The predictive capability of aortic stiffness index for aortic dissection among dilated ascending aortas.

OBJECTIVE: We created a finite element model to predict the probability of dissection based on imaging-derived aortic stiffness and investigated the link between stiffness and wall tensile stress using our model. METHODS: Transthoracic echocardiogram measurements were used to calculate aortic diameter change over the cardiac cycle. Aortic stiffness index was subsequently calculated based on diameter change and blood pressure. A series of logistic models were developed to predict the binary outcome of aortic dissection using 1 or more series of predictor parameters such as aortic stiffness index or patient characteristics. Finite element analysis was performed on a subset of diameter-matched patients exhibiting patient-specific material properties. RESULTS: Transthoracic echocardiogram scans of patients with type A aortic dissection (n = 22) exhibited elevated baseline aortic stiffness index when compared with aneurysmal patients' scans with tricuspid aortic valve (n = 83, P < .001) and bicuspid aortic valve (n = 80, P < .001). Aortic stiffness index proved an excellent discriminator for a future dissection event (area under the curve, 0.9337, odds ratio, 2.896). From the parametric finite element study, we found a correlation between peak longitudinal wall tensile stress and stiffness index (ρ = .6268, P < .001, n = 28 pooled). CONCLUSIONS: Noninvasive transthoracic echocardiogram-derived aortic stiffness measurements may serve as an impactful metric toward predicting aortic dissection or quantifying dissection risk. A correlation between longitudinal stress and stiffness establishes an evidence-based link between a noninvasive stiffness parameter and stress state of the aorta with clinically apparent dissection events.

Aortic Dissection is Determined by Specific Shape and Hemodynamic Interactions.

The aim of this study was to determine whether specific three-dimensional aortic shape features, extracted via statistical shape analysis (SSA), correlate with the development of thoracic ascending aortic dissection (TAAD) risk and associated aortic hemodynamics. Thirty-one patients followed prospectively with ascending thoracic aortic aneurysm (ATAA), who either did (12 patients) or did not (19 patients) develop TAAD, were included in the study, with aortic arch geometries extracted from computed tomographic angiography (CTA) imaging. Arch geometries were analyzed with SSA, and unsupervised and supervised (linked to dissection outcome) shape features were extracted with principal component analysis (PCA) and partial least squares discriminant analysis (PLS-DA), respectively. We determined PLS-DA to be effective at separating dissection and no-dissection patients ([Formula: see text]), with decreased tortuosity and more equal ascending and descending aortic diameters associated with higher dissection risk. In contrast, neither PCA nor traditional morphometric parameters (maximum diameter, tortuosity, or arch volume) were effective at separating dissection and no-dissection patients. The arch shapes associated with higher dissection probability were supported with hemodynamic insight. Computational fluid dynamics (CFD) simulations revealed a correlation between the PLS-DA shape features and wall shear stress (WSS), with higher maximum WSS in the ascending aorta associated with increased risk of dissection occurrence. Our work highlights the potential importance of incorporating higher dimensional geometric assessment of aortic arch anatomy in TAAD risk assessment, and in considering the interdependent influences of arch shape and hemodynamics as mechanistic contributors to TAAD occurrence.

Prediction of bleb formation in intracranial aneurysms using machine learning models based on aneurysm hemodynamics, geometry, location, and patient population.

