Abdominal aortic aneurysm (AAA) is the gradual weakening and dilation of the infrarenal aorta. This disease is progressive, asymptomatic, and can eventually lead to rupture—a catastrophic event leading to massive internal bleeding and possibly death. The mechanical environment present in AAA is currently thought to be important in disease initiation, progression, and diagnosis. In this study, we utilize porohyperelastic (PHE) finite element models (FEMs) to investigate how such modeling can be used to better understand the local biomechanical environment in AAA. A 3D hypothetical AAA was constructed with a preferential anterior bulge assuming both the intraluminal thrombus (ILT) and the AAA wall act as porous materials. A parametric study was performed to investigate how physiologically meaningful variations in AAA wall and ILT hydraulic permeabilities affect luminal interstitial fluid velocities and wall stresses within an AAA. A corresponding hyperelastic (HE) simulation was also run in order to be able to compare stress values between PHE and HE simulations. The effect of AAA size on local interstitial fluid velocity was also investigated by simulating maximum diameters (5.5 cm, 4.5 cm, and 3.5 cm) at the baseline values of ILT and AAA wall permeability. Finally, a cyclic PHE simulation was utilized to study the variation in local fluid velocities as a result of a physiologic pulsatile blood pressure. While the ILT hydraulic permeability was found to have minimal affect on interstitial velocities, our simulations demonstrated a 28% increase and a 20% decrease in luminal interstitial fluid velocity as a result of a 1 standard deviation increase and decrease in AAA wall hydraulic permeability, respectively. Peak interstitial velocities in all simulations occurred on the luminal surface adjacent to the region of maximum diameter. These values increased with increasing AAA size. PHE simulations resulted in 19.4%, 40.1%, and 81.0% increases in peak maximum principal wall stresses in comparison to HE simulations for maximum diameters of 35 mm, 45 mm, and 55 mm, respectively. The pulsatile AAA PHE FEM demonstrated a complex interstitial fluid velocity field the direction of which alternated in to and out of the luminal layer of the ILT. The biomechanical environment within both the aneurysmal wall and the ILT is involved in AAA pathogenesis and rupture. Assuming these tissues to be porohyperelastic materials may provide additional insight into the complex solid and fluid forces acting on the cells responsible for aneurysmal remodeling and weakening.

Purpose Progressively deteriorating visual field is a characteristic feature of primary open-angle glaucoma (POAG), and the biomechanics of optic nerve head (ONH) is believed to be important in its onset. We used porohyperelasticity to model the complex porous behavior of ocular tissues to better understand the effect variations in ocular material properties can have on ONH biomechanics. Methods An axisymmetric model of the human eye was constructed to parametrically study how changes in the permeabilities of retina-Bruch's-choroid complex (kRBC), sclera (ksclera), uveoscleral pathway (kUVSC), and trabecular meshwork (kTM) as well as how changes in the stiffness of the lamina cribrosa (LC) and sclera affect IOP, LC strains, and translaminar interstitial pressure gradients (TLIPG). Results Decreasing kRBC from 5×10−12m/s to 5×10−13m/s increased IOP and LC strains by 17%, and TLIPG by 21%. LC strains increased by 13% and 9% when the scleral and LC moduli were decreased by 48% and 50%, respectively. Conclusions In addition to the trabecular meshwork and uveoscleral pathway, the retina-Bruch's-choroid complex had an important effect on IOP, LC strains, and TLIPG. Changes in kRBC and scleral modulus resulted in nonlinear changes in the IOP, and LC strains especially at the lowest kTM and kUVSC. This study demonstrates that porohyperelastic modeling provides a novel method for computationally studying the biomechanical environment of the ONH. Porohyperelastic simulations of ocular tissues may help provide further insight into the complex biomechanical environment of posterior ocular tissues in POAG.

An aneurysm is a gradual and progressive ballooning of a blood vessel due to wall degeneration. Rupture of abdominal aortic aneurysm (AAA) constitutes a significant portion of deaths in the US. In this study, we describe a technique to reconstruct AAA geometry from CT images in an inexpensive and streamlined fashion. A 3D reconstruction technique was implemented with a GUI interface in MATLAB using the active contours technique. The lumen and the thrombus of the AAA were segmented individually in two separate protocols and were then joined together into a hybrid surface. This surface was then used to obtain the aortic wall. This method can deal with very poor contrast images where the aortic wall is indistinguishable from the surrounding features. Data obtained from the segmentation of image sets were smoothed in 3D using a Support Vector Machine technique. The segmentation method presented in this paper is inexpensive and has minimal user-dependency in reconstructing AAA geometry (lumen and wall) from patient image sets. The AAA model generated using this segmentation algorithm can be used to study a variety of biomechanical issues remaining in AAA biomechanics including stress estimation, endovascular stent-graft performance, and local drug delivery studies.

