Identification and characterisation of regional variations in the material properties of ureter according to microstructure

Mechanical characteristics of the ureter and clinical implications

The ureteric wall is a complex multi-layered structure. The ureter shows variation in passive mechanical properties, histological morphology and insertion forces along the anatomical length. Ureter mechanical properties also vary depending on the direction of tensile testing and the anatomical region tested. Compliance is greatest in the proximal ureter and lower in the distal ureter, which contributes to the role of the ureter as a high-resistance sphincter. Similar to other human tissues, the ureteric wall remodels with age, resulting in changes to the mechanical properties. The passive mechanical properties of the ureter vary between species, and variation in tissue storage and testing methods limits comparison across some studies. Knowledge of the morphological and mechanical properties of the ureteric wall can aid in understanding urine transport and safety thresholds in surgical techniques. Indeed, various factors alter the forces required to insert access sheaths or scopes into the ureter, including sheath diameter, safety wires and medications. Future studies on human ureteric tissue both in vivo and ex vivo are required to understand the mechanical properties of the ureter and how forces influence these properties. Testing of instrument insertion forces in humans with a focus on defining safe upper limits and techniques to reduce trauma are also needed. Last, evaluation of dilatation limits in the mid and proximal ureter and clarification of tensile strength anisotropy in human specimens are necessary.

Predicting oxygen tension along the ureter

Continuous measurement of bladder urine oxygen tension (Po₂) is a method to potentially detect renal medullary hypoxia in patients at risk of acute kidney injury (AKI). To assess its practicality, we developed a computational model of the peristaltic movement of a urine bolus along the ureter and the oxygen exchange between the bolus and ureter wall. This model quantifies the changes in urine Po₂ as urine transits from the renal pelvis to the bladder. The model parameters were calibrated using experimental data in rabbits, such that most of the model predictions are within ±1 SE of the reported mean in the experiment, with the average percent difference being 7.0%. Based on parametric experiments performed using a model scaled to the geometric dimensions of a human ureter, we found that bladder urine Po₂ is strongly dependent on the bolus volume (i.e., bolus volume-to-surface area ratio), especially at a volume less than its physiological (baseline) volume (<0.2 mL). For the model assumptions, changes in peristaltic frequency resulted in a minimal change in bladder urine Po₂ (<1 mmHg). The model also predicted that there exists a family of linear relationships between the bladder-urine Po₂ and pelvic urine Po₂ for different input conditions. We conclude that it may technically be possible to predict renal medullary Po₂ based on the measurement of bladder urine Po₂, provided that there are accurate real-time measurements of model input parameters.NEW & NOTEWORTHY Measurement of bladder urine oxygen tension has been proposed as a new method to potentially detect the risk of acute kidney injury in patients. A computational model of oxygen exchange between urine bolus and ureteral tissue shows that it may be technically possible to determine the risk of acute kidney injury based on the measurement of bladder urine oxygen tension, provided that the measurement data are properly interpreted via a computational model.

Alterations with age in the biomechanical behavior of human ureteral wall: Microstructure-based modeling

The human ureters have not been thoroughly explored from the biomechanics perspective, despite the wealth of such data for other soft-tissue types. This study was motivated by the need to use relevant biomechanical data from human ureters and microstructure-based material formulations for simulations of ureteral peristalsis and stenting. Our starting choice was the four-fiber family model that has proven its validity as a descriptor of the multiaxial response of cardiovascular tissues. The degree of model complexity, required for rigorous fits to passive quasi-static pressure-diameter-force data at several axial stretches, was systematically investigated. Ureteral segments from sixteen human autopsy subjects were evaluated. A diagonal and axial family model allowed equally-good fits as the full model for all age groups and ureteral regions; considerably better than those allowed by the phenomenological Fung-type model whose root-mean-square error of fitting was three-fold greater. This reduced model mimicked the structure seen in histologic sections, namely plentiful diagonal collagen fibers in the lamina propria and axial fibers in the muscle and adventitia. The paucity of elastin fibers and mixed muscle orientation justified the use of isotropic muscle-dominated matrix with small neo-Hookean parameter values. The significantly thicker lamina propria in the lower than the upper ureter of young subjects (312 ± 27 vs. 232 ± 26 μm; mean ± standard error) corroborated the significant regional differences in diagonal-fiber family parameter values. The significant muscle thickening with age (upper ureter: 373 ± 48 vs. 527 ± 67 μm; middle: 388 ± 29 vs. 575 ± 69 μm; lower: 440 ± 21 vs. 602 ± 71 μm) corroborated the significant age-related increase in axial-fiber family parameter values.

