Peak Circumferential Stress Research Articles

Temporal evolution of ascending aortic aneurysm wall stress predicts all-cause mortality.

Diameter-based risk stratification for elective repair of ascending aortic aneurysm fails to prevent type A dissection in many patients. Aneurysm wall stresses may contribute to risk prediction; however, rates of wall stress change over time are poorly understood. Our objective was to examine aneurysm wall stress changes over 3-5 years and subsequent all-cause mortality. Male veterans with <5.5 cm ascending aortic aneurysms and computed tomography at baseline and 3- to 5-year follow-up underwent three-dimensional aneurysm model construction. Peak circumferential and longitudinal wall stresses at systole were calculated using finite element analysis. Temporal trends were assessed by mixed-effects modelling. Changes in aortic wall stresses, diameter and length over time were evaluated as predictors of subsequent 3-year all-cause mortality by Cox proportional hazards modelling. Sixty-two male veterans were included in the study. Yearly changes in geometric and biomechanical measures were 0.12 mm/year (95% confidence interval, 0.04-0.20) for aortic diameter, 0.41 mm/year (0.12-0.71) for aortic length, 1.19 kPa/year -5.94 to 8.33) for peak circumferential stress, and 0.48 kPa/year (-3.89 to 4.84) for peak longitudinal stress. Yearly change in peak circumferential stress was significantly associated with hazard of death-hazard ratio for peak circumferential stress growth per 10 kPa/year, 1.27 (95% CI, 1.02-1.60; P = 0.037); hazard ratio for peak circumferential stress growth ≥ 32 kPa/year, 8.47 (95% CI, 2.42-30; P < 0.001). In this population of nonsurgical aneurysm patients, large temporal changes in peak circumferential stress, but not aortic diameter or length, was associated with all-cause mortality. Biomechanical stress and stress changes over time may be beneficial as additional risk factors for elective surgery in small aneurysms.

A study of crack propagation during blasting under high in-situ stress conditions based on an improved CDEM method

Considering the energy consumption of the crushed zone caused by blasting, the energy dissipation characteristics were investigated through theoretical calculation and a plug-in for calculation of energy reduction was made. As a result, an improved CDEM method suitable for studying blast-induced crack propagation was proposed. The improved CDEM method was then used to numerically simulate the blast-induced crack propagation under three different in-situ stress conditions: uniaxial in-situ stress, biaxial equal in-situ stresses and biaxial unequal in-situ stresses. It was found that the in-situ stress conditions had significant effects on the distribution of the blast-induced cracks, the evolution of blast-induced stresses and the propagation of the blast-induced cracks. At the uniaxial in-situ stress condition, the vertical in-situ stress reduced the tip stresses of the horizontal crack and inhibited the propagation of the horizontal crack. At the biaxial equal in-situ stress condition, the peak radial stresses and circumferential stresses of the gauging points decreased gradually with the increase of in-situ stresses. In particular, the circumferential stresses of the gauging points gradually changed from tension to compression with an increasing in-situ stress. At the biaxial unequal in-situ stress condition, there were different inhibitions on crack propagation in the horizontal direction and the vertical one.

Abstract 15119: Diastolic Peak Wall Stresses in Marfan vs Non-Marfan Ascending Thoracic Aortic Aneurysm: A Propensity Score Analysis

Introduction: Marfan syndrome (MFS) carries a risk of aortic dissection and death. The current aneurysm repair guideline of diameter ≥5cm may neglect up to 15% of MFS patients who dissect at smaller diameters. Since dissection can occur biomechanically when wall stress exceeds tissue strength, patient-specific biomechanics may enhance risk stratification for MFS patients. The goal of this study was to compare aneurysm wall stresses between propensity score-matched MFS and non-MFS aneurysm patients. Methods: The cohort consisted of 5 MFS and 42 non-MFS patients with aortic aneurysm. Preoperative CTA or MR scans were used to create 3D hexahedral meshes. Finite element analysis was performed with MFS and nonMFS material properties to determine peak (99th percentile) diastolic stresses[TE1] . Propensity scores were calculated by logistic regression using age, sex, height, hypertension, diabetes, smoking history, valve type, and sinus diameter. Five pairs were formed by 1:1 optimal matching without replacement. Comparisons used the paired t-test. Results: Diameters were similar at the sinus but lower for MFS at the STJ (3.4±0.5 vs 3.9±0.7cm, p=0.003) and the ascending aorta (3.1±0.5 vs 4.3±0.5cm, p&lt;0.001). Peak circumferential stresses (CS) were similar between MFS and nonMFS aneurysms across all regions. As well, peak longitudinal stresses (LS) were similar between the two cohorts across all regions (Fig 1). Conclusion: Both CS and LS were similar at all regions for MFS vs non-MFS patients. Because LS has been implicated in acute dissection and LS were similar, differences in tissue strength, rather than wall stress, are most likely causes of high dissection rates in MFS due to material weakness from fibrillin mutation.