Although strongly correlated with elevated intraocular pressure, primary open-angle glaucoma (POAG) occurs in normotensive eyes. Mechanical properties of the sclera around the optic nerve head (ONH) may play a role in this disparity. The purpose of this study is to present an automated inverse mechanics based approach to determine the distribution of heterogeneous mechanical properties of the human sclera as derived from its surface deformations arising from pressure inflation experiments. The scleral shell of a 78 year old European Descent male donor eye was utilized to demonstrate the method; the sclera was coated with a speckle pattern on the outer surface and was subjected to inflation pressures of 5, 15, 30, and 45 mmHg. The speckle pattern was imaged at each pressure, and a displacement field was calculated for each pressure step using a previously described sequential digital image correlation (S-DIC) technique. The fiber splay and fiber orientation of the sclera collagen were determined experimentally, and the thickness across the scleral globe was determined using micro CT images. The displacement field from the inflation test was used to calculate the strain and also used as an input for inverse mechanics to determine the heterogeneity of material properties. The scleral geometry was divided into subdomains using the first principal strain. The Holzapfel anisotropic material parameters of matrix and fiber stiffness were estimated within each individual subdomain using an inverse mechanics approach by minimizing the sum of the square of the residuals between the computational and experimental displacement fields. The mean and maximum error in displacement across all subdomains were 8.9 ± 3.0 μm and 13.2 μm, respectively. The full pressure-inflation forward mechanics experiment was done using subdomain-specific mechanical properties on the entire scleral surface. The proposed approach is effective in determining the distribution of heterogeneous mechanical properties of the human sclera in a user-independent manner. Our research group is currently utilizing this approach to better elucidate how scleral stiffness influences those at high risk for POAG.

Polymeric endoaortic paving (PEAP) may improve endovascular repair of abdominal aortic aneurysms (AAA) since it has the potential to treat patients with complex AAA geometries while reducing the incidence of migration and endoleak. Polycaprolactone (PCL)/polyurethane (PU) blends are proposed as PEAP materials due to their range of mechanical properties, thermoformability, and resistance to biodegradation. In this study, the reduction in AAA wall stress that can be achieved using PEAP was estimated and compared to that resulting from stent-grafts. This was accomplished by mechanically modeling the anisotropic response of PCL/PU blends and implementing these results into finite element model (FEM) simulations. We found that at the maximum diameter of the AAA, the 50/50 and 10/90 PCL/PU blends reduced wall stress by 99 and 98%, respectively, while a stent-graft reduced wall stress by 99%. Our results also show that wall stress reduction increases with increasing PEAP thickness and PCL content in the blend ratio. These results indicate that PEAP can reduce AAA wall stress as effectively as a stent-graft. As such, we propose that PEAP may provide an improved treatment alternative for AAA, since many of the limitations of stent-grafts have the potential to be solved using this approach.

Abdominal aortic aneurysm (AAA) is a progressive dilation of the infrarenal aorta and results in a significant alteration in local hemodynamic environment [1]. While an aneurysmal diameter of 5.5cm is typically classified as being of high risk, recent studies have demonstrated that maximum wall stress could be a better indicator of an AAA rupture than maximum diameter [2]. The wall stress is greatly influenced by the blood pressure, aneurysm diameter, shape, wall thickness and the presence of thrombus. The work done by Finol et al. suggested that hemodynamic pressure variations have an insignificant effect on AAA wall stress and that primarily the shape of the aneurysm determines the stress distribution. They noted that for peak wall stress studies the static pressure conditions would suffice as the in vivo conditions. Wang et al have developed an isotropic hyperelastic constitutive model for the intraluminal thrombus (ILT). Such models have been used to study the stress distributions in patient specific AAAs [3, 4].

There is a general consensus that elevation in intraocular pressure (IOP), due to a reduced outflow of aqueous humor, is a major factor leading to primary open-angle glaucoma (POAG). Studies indicated that the damage of the optic nervehead (ONH), due to the biomechanical environment in and around the lamina cribrosa (LC), could be an important event leading to POAG [Morgan]. Since experimentally testing tissues of such small dimensions is difficult, many researchers resorted to computationally model the biomechincal environment inside the eye [Avatar, Kobayashi, Sigal, Uchio, Xu, Tandon]. It also gives the flexibility to parametrically study and isolate the effects of individual tissues on the IOP and LC. Many of these studies involve stress analysis on a hypothetical geometry (for e.g. spherical or axisymmetric hemisphere) using a variety of constitutive models (for e.g. elastic, biphasic etc) to study the static, and dynamic response of the IOP [Tandon, Sigal].

One of the important events leading to loss of vision in primary open-angle glaucoma — the 2 nd leading cause of blindness in the US [1] — is the death of retinal ganglion cells. Previous research has established a strong correlation between elevated intraocular pressure (IOP) and the incidence of glaucoma [2]. Stiffening of ocular tissues (like sclera) and axonal damage in the optic nerve head (ONH) were found to occur in response to elevated IOPs [3, 4]. As such, the biomechanical environment in and around the ONH, which is surrounded by the sclera and through which the visual information exits the eye, could be important in the incidence of this disease. Additionally, race and ethnicity factors were found to affect the incidence of glaucoma [5].