Experimental evaluation of the optimum applied pressure and number of cuffs for artificial sphincter in fluid flow control in an ex vivo model

Abstract

Objective

The aim of this study was to find the optimal force that should be applied to control urine leakage using a non-hydraulic cuff design, and to investigate the relationship between the number of cuffs used in urinary control and the change in demand force.

Method

In this study, five sheep bladders were used to design and build a biomechanical evaluation system. The biomechanical system included the bladder, pressure gauge, pressure regulator system of the bladder, and force application system by the cuff. In this study, we increased the fluid pressure inside the bladder from 0 cmH₂O to 200 cmH₂O (1 cmH₂O=0.098 kPa), and at each point, the force applied by the cuff to prevent fluid leakage was measured and recorded.

Results

The study revealed that a mean tensile force of 2.81 (standard deviation 0.23) N for a single cuff and 1.63 (standard deviation 0.16) N for double cuffs with symmetrical pressure effectively prevented bladder fluid leakage in the pressure range from 75 cmH₂O to 100 cmH₂O (p<0.001). However, there were no significant differences in results when comparing the applied tensile force required by double cuffs with asymmetric tensile force to that of the single cuff use. Furthermore, using three cuffs instead of two did not yield significantly different outcomes.

Conclusion

Using double cuffs with symmetrical pressure had a greater effect compared with a single cuff with the same tensile force. Although multiple cuffs with varying tensile forces were not found to be useful in controlling urine leakage in this study, it is suggested that applying variable forces alternately and rotationally among several cuffs may reduce long-term risks such as atrophy and necrosis. Future in vivo studies are recommended to further evaluate efficiency.

1. Introduction

Nowadays urinary control disorder is one of the most common diseases among men and women around the world, which is subject to various treatment methods, including behavioral therapy, drug therapy, and the use of surgical methods such as the sling procedure and the use of artificial urinary sphincter [1,2]. The gold standard treatment for men, which is also one of the conventional and effective methods for women, is the use of artificial sphincters, which sometimes are associated with complications such as atrophy and necrosis that are mainly caused by applying high and long-term pressure on the urethra [[2], [3], [4]]. A study by Lai et al. [5] showed that in a mean period of 29.6 months, 9.6% of patients who underwent artificial sphincter implantation developed atrophy. Various measures have been taken to reduce the incidence of complications caused by the use of artificial sphincters, including modifying the applied pressure of the sphincter, changing the cuff size, altering the number of cuffs, and designing sphincters with more optimal performance [[6], [7], [8]]. In this study, we aimed to evaluate the performance of a cuff with a novel mechanical and non-hydraulic actuation mechanism, in order to find the optimal traction force required to control urine leakage in a single cuff, double cuffs, and triple cuffs, respectively. In addition, we aimed to check how to control the flow of urine by applying symmetrical and asymmetrical forces in the case of double cuffs.

2. Material and methods

This study received its ethics approval under the number IR.SBMU.UNRC.REC.1401.029 from the Urology and Nephrology Research Center, Research Institute of Urology and Nephrology, Shahid Beheshti University of Medical Sciences. In addition, ARRIVE guideline items were considered throughout the study [9].

The cuff utilized in this study consisted of a set of parallel cables enclosed within an elastic cover. The cables were further covered by a 1 mm thick layer of silicone on the inside and a 0.4 mm thick layer on the outside. The cuff was wrapped around the urethra, and the cables were connected to the traction system via a connecting tube.

For this study, bladder and urethra samples from five male sheep weighing 50±5 (mean±standard deviation [SD]) kg were used. After removing the bladder from each animal, it was stored in a 0.9% normal saline solution at room temperature (27 °C). Tests and measurements were conducted at regular intervals up to 2 h after removal. The urethral diameter for all samples was approximately 7 mm, with a surrounding tissue thickness of 4 mm.