Association of 3-Year All-Cause Mortality and Peak Wall Stresses of Ascending Thoracic Aortic Aneurysms in Veterans

Risk of aortic dissection in ascending thoracic aortic aneurysms is not sufficiently captured by size-based metrics. From a biomechanical perspective, dissection may be initiated when wall stress exceeds wall strength. Our objective was to assess the association between aneurysm peak wall stresses and 3-year all-cause mortality. Finite element analysis was performed in 273 veterans with chest computed tomography for surveillance of ascending thoracic aortic aneurysms. Three-dimensional geometries were reconstructed and models developed accounting for prestress geometries. A fiber-embedded hyperelastic material model was applied to obtain circumferential and longitudinal wall stresses under systolic pressure. Patients were followed up to 3 years following the scan to assess aneurysm repair and all-cause mortality. Fine-Gray subdistribution hazards were estimated for all-cause mortality based on age, aortic diameter, and peak wall stresses, treating aneurysm repair as a competing risk. When accounting for age, subdistribution hazard of mortality was not significantly increased by peak circumferential stresses (p=0.30) but was significantly increased by peak longitudinal stresses (p=0.008). Aortic diameter did not significantly increase subdistribution hazard of mortality in either model (circumferential model: p=0.38; longitudinal model: p = 0.30). The effect of peak longitudinal stresses on subdistribution hazard of mortality was maximized at a binary threshold of 355kPa, which captured 34 of 212(16%) patients with diameter <5 cm, 11 of 36(31%) at 5.0-5.4 cm, and 11 of 25(44%) at ≥5.5 cm. Aneurysm peak longitudinal stresses stratified by age and diameter were associated with increased hazard of 3-year all-cause mortality in a veteran cohort. Risk prediction may be enhanced by considering peak longitudinal stresses.

Regional wall stress differences on tricuspid aortic valve-associated ascending aortic aneurysms.

Ascending thoracic aortic aneurysms (aTAAs) carry a risk of acute type A dissection. Elective repair guidelines are based on diameter, but complications often occur below diameter threshold. Biomechanically, dissection can occur when wall stress exceeds wall strength. Aneurysm wall stresses may better capture dissection risk. Our aim was to investigate patient-specific aTAA wall stresses associated with a tricuspid aortic valve (TAV) by anatomic region. Patients with aneurysm diameter ≥4.0 cm underwent computed tomography angiography. Aneurysm geometries were reconstructed and loaded to systemic pressure while taking prestress into account. Finite element analyses were conducted to obtain wall stress distributions. The 99th percentile longitudinal and circumferential stresses were determined at systole. Wall stresses between regions were compared using one-way analysis of variance with post hoc Tukey HSD for pairwise comparisons. Peak longitudinal wall stresses on aneurysms (n = 204) were 326 [standard deviation (SD): 61.7], 246 (SD: 63.4) and 195 (SD: 38.7) kPa in sinuses of Valsalva, sinotubular junction (STJ) and ascending aorta (AscAo), respectively, with significant differences between AscAo and both sinuses (P < 0.001) and STJ (P < 0.001). Peak circumferential wall stresses were 416 (SD: 85.1), 501 (SD: 119) and 340 (SD: 57.6) kPa for sinuses, STJ and AscAo, respectively, with significant differences between AscAo and both sinuses (P < 0.001) and STJ (P < 0.001). Circumferential and longitudinal wall stresses were greater in the aortic root than AscAo on aneurysm patients with a TAV. Aneurysm wall stress magnitudes and distribution relative to respective regional wall strength could improve understanding of aortic regions at greater risk of dissection in a particular patient.