In vitro study of age-related changes in human ureteral failure properties according to region, direction, and layer

Knowledge of the capacity of the ureteral wall to withstand urodynamic or external stresses is essential to understand ureteral injury and rupture that mostly occur following trauma, but may also be secondary to obstructive uropathy. It has clinical significance as well in the prevention of iatrogenic injury, for example, during ureteroscopy, but no information is available with regard to the age-related failure properties and regional differences have not been systematically described. Uniaxial tensile testing was performed on 166 ureteral rings and strips from 21 humans free of overt urologic disease; histological evaluation was performed. The degree of layer participation to the intact wall failure stress (=tissue strength), peak elastic modulus (=stiffness), and failure stretch (=extensibility) was assessed by examining layer-specific ruptures in the stress-stretch data. Failure stress at and peak elastic modulus before the first (muscle/adventitial) rupture correlated inversely less with age ( p < 0.05 in few regions/directions) than failure stress at the second (mucosal) rupture ( p < 0.05 in the middle and lower ureter), consistent with the decreased mucosal thickness in ≥50-year-old subjects. Failure stretch at both ruptures did not correlate with age ( p > 0.05 in most regions/directions), paralleling elastin content. Correlations with age were more significant in females than males. Failure stress at the second rupture point was higher ( p < 0.05) distally in <50-year-old but not in ≥50-year-old subjects, justified by the increased collagen distally in the former. Directional differences in failure stretches ( p < 0.05 at all ages/regions/genders) were justified by preferentially axial collagen reinforcement. The presented results may establish the foundation for computational models of iatrogenic/accidental ureteral trauma.

A three-dimensional (3D) two-way coupled fluid-structure interaction (FSI) study of peristaltic flow in obstructed ureters

Obstruction in the ureter flow path is one of the most common problems in urinary-related diseases. As the ureter transports the urine using the expansion bolus created by the peristaltic pulses, an obstruction in its path can cause unwanted backflow and can also result in damage to the wall. But in order to understand this further, and specifically to quantify and parametrize the effect of the obstruction in the ureter, a detailed study investigating various level of obstructions in peristaltic ureter flow is necessary. In the current study, full 3D numerical simulations of peristalsis in an obstructed ureter are carried out using a finite element solver along with a two-way coupling between the fluid and structural domain with the arbitrary Eulerian-Lagrangian method. Analysis of the results shows that the larger the obstruction, the higher the wall shear stress and pressure gradient in the fluid. In addition, the amount of backflow increases with increase in the obstruction.

A fluid-structure interaction (FSI)-based numerical investigation of peristalsis in an obstructed human ureter

Urine moves from the kidney to the bladder through the ureter. A series of compression waves facilitates this transport. Due to the highly concentrated mineral deposits in urine, stones are formed in the kidney and travel down through the urinary tract. While passing, a larger stone can get stuck and cause severe damage to ureter wall. Also, stones in the ureter obstructing the urine flow can cause pain and backflow of urine which in turn might require surgical intervention. The current study develops a 2D axisymmetric numerical model to gain an understanding of the ureter obstruction and its effects on the flow, which are critical in assessing the different treatment options. Transient computational analysis involving a two-way fully coupled fluid-structure interaction with the arbitrary Lagrangian-Eulerian method between the ureteral wall and urine flow is conducted with an obstruction in the ureter. The ureter wall is modeled as an anisotropic hyperelastic material, data of which, is based on biaxial tests on human ureter from previous literature, while the incompressible Navier-Stokes equations are solved to calculate urine flow. A finite element-based monolithic solver is used for the simulations here. The obstruction is placed in the fluid domain as a circular stone at the proximal part of the ureter. One of the objectives of this study is to quantify the effect of the ureteral obstruction. A sharp jump in pressure gradient and wall shear stress, as well as retrograde urine flow, is observed as a result of the obstruction.

