A Numerical Study to Precise the Estimation of the “Good” Mound Height in Endoscopic Treatment of VUR
Author(s): Mehdi Shirazi1, Zahra Jahanabadi2, Zeinab Hooshyar5, Zahra Mortazavinia4, Alireza Mehdizadeh3, Mojtaba Taherisadr6
Purpose: it seems that there is no definite and standard index to ascertain the technique and volume of injection in order to increase the success rate of endoscopic VUR treatment. In this study, we introduced a novel method to numerically define the relationship between some indexes in order to determine optimum mound height promise to standardize the injection technique in this kind of treatment.
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Material and method: The main purpose of this study is to construct a finite element simulation of intramural ureter and injected mound which aims to numerically define the relationship between indexes which have influence in VUR endoscopic treatment. Using linearization software and numerically simulation data the relationship between effective indexes has been derived.
Result: By linearization of the effective parameters of different finite element models, the relationship between effective parameters in filling phase is derived as: H=0.7456D+ 0.02174L +0.01846. This equation depicts adequate injected mound height as a function of internal diameter and intramural length, H=f(L,D).
Conclusion: Using numerical simulation, we introduced the novel formula to predict the height of injected mound in endoscopic VUR treatment. As a result of this study, in order to increasing the success rate of this treatment, the ratio of mound height to intramural ureter diameter should be approximately 74% (~3/4).
Keywords: Mound height, Endoscopic treatment, Vesico ureteral reflux, Finite element model
Vesico ureteral reflux (VUR) is a major disorder of childhood declared by retrograde urine flow from the bladder toward the kidney, predisposing patients to UTI and renal scarring and in long term leading to renal insufficiency and hypertension. Current treatment options include close observation with urinary chemoprophylaxis, minimally invasive endoscopic subureteral injection of bulking agents and open or laparoscopic reimplantation. The prophylactic antibiotic approach is proper for uncomplicated reflux (grade I-III without significant renal scarring or breakthrough infection). This type of treatment doesn’t cure VUR and has some problems such as patient noncompliance and increased antibiotic resistance.
Surgical reimplantation of the ureter is an invasive method and already used for patients with high grade reflux, children with breakthrough UTI and children with reflux and developed renal scarring. Endoscopic subureteral injection of bulking agents was introduced by Matouscheck as an evolution in treatment of reflux with a high success rate and minimal invasion, . The substance used as a bulking material should be biocompatible, nonantigenic, nonmigratory and non-toxic with minimal local inflammation. Multiple techniques on injection have been described. These methods include subureteral needle placement, intraureteral needle placement,  or some combinations of these. During recent years, several studies have been done to determine success or failure of endoscopic treatment, and various injection techniques have been introduced. Puri et al., described “volcano” appearance as the main sign of success of injection. The proper shape was demonstrated by adequate coaptation of the ureteral orifice and by its location in the bladder below the ureteral orifice and/or along the waldeyer’s sheat, .
Despite all the studies carried out to determine the success or failure of endoscopic treatment, such as description of volcano appearance, injection technique in combination with hydrodistention, increasing injected volume, use of intraoperative cystogram with a simulated voiding phase, there is no definite and standard index to ascertain the technique and volume of injection in order to increase the success rate. In this study, we introduced a novel method to numerically define the relationship between some indexes in order to determine optimum mound height promise to standardize the injection technique.
- Materials & Method
In this study, a computational simulation was proposed to simulate the intramural ureter and injected mound which aims to numerically define the relationship between indexes which have influence in retrograde urine flow from intramural ureter.
- Geometry of the numerical model
Fig. 1 illustrates diagrammatic representation of the functional anatomy of the ureterovesical junction and urinary anti reflux mechanism as the accumulation of urine within the bladder will lead to the tight closure of the portion of the ureter in between, thus preventing the backward return of urine to the kidneys, urinary anti reflux mechanism. In Fig. 1, P, D, H and L are intravesical pressure, intramural ureter diameter, injected mound height, and intramural ureter length, respectively. Section “A” in Figure 1 depicts the cross section of the intramural ureter with injected mound. The geometrical data of intramural ureter and ureterovesical junction were utilized as per Table 1.
- Materials Properties and Boundary Conditions
Material properties for the intramural ureter wall were assumed to be linear elastic, isotropic, incompressible, and homogeneous with Young’s modulus of 10 Kpa . The injected mound was modeled as a rigid volume as shown in Fig.
3. The boundary conditions for the wall were (1) the bottom surface of intramural wall was fixed at the connection to the internal bladder wall. (2) The upper wall was assumed to be free such that the deformation would occur due to the intravesical pressure and contribute to the tight closure of the portion of the ureter in between.
- Solution Method
The ANSYS 11 software was employed to simulate the intramural ureter with injected mound in voiding phase. The intravesical pressure was applied on the upper portion of the intramural wall. The upper bound of intravesical pressure was considered to be 160 CmH2 O to encompass both male and female intravesical voiding pressures, see Table 1. The 8-Node structural solid mesh has been utilized in finite element model to have adequate adoption with this geometry and material behavior.