Simplified model for static stress analysis of straight eccentric lug subjected to cosine distribution pressure

A simplified analytical method is presented for predicting static stress distribution of the straight eccentric lug subjected to cosine distribution pressure. The method of analysis is only suitable for the situation where the predicted peak stress in the straight eccentric lug remains at or below the yield stress of the steel material used for it. In this method, the Castigliano’s theorem is used to determine the internal force distribution of the straight eccentric lug, and the bipolar coordinate system is introduced to analyze the stress state of the straight eccentric lug under bending moment. By comparing with the FEM’s results, it is proved that the present method can give a good precision in predicting the stress distribution of the straight eccentric lug subjected to the cosine distribution pressure. And the peak circumferential tension stress of the straight eccentric lug is observed at the edge of lug hole above the pin. Moreover, the effects of inner radius/outer radius ratio, distance between the two circle centers, and lug thickness to the peak tension stress are investigated. Based on contrastive analysis, it is concluded that the peak tension stress can be reduced either by increasing the inner radius/outer radius ratio and lug thickness, or by decreasing the distance between the two circle centers.

Association of diameter and wall stresses of tricuspid aortic valve ascending thoracic aortic aneurysms

ObjectiveAscending thoracic aortic aneurysms carry a risk of acute type A dissection. Elective repair guidelines are designed around size thresholds, but the 1-dimensional parameter of maximum diameter cannot predict acute events in small aneurysms. Biomechanically, dissection can occur when wall stress exceeds strength. Patient-specific ascending thoracic aortic aneurysm wall stresses may be a better predictor of dissection. Our aim was to compare wall stresses in tricuspid aortic valve–associated ascending thoracic aortic aneurysms based on diameter. MethodsPatients with tricuspid aortic valve–associated ascending thoracic aortic aneurysm and diameter 4.0 cm or greater (n = 221) were divided into groups by 0.5-cm diameter increments. Three-dimensional geometries were reconstructed from computed tomography images, and finite element models were developed taking into account prestress geometries. A fiber-embedded hyperelastic material model was applied to obtain longitudinal and circumferential wall stress distributions under systolic pressure. Median stresses with interquartile ranges were determined. The Kruskal–Wallis test was used for comparisons between size groups. ResultsPeak longitudinal wall stresses for tricuspid aortic valve–associated ascending thoracic aortic aneurysm were 290 (265-323) kPa for size 4.0 to 4.4 cm versus 330 (296-359) kPa for 4.5 to 4.9 cm versus 339 (320-373) kPa for 5.0 to 5.4 cm versus 318 (293-351) kPa for 5.5 to 5.9 cm versus 373 (363-449) kPa for 6.0 cm or greater (P = 8.7e-8). Peak circumferential wall stresses were 460 (421-543) kPa for size 4.0 to 4.4 cm versus 503 (453-569) kPa for 4.5 to 4.9 cm versus 549 (430-588) kPa for 5.0 to 5.4 cm versus 540 (471-608) kPa for 5.5 to 5.9 cm versus 596 (506-649) kPa for 6.0 cm or greater (P = .0007). ConclusionsCircumferential and longitudinal wall stresses are higher as diameter increases, but size groups had large overlap of stress ranges. Wall stress thresholds based on aneurysm wall strength may be a better predictor of patient-specific risk of dissection than diameter in small ascending thoracic aortic aneurysms.

Damage Evolution and Circumferential Strain Distribution Characteristics of the Bolt-Supported Cavern under Multiple Explosion Sources

The impact of multiple explosion sources on the safety of the underground cavern is enormous. Based on a similarity model test, the finite element software LS-DYNA3D was utilized to analyze the damage evolution and circumferential strain distribution characteristics of the bolt-supported cavern under the seven combinations of concentrated charge explosion sources in three places, including the side of the vault, side arch, and sidewall. The accuracy of the simulation results is verified by comparing them with test results. The research results indicate that the damage of the surrounding rock is mainly caused by the tensile stress wave reflected from the free surfaces and the superposition of the tensile stress wave. The damage of the surrounding rock in the cases of multiple explosion sources is not a simple superposition of that in the cases of a single explosion source. The peak circumferential stress and damage of the surrounding rock in the middle of two explosion sources are significantly greater than that of the cases of the corresponding single explosion source. In the seven cases, the peak circumferential strain of the cavern wall changes from tensile to compressive from the vault to the spandrel. When the explosion occurs on the sidewall, the peak circumferential strain of the floor is tensile.

