the drill speed decreased, which also effectively reduced the risk region of osteonecrosis. These findings can serve as a reference for dentists in choosing drilling parameters for dental implant surgeries.

Computing the stresses and deformations of the human eye components due to a high explosive detonation using fluid-structure interaction model.

INTRODUCTION: In spite the fact that a very small human body surface area is comprised by the eye, its wounds due to detonation have recently been dramatically amplified. Although many efforts have been devoted to measure injury of the globe, there is still a lack of knowledge on the injury mechanism due to Primary Blast Wave (PBW). The goal of this study was to determine the stresses and deformations of the human eye components, including the cornea, aqueous, iris, ciliary body, lens, vitreous, retina, sclera, optic nerve, and muscles, attributed to PBW induced by trinitrotoluene (TNT) explosion via a Lagrangian-Eulerian computational coupling model. MATERIALS AND METHODS: Magnetic Resonance Imaging (MRI) was employed to establish a Finite Element (FE) model of the human eye according to a normal human eye. The solid components of the eye were modelled as Lagrangian mesh, while an explosive TNT, air domain, and aqueous were modelled using Arbitrary Lagrangian-Eulerian (ALE) mesh. Nonlinear dynamic FE simulations were accomplished using the explicit FE code, namely LS-DYNA. In order to simulate the blast wave generation, propagation, and interaction with the eye, the ALE formulation with Jones-Wilkins-Lee (JWL) equation defining the explosive material were employed. RESULTS: The results revealed a peak stress of 135.70kPa brought about by detonation upsurge on the cornea at the distance of 25cm. The highest von Mises stresses were observed on the sclera (267.3kPa), whereas the lowest one was seen on the vitreous body (0.002kPa). The results also showed a relatively high resultant displacement for the macula as well as a high variation for the radius of curvature for the cornea and lens, which can result in both macular holes, optic nerve damage and, consequently, vision loss. CONCLUSION: These results may have implications not only for understanding the value of stresses and strains in the human eye components but also giving an outlook about the process of PBW triggers damage to the eye.

Effects of implant thread profile on insertion stress generation in cortical bone studied by dynamic finite element simulation / 대한치과보철학회지

PURPOSE: The aim of this study was to investigate the effect of implant thread profile on the marginal bone stresses which develop during implant insertion. MATERIALS AND METHODS: Four experimental implants were created by placing four different thread systems on the body (4.1 mm x 10 mm) of the ITI standard implant. The thread types studied in this study included the buttress, v-shape, reverse buttress, and square shape threads. In order to examine the insertion stress generation, 3D dynamic finite element analysis was performed which simulated the insertion process of implants into a 1.2 mm thick cortical bone plate (containing 3.5 mm pilot hole) using a PC-based DEFORM 3D (ver 6.1, SFTC, Columbus, OH, USA) program. RESULTS: Insertion stresses higher than human cortical bone developed around the implants. The level of insertion stresses was much different depending on the thread. Stress level was lowest near the v-shape thread, and highest near the square shaped thread. Difference in the interfacial bone stress level was more noticeable near the valley than the tip of the threads. CONCLUSION: Among the four threads, the v-shape thread was turned out to minimize the insertion stress level and thereby create better conditions for implant osseointegration.