BACKGROUND: Bleb presence in intracranial aneurysms (IAs) is a known indication of instability and vulnerability. OBJECTIVE: To develop and evaluate predictive models of bleb development in IAs based on hemodynamics, geometry, anatomical location, and patient population. METHODS: Cross-sectional data (one time point) of 2395 IAs were used for training bleb formation models using machine learning (random forest, support vector machine, logistic regression, k-nearest neighbor, and bagging). Aneurysm hemodynamics and geometry were characterized using image-based computational fluid dynamics. A separate dataset with 266 aneurysms was used for model evaluation. Model performance was quantified by the area under the receiving operating characteristic curve (AUC), true positive rate (TPR), false positive rate (FPR), precision, and balanced accuracy. RESULTS: The final model retained 18 variables, including hemodynamic, geometrical, location, multiplicity, and morphology parameters, and patient population. Generally, strong and concentrated inflow jets, high speed, complex and unstable flow patterns, and concentrated, oscillatory, and heterogeneous wall shear stress patterns together with larger, more elongated, and more distorted shapes were associated with bleb formation. The best performance on the validation set was achieved by the random forest model (AUC=0.82, TPR=91%, FPR=36%, misclassification error=27%). CONCLUSIONS: Based on the premise that aneurysm characteristics prior to bleb formation resemble those derived from vascular reconstructions with their blebs virtually removed, machine learning models can identify aneurysms prone to bleb development with good accuracy. Pending further validation with longitudinal data, these models may prove valuable for assessing the propensity of IAs to progress to vulnerable states and potentially rupturing.

An exploratory assessment of stretch-induced transmural myocardial fiber kinematics in right ventricular pressure overload.

Right ventricular (RV) remodeling and longitudinal fiber reorientation in the setting of pulmonary hypertension (PH) affects ventricular structure and function, eventually leading to RV failure. Characterizing the kinematics of myocardial fibers helps better understanding the underlying mechanisms of fiber realignment in PH. In the current work, high-frequency ultrasound imaging and structurally-informed finite element (FE) models were employed for an exploratory evaluation of the stretch-induced kinematics of RV fibers. Image-based experimental evaluation of fiber kinematics in porcine myocardium revealed the capability of affine assumptions to effectively approximate myofiber realignment in the RV free wall. The developed imaging framework provides a noninvasive modality to quantify transmural RV myofiber kinematics in large animal models. FE modeling results demonstrated that chronic pressure overload, but not solely an acute rise in pressures, results in kinematic shift of RV fibers towards the longitudinal direction. Additionally, FE simulations suggest a potential protective role for concentric hypertrophy (increased wall thickness) against fiber reorientation, while eccentric hypertrophy (RV dilation) resulted in longitudinal fiber realignment. Our study improves the current understanding of the role of different remodeling events involved in transmural myofiber reorientation in PH. Future experimentations are warranted to test the model-generated hypotheses.

Blebs in intracranial aneurysms: prevalence and general characteristics.

BACKGROUND: Blebs are rupture risk factors in intracranial aneurysms (IAs), but their prevalence, distribution, and associations with clinical factors as well as their causes and effects on aneurysm vulnerability remain unclear. METHODS: A total of 122 blebs in 270 IAs selected for surgery were studied using patient-specific vascular reconstructions from 3D angiographic images. Bleb geometry, location on the aneurysm, and frequency of occurrence in aneurysms at different locations were analyzed. Associations between gender, age, smoking, hypertension, hormone therapy, dental infection, and presence of blebs were investigated. RESULTS: Of all aneurysms with blebs, 77% had a single bleb and 23% had multiple blebs. Only 6% of blebs were at the neck, while 46% were in the body and 48% in the dome. Aneurysms with blebs were larger (p<0.0001), more elongated (p=0.0002), and with wider necks than aneurysms without blebs. Bleb presence was associated with dental infection (p=0.0426) and negatively associated with hormone therapy (p=0.0426) in women. Anterior and posterior communicating arteries had larger percentages of aneurysms with blebs than internal carotid arteries. Patients with a history of hypertension tended to have a larger percentage of aneurysms with blebs. However, these trends did not reach significance in this sample. CONCLUSIONS: Blebs are common in IAs, and most aneurysms harboring blebs have a single bleb. Blebs in the aneurysm neck are rare, but they are equally common in the body and dome. The presence of blebs in IAs was associated with dental infection, and negatively associated with hormone replacement therapy.

Hemodynamics in aneurysm blebs with different wall characteristics.