Fig. 2 illustrates the total deformation of the intramural ureter with injected mound in voiding phase. As shown, the intramural ureter has been closed due to the intravesical pressure. Fig. 3 shows the deformed and undeformed intramural
wall in resting and voiding phase. As shown, in voiding position the injected mound height is high enough for the intramural ureter to be closed due to the intravesical pressure and prevents retrograde urine flow.
In the first attempt, keeping the intramural ureter length constant, by changing intravesical pressure and internal diameter, the adequate injected mound height leading to tight closure of intramural ureter was obtained.
By linearization of the effective parameters of about 30 different finite element models, the primary relationship between injected mound height, intravesical pressure and internal diameter of intramural ureter was defined as bellow, H=f(D,P)
According to this relationship, it has been inferred that intravesical pressure coefficient is minute in comparison with coefficient of other effective parameters and it shows that changing the intravesical pressure plays an insignificant role in adequate injected mound height.
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In order to investigate the more effective indexes on injected mound height, the intramural length has been considered as one of variable parameters in finite element modeling. Varying internal diameter, intramural length and linearization of obtained data, the relationship between parameters was derived as:
H=0.7456D+ 0.02174L +0.01846(5)
This equation depicts optimum mound height as a function of internal diameter and intramural length, H=f(L,D).
Endoscopic treatment of VUR was introduced more than 25 years ago and since then many different substances and injection techniques have been used with different results. Choosing endoscopic treatment is reasonable, as it has many advantages, such as technical simplicity, greater acceptance of patients and parents and significant decrease in post- operation complications. By improving the injection techniques the rate of endoscopic treatment has significantly increased as compared with open ureteral reimplantation. During the recent years, many studies have been done to increase the ability to identify factors predicting success with the endoscopic subureteral injection. At 2003, Puri et al. described “volcano” appearance as the main sign of success of injection. The proper shape was demonstrated by
adequate coaptation of the ureteral orifice and by its location in the bladder below the ureteral orifice and/or along the waldeyer’s sheat.
In different studies conducted by Lavelle et al. and Yucel et al. , it was found that a subjectively proper mound appearance was highly predictive of injection success, but it should be noted that the morphology and location of the mound are not perfectly predictive of injection success or failure, as the mound seems perfectly adequate in some injections but the injection is not successful; also, the imperfect mound morphology does not necessarily imply injection failure. Moreover, Ellworth PI et al. showed no correlation between the presence of a mound on post-injection ultrasound and the success of injection. Some research agrees that an intraureteral injection technique in combination with hydrodistention results in higher success rates but this is controversial,,,. The effect of injected volume on increasing the success rate is also controversial. Mathew D et al. showed that increasing the injection material volume will improve the success of subureteric injection but other centers have shown that higher injection material volume doesn’t necessarily increase the treatment success rate. So the effect of the injected volume in association with improved success rate remain unclear.
To determine the treatment success or failure, Tarry WF et al. described the utility of an intraoperative cystogram with a simulated voiding phase, but they demonstrated that an introperative cystogram can only detect de novo contralateral reflux but is not a proper method to predict the final success of injection. Despite all the mentioned studies, still there is no definite and standard index to ascertain the technique and volume of injection, and also there is no quantitative index for determining the proper mound size and shape. So, we have conducted a study aiming to define an optimum injection mound by finite element modeling.
In this study, we introduced a novel method to numerically define the relationship between some indexes in order to determine optimum mound height promise to standardize the injection technique in VUR treatment. It should be noted that due to every patient individual properties of intramural, it is not logical to consider similar injected mound height for all VUR treatment cases. Thus, it is needed to state the adequate injected mound height based on individual indexes of each case. The Intravesical pressure, intramural ureter diameter and length are some of these effective indexes. By specifying the relationship between all of these effective factors, the adequate and optimum injected mound height can be determined which can contribute to more success in treatment of VUR. In addition, it can help to save the needed injected mound height and make this injection method more economical.
The results of this study show that the intravesical pressure has an insignificant effect on the required injected mound height. However, intramural length plays an important role on it in comparison with Intravesical Pressure. Moreover, the ratio of mound height to intramural ureter diameter is approximately 0.74 (~3/4), which is drawn that in order to have successful VUR treatment, it is needed that about 74% of intramural ureter diameter be filled with the injected mound.
It should be mentioned that in Department of Urology of Shiraz University for the first time, an innovative method, introduced by Taheri et al., is used to measure the injected mound height to assure the adequate injected mound height. In this method by adjusting camera, laparoscopy lens and imaging screen, it would be possible to measure the injected mound height as a ratio of projected picture on the screen.
This study has served some limitation. The realistic cross section of intramural ureter has star shape. However, because of some limitation in finite element modeling, the circular one considered. Furthermore, we considered computational simulation only in voiding phase, although VUR may occur in resting phase.
In this study using numerical simulation, we introduced the novel formula to predict the height of injected mound in endoscopic VUR treatment. As a result of this study, in order to increase the success rate of this tevhnique, the ratio of mound height to intramural ureter diameter should be approximately 74% (~3/4). Moreover, clinical study has been conducted to ascertain the accuracy of this obtained height.
Fig. 1Diagrammatic representation of the functional anatomy of the ureterovesical junction
Fig. 2Intramural ureter displacement in voiding phase
Fig. 3Deformed and undeformed intramural urine wall
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