A mechanical structure was employed to hold the bladder and urethra samples in place. A fluid infusion tube was inserted into the bladder, and connected to a water source that regulated intravesical pressure. Another tube connected to a pressure gauge was also inserted into the bladder to accurately measure internal pressure. The cuffs were also closed around the urethra in the proximal part of it, so that each cuff was connected to an independent pulling device. The mean length was 40 (SD 5) mm and the mean width was 10 (SD 2) mm. The tensile force required to retract the cuff, while not subjected to an external force, was 0.3 N. The cuffs were wrapped around the tube in the area between the neck of the bladder and the proximal side of the urethra (Fig. 1). Evaluation was conducted on each sample in four stages.

Figure 1.

The image showing how to connect the bladder, cuff, and pressure gauge in the biomechanical system.

In the first stage, a single cuff was used to control urine. In the second stage, double cuffs were used with equal force applied to both. In the third stage, double cuffs with unequal applied force were used. Lastly, in the fourth stage, three cuffs with equal applied force were used.

During each stage, the pressure inside the bladder increased from 0 cmH₂O to 200 cmH₂O (1 cmH₂O=0.098 kPa). Simultaneously, we recorded the tension force required for the cuffs to prevent liquid leakage from the bladder.

To examine the impact of four types of cuffs (positive or negative) on the outcome, repeated measures analysis of variance was utilized to compare the means of the outcome when the same samples were measured using various cuff types. In other words, each sample's outcome was measured using four cuffs, each with different pressures. This analysis also assessed the interaction effect between the cuffs and pressure, indicating that the effect of cuff types depends on the level of pressure.

The statistical analysis was performed with SPSS 26.0 (for Windows; SPSS Inc., Chicago, IL, USA). The descriptive results were presented as means with SDs for quantitative variables. In order to eliminate the effects of cuffs, repeated measures analysis of variance was used. For further exploration, pairwise comparisons based on Bonferroni correction were applied. A p-value of less than 0.05 was considered statistically significant.

3. Results

The results of our study revealed a significant interaction effect between the number of cuffs and pressure on the outcome (p<0.001). To further explore the interaction effect, additional tests were conducted for each level of pressure. The results showed that there was a significant difference in bladder leakage control between the cuffs for all pressure levels (p<0.001). Furthermore, none of our investigated samples exhibited any bladder leakage with intravesical pressure up to 10 cmH₂O without any force being applied to the urethra by the cuff.

In the first stage (single cuff), when the water pressure inside the bladder reached 15 cmH₂O, the mean force required to prevent leakage was 0.52 N. As the intravesical fluid pressure increased, the force needed to be applied to the cuff in order to prevent leakage also increased. In fact, there was a linear relationship between intravesical pressure and cuff force. Specifically, from 15 cmH₂O to 45 cmH₂O (for every 5 cmH₂O increase), there was a corresponding increase of 0.1 N in force demand. Similarly, from 45 cmH₂O to 85 cmH₂O, there was an approximate increase of 0.2 N in the tensile force demand to control leakage. From the pressure of 85 cmH₂O to 110 cmH₂O, approximately an additional 0.1 N should be added to the tension force of the cuff for every 1 cmH₂O increase in intravesical pressure. At a pressure higher than 150 cmH₂O, the pressure-force diagram showed a plateau where there was minimal change in tensile force required by the cuff despite increasing intravesical pressure. The mean force required at the higher pressure was found to be approximately 3.42 (SD 0.30) N (Fig. 2).

Figure 2.

The relationship between the tensile force applied to the cuff and the pressure inside the bladder across experimental stages. 1 cmH₂O=0.098 kPa.

In the second stage (double cuffs with symmetric force), when both cuffs were used to prevent liquid leakage, the mean tensile force applied was measured at 0.53 N at 15 cmH₂O and increased to 1.57 N at 85 cmH₂O. The relationship between increasing intravesical pressure and cuff force, up until the pressure of 150 cmH₂O, showed a linear pattern with an approximate intravesical pressure-cuff force ratio of 1:1.3. However, at a pressure higher than this threshold, the pressure-force diagram plateaued and indicated that the mean force required was 2.66 N.