BACKGROUND: Blebs are important secondary structures of intracranial aneurysms associated with increased rupture risk and can affect local wall stress and hemodynamics. Mechanisms of bleb development and evolution are not clearly understood. We investigate the relationship between blebs with different wall characteristics and local hemodynamics and rupture sites. METHODS: Blebs with different wall appearances in intra-operative videos were analyzed with image-based computational fluid dynamics. Thin red blebs were compared against thick atherosclerotic/hyperplastic white/yellow blebs. Rupture points were identified in videos of ruptured aneurysms harboring blebs. RESULTS: Thin blebs tended to be closer to the inflow than atherosclerotic blebs of the same aneurysm (P=0.0234). Blebs near the inflow had higher velocity (P=0.0213), vorticity (P=0.0057), shear strain rate (P=0.0084), wall shear stress (WSS) (P=0.0085), and WSS gradient (P=0.0151) than blebs far from the inflow. In a subset of 12 ruptured aneurysms harboring blebs, rupture points were associated with thin blebs in 42% of aneurysms, atherosclerotic blebs in 25%, and were away from blebs in the remaining 33%. CONCLUSIONS: Not all blebs are equal; some have thin translucent walls while others have thick atherosclerotic walls. Thin blebs tend to be located closer to the inflow than atherosclerotic blebs. Blebs near the inflow are exposed to stronger flows with higher and spatially variable WSS than blebs far from the inflow which tend to have uniformly lower WSS. Aneurysms can rupture at thin blebs, atherosclerotic blebs, and even away from blebs. Further study of wall failure in aneurysms with different bleb types is needed.

Computational modeling of the strength of the ascending thoracic aortic media tissue under physiologic biaxial loading conditions.

Type A Aortic Dissection (TAAD) is a life-threatening condition involving delamination of ascending aortic media layers. While current clinical guidelines recommend surgical intervention for aneurysm diameter > 5.5 cm, high incidence of TAAD in patients below this diameter threshold indicates the pressing need for improved evidence-based risk prediction metrics. Construction of such metrics will require the knowledge of the biomechanical failure properties of the aortic wall tissue under biaxial loading conditions. We utilized a fiber-level finite element based structural model of the aortic tissue to quantify the relationship between aortic tissue strength and physiologically relevant biaxial stress state for nonaneurysmal and aneurysmal patient cohorts with tricuspid aortic valve phenotype. We found that the model predicted strength of the aortic tissue under physiologic biaxial loading conditions depends on the stress biaxiality ratio, defined by the ratio of the longitudinal and circumferential components of the tissue stress. We determined that predicted biaxial tissue strength is statistically similar to its uniaxial circumferential strength below biaxiality ratios of 0.68 and 0.69 for nonaneurysmal and aneurysmal cohorts, respectively. Beyond this biaxiality ratio, predicted biaxial strength for both cohorts reduced drastically to a magnitude statistically similar to its longitudinal strength. We identified fiber-level failure mechanisms operative under biaxial stress state governing aforementioned tissue failure behavior. These findings are an important first step towards the development of mechanism-based TAAD risk assessment metrics for early identification of high-risk patients.

Effect of localized tendon remodeling on supraspinatus tear propagation.

Rotator cuff tear propagation is multifactorial and may be due to localized changes in mechanical properties from tendon remodeling based on the inhomogeneous stresses experienced by a tendon with a tear. The objective of this study was to investigate the effect of localized tendon remodeling on tear propagation for simulated supraspinatus tendon tears. A validated computational model of a supraspinatus tendon using subject-specific geometry and material properties with a 1 cm wide anterior tear was used. The medial edge of the supraspinatus tendon was displaced 5 mm to induce tear propagation and cohesive elements were used to model tear propagation. Four remodeling scenarios were investigated: (1) Baseline (no remodeling), (2) Positive remodeling (increased fiber stiffness) and (3) Negative remodeling (decreased fiber stiffness) at tear tips, and (4) Negative remodeling along the medial-lateral tear edge. Output parameters included the amount of tear propagation, critical load to propagate the tear, and maximum principal stress at the tear tips. Positive remodeling at the tear tips resulted in the largest amount of tear propagation (18.4 mm), highest peak maximum principal stress (25.2 MPa), and lowest critical load to propagate the tear (249N). Conversely, negative remodeling at the tear tips resulted in the least amount of tear propagation (16 mm), lowest peak maximum principal stress (17.6 MPa) and highest critical load to propagate the tear (278N). Overall, remodeling at the tear tips has the greatest effect on tear propagation. Therefore, a better method for clinicians to measure tendon stiffness at the tear tips would be helpful to improve outcome of patients.

