A Clinical and Finite Elements Study of Stress Urinary Incontinence in Women Using Fluid-Structure Interactions

Abstract
Stress Urinary Incontinence (SUI) or urine leakage from urethra occurs due to an increase in abdominal pressure resulting from stress like a cough or jumping height. SUI is more frequent among post- menopausal women. In the absence of bladder contraction, vesical pressure exceeds from urethral pressure leading to urine leakage. Despite a large number of patients diagnosed with this problem, few studies have investigated its function and mechanics. The main goal of this study is to model bladder and urethra computationally under an external pressure like sneezing. Finite Element Method and Fluid- Structure Interactions are utilized for simulation. Linear mechanical properties assigned to the bladder and urethra and pressure boundary conditions are indispensable in this model. The results show good accordance between the clinical data and predicted values of the computational models, such as the pressure at the center of the bladder. This indicates that numerical methods and simplified physics of biological systems like inferior urinary tract are helpful to achieve the results similar to clinical results, in order to investigate pathological conditions.
Key words: Computational Fluid Dynamics, Urinary Tract, Stress Urinary Incontinence, Finite Element Method, Fluid-Structure Interaction Introduction
Recently, diseases associated with urine and genital tract, with the general term urology, are more prevalent among both men and women older than 40 years old. The most critical subset of these problems is urinary incontinence [1]. Stress Urinary Incontinence is very common among women, which occurs due to a mechanical pressure like sneezing or jumping height. In this type of incontinence, any activity that increases abdominal pressure (including laughing, coughing, sneezing, and straining) leads to urine leakage as a result of urethral sphincter weakness. Respecting to the published reports on 2001, costs of urinary incontinence treatments exceeded 16.3 billion US dollar. The reports indicate 75% of total costs were spent on diagnosis and treatment for women [1].
SUI is more prevalent in women [1]. An increase in abdominal pressure in the absence of bladder contraction raises the vesical pressure to a level that exceeds urethral pressure, leading to involuntary loss of urine which mainly characterizes SUI. Abdominal pressure increases due to a mechanical incident like laughing, sneezing, jumping height, or any other tension in the body [1]. This explains why the SUI is considered as a mechanical force.
Although the main reason of SUI remains unknown, a large number of physicians believe that SUI is caused by injuries to the pelvic floor neuro-musculature, which mainly happen among those who have given birth vaginally [1]. While not a life threatening condition, SUI can detrimentally impact the quality of life. Up to now, biomechanical studies over female incontinence are dominated by three theories; including pressure transmission theory proposed by Enhoring [2], the integral theory proposed by Petrus and Ulmestan [3], and the hammock theory proposed by Delancy and Ashton-Miller [4]. Moreover, all these three theories seem paradoxical and within themselves, there is no consistency about involved structures and tissues. On the other hand, analytical studies and organic structures’ modeling is a practical way to study biomechanical phenomena.
In the current study, complications of the numerical model could challenge the researcher. In the current paper, the urine flow in the urinary tract is investigated. It is more common to investigate a part of urinary tract since scrutinizing all parts is very complex and time-consuming. Here, the modeling is mainly focused on bladder and urethra. The preferred computational parameter is the fluid (urine) pressure which is compared to clinical data. Pressure is the considered parameter since it is available in urodynamic measurement and it has varied measuring methods.
There are few reports of finite element analysis about SUI. Kim proposed a 2-dimensional model to study urethral closure during stress [5]. Kim proposed an axisymmetric model of the urethra and pelvic floor to find proper dimensions for his model. In Kim’s model, a catheter was placed in the urethra. This model was developed to analyze active (muscle contraction) or passive (pressure transmission) contributions to urethral cl

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