Abstract 16806: Changes in Wall Stress in Ascending Thoracic Aortic Aneurysms From Systole to Diastole

Introduction: Aortic rupture or dissection of ascending thoracic aortic aneurysms (aTAA) is a life-threatening pathology that can occur when wall stress exceeds wall strength. Surgical guidelines for repair is when aneurysm max diameter ≥5.5 cm. However, rupture has been documented at various sizes, including 60% &lt;4.5cm. Wall stress may be a better predictor of dissection than diameter, using finite element analysis (FEA) of patient-specific aTAA models to assess patient-specific risk of dissection. The purpose of this study was to determine changes in aneurysm FEA peak wall stresses in patients during the cardiac cycle from systole to diastole. Methods: Preoperative aTAA patients (n = 20) underwent ECG-gated computed tomography (CT) scans at systolic and diastolic phases. Patient-specific 3D aneurysm geometries were reconstructed from the imaging data at both phases and after accounting for pre-stress, underwent FEA using LS-DYNA solver. Wall stress distribution and magnitude were determined using user-defined fiber-embedded material model at systole (120mmHg) and diastole (80mmHg). Aortic volume changes during the cardiac cycle were determined using computer-aided design tools. Results: From the diastolic model, the 99 th percentile circumferential stresses were 348.8 ±105.6 kPa and 186.6 ±31.4 kPa for the sinus and ascending aorta respectively, while for the systolic model, they were 500.7 ±183.7 kPa (p=0.00072) and 296.5 ±64.4 kPa (p &lt; .00001), respectively. The 99th percentile longitudinal stresses from the diastolic model were 223.2 ±75.4 kPa and 126.1 ±20.4 kPa for the sinus and ascending aorta, respectively, while for the systolic model, they were 336.3 ±78.4 kPa (p &lt; .00001), and 235.2 ±48.0 kPa (p &lt; .00001), respectively. Mean volume change of the ascending aorta in percentage between systolic and diastolic phases was 6.34%. Conclusions: Peak circumferential and longitudinal stresses changed significantly from systole to diastole. For patients with significant hypertension, these differences in wall stress may be magnified such that sudden or significant hypertension may result in wall stress values that exceed tissue strength leading to dissection.

Abstract 16886: Wall Stress Profiles in Tricuspid Aortic Valve Associated Ascending Thoracic Aortic Aneurysms: The Effect of Surgical Threshold Diameter ≥5.5cm

Introduction: Ascending thoracic aortic aneurysms (aTAAs) carry a risk of dissection. Elective repair guidelines are designed around size thresholds, but the one-dimensional parameter is insufficient to predict acute events in small aneurysms. Biomechanically, aortic events can occur when wall stress exceeds wall strength. Patient-specific aTAA wall stresses may be a better predictor of complications. Our aim was to compare wall stresses in aTAAs associated with a tricuspid aortic valve (TAV) based on diameter. Methods: Patients with TAV-aTAA and diameter &gt;4.0cm (n=448) were divided into groups by 0.5 cm diameter increments. Pre-stress three-dimensional aneurysm geometries were reconstructed from ECG-gated computer tomography images. A fiber-embedded hyperelastic material model was applied to obtain longitudinal and circumferential wall stress distributions under systolic pressure. Medians with interquartile ranges are reported. The Kruskal-Wallis test is used for comparisons between size groups. Results: Peak longitudinal wall stresses for TAV-aTAA were 308[282-338] kPa for size 4.0-4.4cm vs 341[309-362] kPa for 4.5-4.9cm vs 339[289-370] kPa for 5.0-5.4cm vs 319[297-355] kPa for 5.5-5.9cm vs 373[364-449] for 6.0cm (p=0.003). Peak circumferential wall stresses were 487[448-579] kPa for size 4.0-4.4cm vs 516[473-619] kPa for 4.5-4.9cm vs 506[422-580] kPa for 5.0-5.4cm vs 540[468-591] kPa for 5.5-5.9cm vs 565[506-634] for &gt;6.0cm (p=0.19) (figure). 95th-percentile longitudinal peak stress for TAV-aTAA &lt;5.5cm vs ≥5.5cm is 408 vs 465 kPa. Conclusions: Longitudinal wall stresses are higher as diameter increases. The 95% percentile longitudinal peak stress for diameter ≥5.5cm is ~450 kPa, which correlates with established ~5% dissection risk for size ≥5.5cm. Wall stress thresholds may be a better predictor of patient-specific risk of dissection than diameter and require testing in clinical trials.