Novel simulations to determine the impact of superficial perineal structures on vaginal delivery.

This study's aim was to determine whether the inclusion of superficial perineal structures in a finite-element simulation of vaginal delivery impacts the pubovisceral muscle and perineal body, two common sites of birth-related injury. The hypothesis, inferred from prevailing literature, was that these structures would have minimal influence (differences less than ±10%). Two models were made using the Visible Human Project's female cadaver to create a rigid, fixed pelvis, musculature held by spring attachments to that pelvis, and a rigid, ellipsoidal fetal head prescribed with an inferior displacement to simulate delivery. Injury site stretch ratios and fetal head and perineal body displacements and angles of progression were compared between the Omitted Model (which excluded the superficial perineal structures as is common practice) and the Included Model (which included them). Included Model stretch ratios were +107%, -9.84% and -14.6% compared to Omitted Model perineal body and right and left pubovisceral muscles, respectively. Included Model peak perineal body inferior displacement was +72.5% greater while similar anterior-posterior displacements took longer to reach. These results refute our hypothesis, suggesting superficial perineal structures impact simulations of vaginal delivery by inhibiting perineal body anterior-posterior displacement, which stretches and inferiorly displaces the perineal body.

Computational modeling reveals the relationship between intrinsic failure properties and uniaxial biomechanical behavior of arterial tissue.

Biomechanical failure of the artery wall can lead to rupture, a catastrophic event with a high rate of mortality. Thus, there is a pressing need to understand failure behavior of the arterial wall. Uniaxial testing remains the most common experimental technique to assess tissue failure properties. However, the relationship between intrinsic failure parameters of the tissue and measured uniaxial failure properties is not fully established. Furthermore, the effect of the experimental variables, such as specimen shape and boundary conditions, on the measured failure properties is not well understood. We developed a finite element model capable of recapitulating pre-failure and post-failure uniaxial biomechanical response of the arterial tissue specimen. Intrinsic stiffness, strength and fracture toughness of the vessel wall tissue were used as the input material parameters to the model. Two uniaxial testing protocols were considered: a conventional setup with a rectangular specimen held at the grips by cardboard inserts, and the other used a dogbone specimen with soft foam inserts at the grips. Our computational study indicated negligible differences in the peak stress and post-peak mechanical behavior between these two testing protocols. It was also found that the tissue experienced only modest localized failure until higher levels of applied stretch beyond the peak stress. A robust cohesive model was capable of modeling the post-peak biomechanical response, which was primarily governed by tissue fracture toughness. Our results suggest that the post-peak region, in conjunction with the peak stress, must be considered to evaluate the complete biomechanical failure behavior of the soft tissue.

A Validated, Specimen-Specific Finite Element Model of the Supraspinatus Tendon Mechanical Environment.

