Stress Analysis of Aortic Vessels under Stent Graft / 医用生物力学

Objective To simulate the interaction between the stent graft (SG) and the aortic wall with finite element (FE) analysis by considering the influence of residual stress field, so as to study the stent influence on stress distributions of the aortic wall. Methods The three-dimensional (3D) residual stress field was generated in an idealized bi-layered thick-wall aortic model via a stress-driven anisotropic growth model by reducing the transmural stress gradient. Upon virtually deploying the SG, the stress on the aortic wall was calculated. Results The 3D residual stress field, corresponding to an opening angle of 117.5°, was shown to reduce the transmural stress gradient in both the circumferential and axial directions. The maximum stress was found at the contact area between aortic wall and wave peak of the stent. At 20% oversize ratio of the stent, the maximum stresses on the aortic wall in circumferential and axial direction were 412 and 132 kPa, respectively, while the in-plane shear stresses σrθ and σrz were both 78 kPa. Under residual stress, the maximum radial, circumferential and axial stresses were decreased by 14.9%, 40.5% and 33.8%, respectively, while the maximum shear stresses σrθ ，σrz，σθz were reduced by 2.5%, 7.1% and 27%, respectively. With the increase of oversize ratio from 10% to 20%, the maximum radial, circumferential and axial stresses were increased by 316%, 129% and 41%, respectively, while the maximum shear stresses σrθ ，σrz，σθz were increased by 661%, 450% and 466%, respectively. Conclusions The residual stress can effectively reduce the transmural stress gradient. Both the residual stress and the oversize ratio of the stent play an important role in modulating the wall stress distribution and the maximum stress.

In Vivo Stress Analysis of an Atherosclerotic Plaque at Carotid Bifurcation / 医用生物力学

Objective To investigate the in vivo stress distribution of the atherosclerotic plaque at carotid bifurcation, so as to provide references for the mechanical mechanisms of plaque rupture at carotid bifurcation and the design for further medical treatment. Methods The three-dimensional geometric model of carotid bifurcation and plaque were established according to average geometric parameters of human carotid bifurcation. Residual stress of the carotid bifurcation and plague was reestablished with “thermal-structure” coupling method, and in vivo stresses of vessels with the plaque at carotid bifurcation under blood pressure and blood flow were calculated. Results Both the maximum principal stress and elastic shear stress concentrated on the shoulder of the plaque. Elastic shear stress increased with the increase of stenosis ratio and blood pressure. Wall shear stress in the upstream of the plaque was considerably higher than that of the downstream. The distribution of oscillatory shear index(OSI) was quite the opposite. The changing patterns of the elastic shear stress and flow shear stress were quite different with the change of stenosis ratios. Conclusions Tension grew gradually from the centrality to shoulder surface of the plaque. The centrality of the plaque might bear compression when the stenosis was very severe. The periodic variation of the structural stress might cause structural fatigue of the plaque, thus increasing the rupture risk. Distinction of the component and vulnerability of the plaque between upstream and downstream might be caused by differences in hemodynamic parameters of the plaque between upstream and downstream.

The effet of cooling rate on the residual stresses in the veneer ceramics of zirconia-ceramic restorations: a literature review / 대한치과보철학회지

Nowadays, dental zirconia is widely used as a framework material for a fixed dental prosthesis as well as a single restoration. However, clinical studies have reported high incidence of veneer chipping of zirconia-ceramic restorations compared to that of metal-ceramics. Several factors were raised as the possible causes of veneer ceramic chipping, however, it is still in debate. Recently, residual stresses in the veneer ceramics after cooling process gathers attention as one possible cause of chipping and many studies reported that the rate of cooling significantly influenced the types and the amount of residual stress. The purpose of current review was to briefly describe the effect of cooling rate on the residual stress in zirconia-ceramics. It was also described that the different behavior of residual stress between zirconia-ceramics and metal-ceramics following different cooling rate.