In the third stage (double cuffs with asymmetric force), the mean tensile force applied to the cuffs at the pressure of 15 cmH₂O inside the bladder was 0.52 N. The pressure-force ratio in this stage closely resembled that in the first stage, where only one cuff was used. The tensile force required in this stage at 85 cmH₂O was 2.65 N, and the approximate mean force required at a pressure higher than 150 cmH₂O was 3.46 (SD 0.90) N.

In the fourth stage (triple cuffs), when the pressure inside the bladder was set to 15 cmH₂O, 85 cmH₂O, and 150 cmH₂O, the tensile force applied to the cuffs was measured as 0.52 N, 1.57 N, and 2.49 N, respectively. The pressure-force diagram pattern observed in this stage corresponded to that of the second stage where double cuffs were used with symmetrical force.

Furthermore, comparing data of the first stage with the fourth stage, it was found that at the pressure range from 25 cmH₂O to ≤150 cmH₂O, there was a significant difference in applied tensile force required by the cuff system to prevent leakage (p<0.001). This significant difference also persisted in the higher pressure range from 150 cmH₂O up to 200 cmH₂O (p=0.004) (Table 1).

Table 1.

Pairwise comparisons adjusted by the Bonferroni correction for multiple tests.

Pressure,cmH2Oa	Applied tensile force required by the cuff system to prevent leakage, N				p-Value
	Single cuff	Double cuffs withsymmetric force	Double cuffs withasymmetric force	Triple cuffs	p1	p2	p3	p4	p5
0 to ≤25	0.33±0.32	0.34±0.28	0.32±0.29	0.31±0.28	0.7	0.7	1	1	0.13
25< to ≤50	1.02±0.18	0.81±0.12	1.00±0.21	0.79±0.12	<0.001	1	<0.001	1	<0.001
50< to ≤75	1.87±0.27	1.17±0.15	1.88±0.29	1.20±0.27	<0.001	1	<0.001	1	<0.001
75< to ≤100	2.81±0.23	1.63±0.16	2.79±0.25	1.63±0.14	<0.001	1	<0.001	1	<0.001
100< to ≤125	3.15±0.04	2.00±0.16	3.16±0.05	1.88±0.37	<0.001	1	<0.001	1	<0.001
125< to ≤150	3.27±0.06	2.32±0.17	3.28±0.05	2.30±0.17	<0.001	1	<0.001	1	<0.001
150< to ≤200	3.42±0.03	2.66±0.17	3.46±0.09	2.66±0.21	0.008	1	0.002	1	0.004

p1, single cuff vs. double cuffs with symmetric force; p2, single cuff vs. double cuffs with asymmetric force; p3, double cuffs with symmetric force vs. double cuffs with asymmetric force; p4, double cuffs with symmetric force vs. triple cuffs; p5, single cuff vs. triple cuffs.

^a1 cmH₂O=0.098 kPa.

There is a significant disparity in the tensile force required between the first stage and the second stage at the pressure ranges from 25 cmH₂O to ≤50 cmH₂O, 50 cmH₂O to ≤75 cmH₂O, 75 cmH₂O to ≤100 cmH₂O, 100 cmH₂O to ≤125 cmH₂O, 125 cmH₂O to ≤150 cmH₂O (all p<0.001). Additionally, there is also a notable difference within the pressure range from 150 cmH₂O to ≤200 cmH₂O (p=0.008).

There is no significant distinction between the results of the first stage and the third stage at different pressure ranges (all p>0.5). Similarly, there is no significant difference when comparing the applied tensile force required by triple cuffs with the double cuffs with symmetric tensile force (p=1).