Rotator cuff tears are a significant clinical problem previously investigated by unvalidated computational models that either use simplified geometry or isotropic elastic material properties to represent the tendon. The objective of this study was to develop an experimentally validated, finite element model of supraspinatus tendon using specimen-specific geometry and inhomogeneous material properties to predict strains in intact supraspinatus tendon at multiple abduction angles. Three-dimensional tendon surface strains were determined at 60 deg, 70 deg, and 90 deg of glenohumeral abduction for articular and bursal surfaces of supraspinatus tendon during cyclic loading (5-200 N, 50 cycles, 20 mm/min) to serve as validation data for computational model predictions. A finite element model was developed using the tendon geometry and inhomogeneous material properties to predict surface strains for loading conditions mimicking experimental loading conditions. Experimental strains were directly compared with computational model predictions to validate the model. Overall, the model successfully predicted magnitudes of strains that were within the experimental repeatability of 3% strain of experimental measures on both surfaces of the tendon. Model predictions and experiments showed the largest strains to be located on the articular surface (∼8% strain) between the middle and the anterior edge of the tendon. Importantly, the reference configuration chosen to calculate strains had a significant effect on strain calculations, and therefore, must be defined with an innovative optimization algorithm. This study establishes a rigorously validated specimen-specific (both geometry and material properties) computational model using novel surface strain measurements for the use in investigating the function of the supraspinatus tendon and to ultimately predict the propagation of supraspinatus tendon tears based on the tendon's mechanical environment.

Swine Vagina Under Planar Biaxial Loads: An Investigation of Large Deformations and Tears.

Vaginal tears are very common and can lead to severe complications such as hemorrhaging, fecal incontinence, urinary incontinence, and dyspareunia. Despite the implications of vaginal tears on women's health, there are currently no experimental studies on the tear behavior of vaginal tissue. In this study, planar equi-biaxial tests on square specimens of vaginal tissue, with sides oriented along the longitudinal direction (LD) and circumferential direction (CD), were conducted using swine as animal model. Three groups of specimens were mechanically tested: the NT group (n = 9), which had no pre-imposed tear, the longitudinal tear (LT) group (n = 9), and the circumferential tear (CT) group (n = 9), which had central pre-imposed elliptically shaped tears with major axes oriented in the LD and the CD, respectively. Through video recording during testing, axial strains were measured for the NT group using the digital image correlation (DIC) technique and axial displacements of hook clamps were measured for the NT, LT, and CT groups in the LD and CD. The swine vaginal tissue was found to be highly nonlinear and somewhat anisotropic. Up to normalized axial hook displacements of 1.15, no tears were observed to propagate, suggesting that the vagina has a high resistance to further tearing once a tear has occurred. However, in response to biaxial loading, the size of the tears for the CT group increased significantly more than the size of the tears for the LT group (p = 0.003). The microstructural organization of the vagina is likely the culprit for its tear resistance and orientation-dependent tear behavior. Further knowledge on the structure-function relationship of the vagina is needed to guide the development of new methods for preventing the severe complications of tearing.

Predissection-derived geometric and distensibility indices reveal increased peak longitudinal stress and stiffness in patients sustaining acute type A aortic dissection: Implications for predicting dissection.

OBJECTIVE: To assess ascending aortic distensibility and build geometry and distensibility-based patient-specific stress distribution maps in patients sustaining type A aortic dissection (TAAD) using predissection noninvasive imaging. METHODS: Review of charts from patients undergoing surgical repair of TAAD (n = 351) led to the selection of a subset population (n = 7) with 2 or more predissection computed tomography angiography scans and echocardiograms at least 1 year before dissection. Ascending aortic wall biomechanical properties (aortic strain, distensibility, and stiffness) were compared with age- and size-matched nondissected nonaneurysmal controls. Patient-specific aortic strain served as an input in aortic geometry-based simulated 3-dimensional reconstructions to generate longitudinal and circumferential wall stress maps. Inspection of perioperative dissection scans and intraoperative visual examination confirmed primary tear locations. RESULTS: Predissection echocardiography revealed ascending aortas of patients sustaining TAAD to exhibit decreased aortic wall strain (14.50 ± 1.13% vs 8.49 ± 1.08%; P < .01), decreased distensibility (4.26 ± 0.44 vs 2.39 ± 0.33 10-6 cm2·dyne-1; P < .01), increased stiffness (3.84 ± 0.24 vs 7.48 ± 1.05; P < .001), and increased longitudinal wall stress (246 ± 22 vs 172 ± 37 kPa; P < .01). There was no significant difference in circumferential wall stress. Predissection computed tomography angiography models revealed overlap between regions of increased longitudinal wall stress and primary tear sites. CONCLUSIONS: Using predissection imaging, we identified increased stiffness and longitudinal wall stress in ascending aortas of patients with dissection. Patient-specific imaging-derived biomechanical property maps like these may be instrumental toward designing better prediction models of aortic dissection potential.