Wall stress analyses in patients with ≥5 cm versus

ObjectiveCurrent guidelines for elective surgery of ascending thoracic aortic aneurysms (aTAAs) use aneurysm size as primary determinant for risk stratification of adverse events. Biomechanically, dissection may occur when wall stress exceeds wall strength. Determining patient-specific aTAA wall stresses by finite element analysis can potentially predict patient-specific risk of dissection. This study compared peak wall stresses in patients with ≥5.0 cm versus <5.0 cm aTAAs to determine correlation between diameter and wall stress. MethodsPatients with aTAA ≥5.0 cm (n = 47) and <5.0 cm (n = 53) were studied. Patient-specific aneurysm geometries obtained from echocardiogram-gated computed tomography were meshed and prestress geometries determined. Peak wall stresses and stress distributions were determined using LS-DYNA finite element analysis software (LSTC Inc, Livermore, Calif), with user-defined fiber-embedded material models under systolic pressure. ResultsPeak circumferential stresses at systolic pressure were 530 ± 83 kPa for aTAA ≥5.0 cm versus 486 ± 87 kPa for aTAA <5.0 cm (P = .07), whereas peak longitudinal stresses were 331 ± 57 kPa versus 310 ± 54 kPa (P = .08), respectively. For aTAA ≥5.0 cm, correlation between peak circumferential stresses and size was 0.41, whereas correlation between peak longitudinal wall stresses and size was 0.33. However, for aTAA <5.0 cm, correlation between peak circumferential stresses and size was 0.23, whereas correlation between peak longitudinal stresses and size was 0.14. ConclusionsPeak patient-specific aTAA wall stresses overall were larger for ≥5.0 cm than aTAA <5.0 cm. Although some correlation between size and peak wall stresses was found in aTAA ≥5.0 cm, poor correlation existed between size and peak wall stresses in aTAA <5.0 cm. Patient-specific wall stresses are particularly important in determining patient-specific risk of dissection for aTAA <5.0 cm.

Modeling of Mechanical Stress Exerted by Cholesterol Crystallization on Atherosclerotic Plaques.

Plaque rupture is the critical cause of cardiovascular thrombosis, but the detailed mechanisms are not fully understood. Recent studies have found abundant cholesterol crystals in ruptured plaques, and it has been proposed that the rapid expansion of cholesterol crystals in a limited space during crystallization may contribute to plaque rupture. To evaluate the effect of cholesterol crystal growth on atherosclerotic plaques, we modeled the expansion of cholesterol crystals during the crystallization process in the necrotic core and estimated the stress on the thin cap with different arrangements of cholesterol crystals. We developed a two-dimensional finite element method model of atherosclerotic plaques containing expanding cholesterol crystals and investigated the effect of the magnitude and distribution of crystallization on the peak circumferential stress born by the cap. Using micro-optical coherence tomography (μOCT), we extracted the cross-sectional geometric information of cholesterol crystals in human atherosclerotic aorta tissue ex vivo and applied the information to the model. The results demonstrate that (1) the peak circumference stress is proportionally dependent on the cholesterol crystal growth; (2) cholesterol crystals at the cap shoulder impose the highest peak circumference stress; and (3) spatial distributions of cholesterol crystals have a significant impact on the peak circumference stress: evenly distributed cholesterol crystals exert less peak circumferential stress on the cap than concentrated crystals.

A parametric study of the elastic stress distribution in a pin-loaded tube with multiple pinning

This paper describes a numerical study of the stress distribution in a pin-loaded tube, loaded in tension, where more than one pin is used. The effect of changing the inter-pin distances, pin diameters and pin orientation (i.e. in-line or crossed) have been analysed and discussed. The study shows that there are significant through thickness effects as a result of out-of-plane deformations coupled with an unequal load distribution between each of the pins. Various configurations of pin positions are compared, and it is demonstrated that the joint design can be optimised by crossing, changing the diameter or moving particular pins. It is shown that constructing a joint employing three pins has advantages over a two pin arrangement and that the peak circumferential stress around a pin hole can be reduced by approximately 42% compared with using only a single pin: especially significant when fatigue loadings are considered.

Imaging and analysis of microcalcifications and lipid/necrotic core calcification in fibrous cap atheroma.