4. Discussion

Several studies have been conducted to rectify the necessary issues or discover appropriate methods for reducing complications caused by the use of artificial sphincters, such as atrophy and the need for reconstructive surgery [[10], [11], [12]]. A study by Ahyai et al. [13], which compared the single-cuff with double-cuff usage in 180 patients, found that the incidence of complications was similar in both groups (p>0.05). However, urinary control was better in the double-cuff group (p≤0.02). The likelihood of requiring explantation was higher in the double-cuff group (p=0.003) [13]. Another study on four cadavers demonstrated a positive correlation between the circumference of the urethra and the pressure needed to control urine, indicating that the site of cuff implantation can significantly impact side effects (Spearman’s rank correlation coefficient=0.5) [14].

Other studies comparing single-cuff downsizing versus tandem cuff placement, or transcorporal cuffs versus double cuffs with symmetric pressure did not show any significant differences in side effect incidence or device survival [15,16].

Various non-medical simulations have been conducted to evaluate pressure entering the urethra point by point [17]. These evaluations reveal that the structural form and functional mechanism of cuffs cause pressure congestion points in tissue. This asymmetric distribution of pressure diminishes the effectiveness of cuff performance at different pressures and increases the risk of atrophy and necrosis at these congestion points [18,19].

In our study, we utilized a sphincter featuring a novel mechanism that effectively reduces the occurrence of pressure points by uniformly applying pressure to the urethral environment. Previous research has indicated that when a pressure of only 15 cmH₂O is applied, the two walls of the duct come into contact, thereby preventing urine leakage [17]. To test this hypothesis, we assessed whether decreasing the applied pressure and increasing the contact surface through multiple cuffs could potentially reduce the required pressure for conduit function [17,20]. We found that when double cuffs were used with equal tension applied to both, less tension was needed to prevent leakage compared with using only one cuff. However, the increase in the number of cuffs had a greater impact. The results from double cuffs with asymmetric pressure did not show a significant improvement compared with using just one cuff. Therefore, considering the importance of reducing tissue surface during the surgery and maintaining the required pressure, it appears that using double cuffs with symmetrical pressure is preferable over multiple cuffs based on current information.

However, it seems that if several cuffs are used and pressure is applied to the urethra in a rotating manner using the milking mechanism, it can reduce hypoxia and point pressure on the urethral tissue, thus preventing complications such as atrophy and necrosis (Fig. 3).

Figure 3.

The schematic image showing the milking mechanism using several cuffs (a, b, and c) tied around the urethra. By contracting one or double cuffs from the proximal side of the urethra to the distal side, urine leakage did not occur and over time it distributed the pressure in different parts of the urethra, which prevented complications such as atrophy and necrosis.

One limitation of this study is the type of urethra used. Differences in structure and tissue thickness of the urethra can affect the calculated numbers at different stages, making comparisons with humans somewhat unrealistic. Additionally, although this project tested and evaluated tissue immediately after autopsy, there may be differences in tissue conditions in living organisms due to body temperature and blood circulation. It is recommended that future studies evaluate the milking mechanism when multiple cuffs are used and examine its tissue reaction.

5. Conclusion

Urinary control disorder is a prevalent issue in modern society, and artificial sphincters are often employed as a treatment method. This study demonstrates that the utilization of cuffs featuring a non-hydraulic mechanical mechanism, which applies uniform and equal pressure throughout the tube, effectively prevents urine leakage and the development of pressure points. Moreover, when double cuffs are employed with symmetric force application, as opposed to a single cuff, it significantly reduces the necessary applied pressure and mitigates complications associated with the artificial urinary sphincter.

Author contributions

Study concept and design: Pourya Shokri, Shahin Tabatabaei.

Data acquisition: Pourya Shokri, Shahin Tabatabaei, Seyed Amir Mohsen Ziaee.

Data analysis: Nasrin Broumandnia.

Drafting of manuscript: Pourya Shokri, Shahin Tabatabaei, Seyed Amir Mohsen Ziaee, Nasrin Broumandnia.

Critical revision of the manuscript: Pourya Shokri, Shahin Tabatabaei.

Conflicts of interest

The authors declare no conflict of interest.