Effects of Tendon Degeneration on Predictions of Supraspinatus Tear Propagation.

Rotator cuff tendons undergo degeneration with age, which could have an impact on tear propagation. The objective of this study was to predict tear propagation for different levels of tissue degeneration using an experimentally validated finite element model of a supraspinatus tendon. It was hypothesized that greater amounts of degeneration will result in tear propagation at lower loads than tendons with less degeneration. Using a previously-validated computational model of supraspinatus tendon, 1-cm tears were introduced in the anterior, middle, and posterior thirds of the tendon. Cohesive elements were assigned subject-specific failure properties to model tear propagation, and tendon degeneration ranging from "minimal" to "severe" was modeled by modifying its mechanical properties. Tears in tendons with severe degeneration required the smallest loads to propagate (122-207 N). Posterior tears required greater loads compared to middle and anterior tears at all levels of degeneration. Stress and strain required for tear propagation decreased substantially with degeneration, ranging from 8.5 MPa and 32.6% strain for minimal degeneration and 0.6 MPa and 4.5% strain for severe degeneration. Overall, this work indicates that greater amounts of tendon degeneration lead to greater risk of tear propagation, supporting the need for early detection and treatment of rotator cuff tears.

Necking and drawing of rubber-plastic bilayer laminates.

We examine the stretching behavior of rubber-plastic composites composed of a layer of styrene-ethylene/propylene-styrene (SEPS) rubber, bonded to a layer of linear low density polyethylene (LLDPE) plastic. Dog-bone shaped samples of rubber, plastic, and rubber-plastic bilayers with rubber : plastic thickness ratio in the range of 1.2-9 were subjected to uniaxial tension tests. The degree of inhomogeneity of deformation was quantified by digital image correlation analysis of video recordings of these tests. In tension, the SEPS layer showed homogeneous deformation, whereas the LLDPE layer showed necking followed by stable drawing owing to its elastoplastic deformation behavior and post-yield strain hardening. Bilayer laminates showed behavior intermediate between the plastic and the rubber, with the degree of necking and drawing reducing as the rubber : plastic ratio increased. A simple model was developed in which the force in the bilayer was taken as the sum of forces in the plastic and the rubber layers measured independently. By applying a mechanical energy balance to this model, the changes in bilayer necking behavior with rubber thickness could be predicted qualitatively.

Mechanics of anesthetic needle penetration into human sciatic nerve.

Nerve blocks are frequently performed by anesthesiologists to control pain. For sciatic nerve blocks, the optimal placement of the needle tip between its paraneural sheath and epineurial covering is challenging, even under ultrasound guidance, and frequently results in nerve puncture. We performed needle penetration tests on cadaveric isolated paraneural sheath (IPS), isolated nerve (IN), and the nerve with overlying paraneural sheath (NPS), and quantified puncture force requirement and fracture toughness of these specimens to assess their role in determining the clinical risk of nerve puncture. We found that puncture force (123 ±â€¯17 mN) and fracture toughness (45.48 ±â€¯9.72 J m-2) of IPS was significantly lower than those for NPS (1440 ±â€¯161 mN and 1317.46 ±â€¯212.45 Jm-2, respectively), suggesting that it is not possible to push the tip of the block needle through the paraneural sheath only, without pushing it into the nerve directly, when the sheath is lying directly over the nerve. Results of this study provide a physical basis for tangential placement of the needle as the ideal situation for local anesthetic deposition, as it allows for the penetration of the sheath along the edge of the nerve without entering the epineurium.

Structural modeling reveals microstructure-strength relationship for human ascending thoracic aorta.