The presence of microcalcifications (µCalcs) >5 µm within the cap of human fibroatheroma has been shown to produce a 200-700% increase in peak circumferential stress, which can transform a stable plaque into a vulnerable one, whereas µCalcs < 5 µm do not appear to increase risk. We quantitatively examine the possibility to distinguish caps with µCalcs > 5 µm based on the gross morphological features of fibroatheromas, and the correlation between the size and distribution of µCalcs in the cap and the calcification in the lipid/necrotic core beneath it. Atherosclerotic lesions (N = 72) were imaged using HR-μCT at 2.1-μm resolution for detailed analysis of atheroma morphology and composition, and validated using non-decalcified histology. At 2.1-μm resolution one observes four different patterns of calcification within the lipid/necrotic core, and is able to elucidate the 3D spatial progression of the calcification process using these four patterns. Of the gross morphological features identified, only minimum cap thickness positively correlated with the existence of µCalcs > 5 µm in the cap. We also show that µCalcs in the cap accumulate in the vicinity of the lipid/necrotic core boundary with few on the lumen side of the cap. HR-μCT enables three-dimensional assessment of soft tissue composition, lipid content, calcification patterns within lipid/necrotic cores and analysis of the axial progression of calcification within individual atheroma. The distribution of µCalcs within the cap is highly non-uniform and decreases sharply as one proceeds from the lipid pool/necrotic core boundary to the lumen.

Effect of tissue properties, shape and orientation of microcalcifications on vulnerable cap stability using different hyperelastic constitutive models

Approximately half of all cardiovascular deaths associated with acute coronary syndrome occur when the thin fibrous cap tissue overlying the necrotic core in a coronary vessel is torn, ripped or fissured under the action of high blood pressure. From a biomechanics point of view, the rupture of an atheroma is due to increased mechanical stresses in the lesion, in which the ultimate stress (i.e. peak circumferential stress (PCS) at failure) of the tissue is exceeded. Several factors including the cap thickness, morphology, residual stresses and tissue composition of the atheroma have been shown to affect the PCS. Also important, we recently demonstrated that microcalcifications (μCalcs>5µm are a common feature in human atheroma caps, which behave as local stress concentrators, increasing the local tissue stress by at least a factor of two surpassing the ultimate stress threshold for cap tissue rupture. In the present study, we used both idealized µCalcs with spherical shape and actual µCalcs from human coronary atherosclerotic caps, to determine their effect on increasing the circumferential stress in the fibroatheroma cap using different hyperelastic constitutive models. We have found that the stress concentration factor (SCF) produced by μCalcs in the fibroatheroma cap is affected by the material tissue properties, μCalcs spacing, aspect ratio and their alignment relative to the tensile axis of the cap.

Stress Distribution in a Bilayer Elastic Model of a Coronary Artery

In earlier studies on stress distribution in arteries, a monolayer wall model was often used. An arterial wall consists of three layers, the intima, the media, and the adventitia. The intima is mechanically negligible as a stress supporting layer against the blood pressure in young healthy vessels, although it is important as an interface between blood and arterial wall. The media and adventitia layers are considered to support blood pressure. Recently, residual strain and a constitutive law for porcine coronary arteries have been investigated in separated media and adventitia. Using the data obtained through these investigations, a stress analysis considering residual stress (strain) in each layer was performed in this study, and residual strain and stress were computed for a bilayer model. The circumferential residual stress was compressive in the inner region, tensile in the outer region, and had discontinuity at the boundary between the media and adventitia. A peak circumferential stress occurred in the media at the boundary between the media and adventitia under a physiological condition, and an almost flat distribution was obtained in the adventitia. This pattern does not change under a hypertensive condition. These results suggest that a remodeling with hypertension occurs in the media.

The explosive growth of small voids in vulnerable cap rupture; cavitation and interfacial debonding

While it is generally accepted that ruptures in fibrous cap atheromas cause most acute coronary deaths, and that plaque rupture occurs in the fibrous cap at the location where the tissue stress exceeds a certain critical peak circumferential stress, the exact mechanism of rupture initiation remains unclear. We recently reported the presence of multiple microcalcifications (μCalcs) <50μm diameter embedded within the fibrous cap, μCalcs that could greatly increase cap instability by introducing up to a 5-fold increase in local tissue stress. Here, we explore the hypothesis that, aside from cap thickness, μCalc size and interparticle spacing are principal determinants of cap rupture risk. Also, we propose that cap rupture is initiated near the poles of the μCalcs due to the presence of tiny voids that explosively grow at a critical tissue stress and then propagate across the fibrous cap. We develop a theoretical model based on classic studies in polymeric materials by Gent (1980), which indicates that cavitation as opposed to interfacial debonding is the more likely mechanism for cap rupture produced by μCalcs <65μm diameter. This analysis suggests that there is a critical μCalc size range, from 5μm to 65μm, in which cavitation should be prevalent. This hypothesis for cap rupture is strongly supported by our latest high resolution μCT studies in which we have observed trapped voids in the vicinity of μCalcs within fibrous caps in human coronaries.