Regional and age-dependent residual strains, curvature, and dimensions of the human ureter

The ureters are retroperitoneal structures controlling urine transport from the kidneys to the bladder. Because of the relative scarcity of data on the biomechanical properties of human ureter and the established importance of knowing these properties for understanding its physiology, we initiated biomechanical studies in cadaveric tissue. Herein, we report definite zero-stress/no-load geometrical characterization at 15 regions along the ureter of human cadavers aged 23-82 years, estimating the opening angle, circumferential residual strains, axial curvature, and dimensional parameters. Opening angle decreased along the proximal 25% of ureter, increased and reached a maximum near the mid-ureter, and then gradually decreased toward the vesicoureteral junction (young: p < 0.05; middle-aged: p < 0.05; old: p > 0.05; males: p < 0.05; females: p < 0.05). Similar were the regional distributions of residual strain at the interface between epithelium-lamina propria and muscle and of internal but not external residual strain. Wall thickness increased steadily with aging ( p < 0.05 at few regions), while ureteral circumference did not ( p > 0.05 at most regions) and opening angle decreased ( p < 0.05 at several regions). Consistent with Fung's stress-growth law, the muscle layer thickened with age unlike the epithelium-lamina propria that thinned ( p < 0.05 at most regions for both thicknesses). Moderate-to-strong direct correlations of residual strain difference (= external - internal) with opening angle, wall thickness, and curvature were found in most ureters. The presented data will provide insight into the biomechanical response of ureter under zero/low-stress conditions and the relationship between ureteral remodeling and aging. Importantly, they may also be used to inform finite element models and computational studies simulating the ureter.

Bioinspired coupled helical coils for soft tissue engineering of tubular structures - Improved mechanical behavior of tubular collagen type I templates

The design of constructs for tubular tissue engineering is challenging. Most biomaterials need to be reinforced with supporting structures such as knittings, meshes or electrospun material to comply with the mechanical demands of native tissues. In this study, coupled helical coils (CHCs) were manufactured to mimic collagen fiber orientation as found in nature. Monofilaments of different commercially available biodegradable polymers were wound and subsequently fused, resulting in right-handed and left-handed polymer helices fused together in joints where the filaments cross. CHCs of different polymer composition were tested to determine the tensile strength, strain recovery, hysteresis, compressive strength and degradation of CHCs of different composition. Subsequently, seamless and stable hybrid constructs consisting of PDSII® USP 2-0 CHCs embedded in porous collagen type I were produced. Compared to collagen alone, this hybrid showed superior strain recovery (93.5±0.9% vs 71.1±12.6% in longitudinal direction; 87.1±6.6% vs 57.2±4.6% in circumferential direction) and hysteresis (18.9±2.7% vs 51.1±12.0% in longitudinal direction; 11.5±4.6% vs 46.3±6.3% in circumferential direction). Furthermore, this hybrid construct showed an improved Young's modulus in both longitudinal (0.5±0.1MPavs 0.2±0.1MPa; 2.5-fold) and circumferential (1.65±0.07MPavs (2.9±0.3)×10^-2MPa; 57-fold) direction, respectively, compared to templates created from collagen alone. Moreover, hybrid template characteristics could be modified by changing the CHC composition and CHCs were produced showing a mechanical behavior similar to the native ureter. CHC-enforced templates, which are easily tunable to meet different demands may be promising for tubular tissue engineering.

STATEMENT OF SIGNIFICANCE

Most tubular constructs lack sufficient strength and tunability to comply with the mechanical demands of native tissues. Therefore, we embedded coupled helical coils (CHCs) produced from biodegradable polymers - to mimic collagen fiber orientation as found in nature - in collagen type I sponges. We show that the mechanical behavior of CHCs is very similar to native tissue and strengths structurally weak tubular constructs. The production procedure is relatively easy, reproducible and mechanical features can be controlled to meet different mechanical demands. This is promising in template manufacture, hence offering new opportunities in tissue engineering of tubular organs and preventing graft failure.