Two methods of constructing the residual stress on vessel and its application / 医用生物力学

Objective To study distributions of the residual stresses on blood vessel, and to provide the biomechanical basis for the finite element research on stress changes in blood vessel. Methods The semi-inverse method was used for getting distributions of residual stresses on blood vessel and the result was compared with that from Commercial Software ABAQUS. On that basis, the interaction between the vessel and implanted stent was simulated to get the influence of residual stress on blood vessel. Results Distributions of three normal stresses on blood vessel obtained from the two methods were almost consistent. It is the residual stress that led to the change of vessel stress distributions, and the position of maximal stress was transformed from the inside to the outside of the vessel. Conclusions Assuming that the vessel has homogeneity and isotropic, both the semi-inverse method and the finite element method would get the similar residual stress distributions; significant differences existed between the stress states of vessel with or without considering residual stress. The stress at the inside of vessel was reduced obviously. Consideration of residual stress will be helpful for understanding the real stress state of vessel after stent deployment and providing references for the optimization design of stent.

Analysis of residual stress between core and veneer ceramics by finite element method / 实用口腔医学杂志

Objective:To study the residual stress distribution through the thickness of bilayered dental ceramic subjected to thermal stress, in order to improve the restoration. Methods: The finite element model of bilayered dental ceramic was set up based on International Organization for Standardization(ISO) 96936:1999. The residual stresses were calculated in viscoelastic and elastic phases during cooling of dental ceramic to analyze the residual stress distribution. Results: The deformation of core was greater than the veneer ceramics during the cooling period of dental ceramic. The residual stress increased with the decreasing of the temperature approaching the interface of core and veneer ceramics. But it decreased with the increasing of the thickness of veneer ceramics.Conclusion: Thermal compatibility of core and veneer ceramics is very important to the residual stress distribution in the bilayered dental ceramic, which may benefit to All-ceramic restorations. The viscoelastic behavior of ceramic should be taken into account in the thermal compatibility.

Comparison of the residual stress of the nanofilled composites / 대한치과보존학회지

"Residual stress" can be developed during polymerization of the dental composite and it can be remained after this process was completed. The total amount of the force which applied to the composite restoration can be calculated by the sum of external and internal force. For the complete understanding of the restoration failure behavior, these two factors should be considered. In this experiment, I compared the residual stress of the recently developed nanofilled dental composite by ring slitting methods. The composites used in this study can be categorized in two groups, one is microhybrid type-Z250, as control group, and nanofilled type-Grandio, Filtek Supreme, Ceram-X, as experimental ones. Composite ring was made and marked two reference points on the surface. Then measure the change of the distance between these two points before and after ring slitting. From the distance change, average circumferential residual stress (sigmatheta) was calculated. In 10 minutes and 1 hour measurement groups, Filtek Supreme showed higher residual stress than Z250 and Ceram-X. In 24 hour group, Filtek showed higher stress than the other groups. Following the result of this experiment, nanofilled composite showed similar or higher residual stress than Z250, and when comparing the Z250 and Filtek Supreme, which have quite similar matrix components, Filtek Supreme groups showed higher residual stress.

Effects of endothelial nitric oxide synthase on zero stress state of carotid arteries in rats of different ages / 中国普通外科杂志

Objective To investigate the distribution and effect of endothelial nitric oxide synthase (eNOS) on the zero-stress state of carotid arteries in rats at different ages.Methods Morphometry and microstructure of carotid artery in 3-month-old,6-month-old and 12-month-old Sprague-Dawley rats were studied quantitatively by histological method and computer image analysis.Immunohistochemistry and the NADPH-diaphorase reaction were used to localize the endothelial nitric oxide synthas (eNOS) in different age rats.The changes of opening angle in the zero-stress state was also observed.Results With increasing age,there was a significant reconstruction changes in morphometric parameters and microstructure parameters of the carotid arteries in rats.While the expression of eNOS protein of endothelium and vascular smooth muscle cells were decreased.Compared with 3-month-old rats,the zero-stress state.of opening angle in 6-month-old and 12-month-old group were increased.However,pre-incubation of vessel rings with L-arginine (30 mmol/L),a substrate of eNOS,attenuated the changes of the zero-stress state.Conclusions The results suggest that the carotid arterial wall in different age rats shows unequal growth and different zero-stress state eNOS may be involved in the age-related changes of zero-stress state of the carotid artery in aged rats.The results may provid an experimental base for vascular transplantation and vascular reconstruction.