High lethality of aortic dissection necessitates accurate predictive metrics for dissection risk assessment. The not infrequent incidence of dissection at aortic diameters <5.5 cm, the current threshold guideline for surgical intervention (Nishimura et al., 2014), indicates an unmet need for improved evidence-based risk stratification metrics. Meeting this need requires a fundamental understanding of the structural mechanisms responsible for dissection evolution within the vessel wall. We present a structural model of the repeating lamellar structure of the aortic media comprised of elastic lamellae and collagen fiber networks, the primary load-bearing components of the vessel wall. This model was used to assess the role of these structural features in determining in-plane tissue strength, which governs dissection initiation from an intimal tear. Ascending aortic tissue specimens from three clinically-relevant patient populations were considered: non-aneurysmal aorta from patients with morphologically normal tricuspid aortic valve (CTRL), aneurysmal aorta from patients with tricuspid aortic valve (TAV), and aneurysmal aorta from patients with bicuspid aortic valve (BAV). Multiphoton imaging derived collagen fiber organization for each patient cohort was explicitly incorporated in our model. Model parameters were calibrated using experimentally-measured uniaxial tensile strength data in the circumferential direction for each cohort, while the model was validated by contrasting simulated tissue strength against experimentally-measured strength in the longitudinal direction. Orientation distribution, controlling the fraction of loaded collagen fibers at a given stretch, was identified as a key feature governing anisotropic tissue strength for all patient cohorts.

Effects of tear size and location on predictions of supraspinatus tear propagation.

Rotator cuff tears remain a significant clinical problem with a high incidence rate and severe clinical burden. Previous computational models developed to study rotator cuff tears have not modeled tissue damage and tear propagation. The objective of this study was to predict tear propagation for various combinations of tear size and location using an experimentally validated finite element model of supraspinatus tendon. It was hypothesized that larger rotator cuff tears propagate at lower loads than smaller tears, and that posterior tears require higher loads to propagate than anterior tears. Using a previously validated computational model of supraspinatus tendon, tears of size 0.5-1.5 cm were introduced to the tendon geometry in the anterior, middle, and posterior tendon thirds. Cohesive elements were assigned subject-specific failure properties and used to model tissue damage and tear propagation. A displacement of 5 mm was applied to the medial tendon edge to induce tear propagation. Model outputs included critical load required to propagate the tear, and principal stress and maximum principal strain at the anterior and posterior tear tips. For all tear sizes, posterior tears required the highest loads to propagate (247-567 N). Anterior tears generally required the least load to propagate (171-280 N). Stress and strain were larger on the articular side (maximum 33.9% articular strain vs 27.8% bursal strain). Overall, larger tears located in the anterior supraspinatus tendon that interrupt the rotator cable are most at risk for tear propagation, and should be carefully followed by clinicians when considering treatment options.

Stretching-induced wrinkling in plastic-rubber composites.

We examine the mechanics of three-layer composite films composed of an elastomeric layer sandwiched between two thin surface layers of plastic. Upon stretching and releasing such composite films, they develop a highly wrinkled surface texture. The mechanism for this texturing is that during stretching, the plastic layers yield and stretch irreversibly whereas the elastomer stretches reversibly. Thus upon releasing, the plastic layers buckle due to compressive stress imposed by the elastomer. Experiments are conducted using SEPS elastomer and 50 micron thick LLDPE plastic films. Stretching and releasing the composites to 2-5 times their original length induces buckles with wavelength on the order of 200 microns, and the wavelength decreases as the stretching increases. FEM simulations reveal that plastic deformation is involved at all stages during this process: (1) during stretching, the plastic layer yields in tension; (2) during recovery, the plastic layer first yields in-plane in compression and then buckles; (3) post-buckling, plastic hinges are formed at high-curvature regions. Homogeneous wrinkles are predicted only within a finite window of material properties: if the yield stress is too low, the plastic layers yield in-plane, without wrinkling, whereas if the yield stress is too high, non-homogeneous wrinkles are predicted. This approach to realizing highly wrinkled textures offers several advantages, most importantly the fact that high aspect ratio wrinkles (amplitude to wavelength ratios exceeding 0.4) can be realized.