Effect of ovariectomy and Sideritis euboea extract administration on large artery mechanics, morphology, and structure in middle-aged rats

BACKGROUND

Arterial function is regulated by estrogen, but no consistent pattern of arterial mechanical remodeling in response to depleted estrogen levels is available.

OBJECTIVE

To examine long-term effects of ovariectomy (OVX) on the mechanical properties, morphology, and histological structure of the carotid artery in middle-aged rats and a potentially protective effect of Sideritis euboea extract (SID), commonly consumed as "mountain tea".

METHODS

10-month-old female Wistar rats were allocated into control (sham-operated), OVX, OVX+SID, and OVX+MALT (maltodextrin; excipient used for dilution of SID) groups. They were sacrificed after 6 months and their carotid arteries were submitted to inflation/extension tests and to dimensional and histological evaluation.

RESULTS

Remodeling in OVX rats was characterized by a decreased in situ axial extension ratio, along with increased opening angle, thickness, and area of the vessel wall and of its medial layer, but unchanged lumen diameter. Compositional changes involved increased elastin/collagen densities. Characterization by the "four-fiber" microstructure-motivated model revealed similar in situ biaxial response of carotid arteries in OVX and control rats.

CONCLUSIONS

Carotid artery remodeling in OVX rats was largely consistent with hypertensive remodeling, despite the minor arterial pressure changes found, and was not altered by administration of SID, despite previous evidence of its osteo-protective effect.

Age- and region-related changes in the biomechanical properties and composition of the human ureter

The ureter has been largely overlooked heretofore in the study of the biomechanics of soft biological tissues, although there has been significant motivation to use its biomechanical properties as inputs to mathematical models of ureteral function. Herein, we used histological analysis for quantification of collagen contents and thickness/area of ureteral layers, with concomitant geometrical analysis of zero-stress and no-load states, and inflation/extension testing to biomechanically characterize with the Fung-type model the ureters from cadavers. The effects of age and gender on the regional distribution of those properties were examined. Tissue properties did not differ (p>0.05) between the left and right ureter. Regional heterogeneity was established that was profoundly age-related but seldom gender-related, based on the following evidence: 1) In younger subjects, the axial stress-circumferential strain curves of upper ureter were shifted to smaller stresses and model parameter a₂ representing axial stiffness was smallest (p<0.05), i.e. upper ureter was the least stiff region axially; 2) upper ureter underwent axial stiffening with advanced age, evidenced by the increasing (p<0.05) parameter a₂, and the stress-strain curves were uniformly exhibited along the ureter, evidenced by the non-varying (p>0.05) parameters C,a₁,a₂,anda₄; 3) aging raised (p<0.05) the collagen content of upper ureter to favor a near-uniform regional distribution; 4) wall thickness increased with age, unlike the opening angle and residual strains, reflecting the thickening of outer (muscular) vs. inner (mucosal) layers in aged subjects, with significant differences (p<0.05) in some regions; and 5) gender affected little (p>0.05) the opening angle and morphometry of no-load and zero-stress states.

Fluid Structural Analysis of Urine Flow in a Stented Ureter

Many urologists are currently studying new designs of ureteral stents to improve the quality of their operations and the subsequent recovery of the patient. In order to help during this design process, many computational models have been developed to simulate the behaviour of different biological tissues and provide a realistic computational environment to evaluate the stents. However, due to the high complexity of the involved tissues, they usually introduce simplifications to make these models less computationally demanding. In this study, the interaction between urine flow and a double-J stented ureter with a simplified geometry has been analysed. The Fluid-Structure Interaction (FSI) of urine and the ureteral wall was studied using three models for the solid domain: Mooney-Rivlin, Yeoh, and Ogden. The ureter was assumed to be quasi-incompressible and isotropic. Data obtained in previous studies from ex vivo and in vivo mechanical characterization of different ureters were used to fit the mentioned models. The results show that the interaction between the stented ureter and urine is negligible. Therefore, we can conclude that this type of models does not need to include the FSI and could be solved quite accurately assuming that the ureter is a rigid body and, thus, using the more simple Computational Fluid Dynamics (CFD) approach.