A mechanistic analysis of the role of microcalcifications in atherosclerotic plaque stability: potential implications for plaque rupture

The role of microcalcifications (μCalcs) in the biomechanics of vulnerable plaque rupture is examined. Our laboratory previously proposed (Ref. 44), using a very limited tissue sample, that μCalcs embedded in the fibrous cap proper could significantly increase cap instability. This study has been greatly expanded. Ninety-two human coronary arteries containing 62 fibroatheroma were examined using high-resolution microcomputed tomography at 6.7-μm resolution and undecalcified histology with special emphasis on calcified particles <50 μm in diameter. Our results reveal the presence of thousands of μCalcs, the vast majority in lipid pools where they are not dangerous. However, 81 μCalcs were also observed in the fibrous caps of nine of the fibroatheroma. All 81 of these μCalcs were analyzed using three-dimensional finite-element analysis, and the results were used to develop important new clinical criteria for cap stability. These criteria include variation of the Young's modulus of the μCalc and surrounding tissue, μCalc size, and clustering. We found that local tissue stress could be increased fivefold when μCalcs were closely spaced, and the peak circumferential stress in the thinnest nonruptured cap (66 μm) if no μCalcs were present was only 107 kPa, far less than the proposed minimum rupture threshold of 300 kPa. These results and histology suggest that there are numerous μCalcs < 15 μm in the caps, not visible at 6.7-μm resolution, and that our failure to find any nonruptured caps between 30 and 66 μm is a strong indication that many of these caps contained μCalcs.

Failure of non-conventional composite pipes — an experimental and numerical approach

This paper discusses an experimental and numerical study to investigate the failure behavior of innovative and newly designed non-conventional cross-sectional fiber reinforced composite pipes subjected to internal pressure and bending loads. An adaptive filament winder for non-conventional pipes is exclusively designed and built to fabricate the test samples used in this investigation. Experiments are conducted on triangular and rectangular cross-sectioned samples as per ASTM standards to find the internal burst pressure, bending strength, and failure modes of the pipes. Numerical analysis for the pipe loading process has been developed based on the finite element method for linear orthotropic conditions for composite pipes. The finite element analysis is used to build the model and predict the stresses imposed on the non-conventional pipes. The relationships between the applied internal pressure and peak circumferential stress, bending load, and bending strength with reference to the fillet radius are determined; and generally a good correlation is found between the experimental and numerical results. This study has extended the use of non-conventional composite pipes in structural applications.

Computational Stress Analysis of Atherosclerotic Plaques in ApoE Knockout Mice

The aortic sinus lesions of apolipoprotein E knockout (ApoE KO) mice seldom show any signs of fibrous cap disruption, whereas cap ruptures have been recently reported in the proximal part of their brachiocephalic arteries (BCA). We use histology based finite element analysis to evaluate peak circumferential stresses in aortic and BCA lesions from six 42-56 week-old fat-fed ApoE KO mice. This analysis is able to both explain the greater stability of aortic lesions in mice and provide new insight into the BCA lesion as a model for the stability of human lesions with and without microcalcifications in their fibrous caps. The predicted average peak stress in fibrous caps of aortic lesions of 205.8 kPa is significantly lower than the average value of maximum stresses of 568.8 kPa in BCA caps. The aortic plaque stresses only slightly depend on the cap thickness, while BCA lesions demonstrate an exponential growth of peak cap stresses with decreasing cap thickness similar to human vulnerable plaques. Murine BCA ruptured lesions with mean cap thickness of 2 microm show stresses approximately 1400 kPa, three times higher than human ruptured plaques with a mean cap thickness of 23 microm without microcalcifications in the cap, but nearly identical to the peak stress around an elongated microcalcification with aspect ratio 2 in a human thin cap approximately 50 microm thick. We predict biomechanical stress patterns in mouse BCA close to human vulnerable plaques without microcalcification in the cap, while aortic lesions show stress tendency similar to stable lesions in human.