The accumulation of particles in ureteric stents is mediated by flow dynamics: Full-scale computational and experimental modeling of the occluded and unoccluded ureter

Abstract

Ureteric stents are clinically deployed to restore urinary drainage in the presence of ureteric occlusions. They consist of a hollow tube with multiple side-holes that enhance urinary drainage. The stent surface is often subject to encrustation (induced by crystals-forming bacteria such as Proteus mirabilis) or particle accumulation, which may compromise stent's drainage performance. Limited research has, however, been conducted to evaluate the relationship between flow dynamics and accumulation of crystals in stents. Here, we employed a full-scale architecture of the urinary system to computationally investigate the flow performance of a ureteric stent and experimentally determine the level of particle accumulation over the stent surface. Particular attention was given to side-holes, as they play a pivotal role in enhancing urinary drainage. Results demonstrated that there exists an inverse correlation between wall shear stress (WSS) and crystal accumulation at side-holes. Specifically, side-holes with greater WSS levels were those characterized by inter-compartmental fluid exchange between the stent and ureter. These "active" side-holes were located either nearby ureteric obstructions or at regions characterized by a physiological constriction of the ureter. Results also revealed that the majority of side-holes (>60%) suffer from low WSS levels and are, thus, prone to crystals accumulation. Moreover, side-holes located toward the proximal region of the ureter presented lower WSS levels compared to more distal ones, thus suffering from greater particle accumulation. Overall, findings corroborate the role of WSS in modulating the localization and extent of particle accumulation in ureteric stents.

FIG. 1.

(a) Contours of the velocity magnitude (in m/s) taken at different positions along the model and computed numerically over the model mid-plane. Insets show zoomed-in views at different regions along the ureter (from proximal to distal). These include (b) holes 6–11, (c) holes 12–16, and (d) holes 33–37. Both intra- and extra-luminal compartments nearby a side-hole are shown. Results refer to the unobstructed model.

FIG. 2.

(a) Contours of the WSS magnitude (in Pa) over the stent internal wall and the lateral walls of side-holes, as computed numerically. Zoomed-in views of the WSS contours at both the proximal (holes 1–8) and distal (holes 23–42) regions of the unobstructed and stented model are also shown. (b) Mean values of the WSS magnitude (in Pa) acting over the wall of each individual side-hole from hole 1 to hole 42. Red circles are used to indicate corresponding holes between figures (a) and (b). (c) Contours of the WSS magnitude (in Pa) over the stent external wall, as well as zoomed-in views of the WSS contours at both the proximal and distal regions of the unobstructed model.

FIG. 3.

(a) Mean values of the WSS magnitude (in Pa), calculated numerically for each individual side-hole (from hole 1 to hole 42). (b) % coverage area occupied by crystals at side-holes located in specific regions of interest within the model (i.e., hole 6, hole 7, hole 15, hole 37, and hole 38). The end point binarized images of particles accumulated at side-holes are also reported next to the corresponding coverage area data points. Black pixels in the images correspond to the presence of particles. (c) Plot of the mean coverage area (in %, measured experimentally) as a function of the mean wall shear stress (in Pa, determined numerically) taken at selected side-holes of the stent (corresponding to holes 6, 7, 15, 37, and 38). The red line corresponds to a non-linear (exponential) fit of the data points.

FIG. 4.

(a) The mean % coverage area occupied by accumulated particles, quantified as an average of values taken at side-holes located in specific regions (or domains) of interest within the model (i.e., defined as proximal, middle, or distal). (b) Contours of the WSS magnitude over the outer (top) and inner (bottom) surface of the stent. Red boxes indicate the corresponding regions of interest over which the mean % coverage area was quantified. (c) The top view of the model before and after particle-accumulation experiments with indicated the regions over which the mean % coverage area was quantified.

FIG. 5.

(a) Contours of the velocity magnitude (in m/s) computed numerically over the model mid-plane. Insets show zoomed-in views of the contours at different regions along the ureter (from proximal to distal). These include (b) holes 6–11, (c) holes 12–16, and (d) holes 33–37. Both intra- and extra-luminal compartments nearby a side-hole are shown. Results refer to the obstructed model.

FIG. 6.

(a) Contours of the WSS magnitude (in Pa) over the stent internal wall, as well as WSS contours for both the proximal (holes 1–8) and distal (holes 23–42) regions (obstructed model). (b) Mean values of the WSS magnitude (in Pa) for each individual side-hole, from hole 1 to hole 42. Red circles are used to indicate correspondence of side-holes between (a) and (b). (c) Contours of the WSS magnitude (in Pa) over the stent external wall and zoomed-in views at both the proximal and distal regions (obstructed model).

FIG. 7.

(a) CAD drawing of the stent, where the geometrical properties are taken from the commercial double-J stent UniversaVR (CookVR Medical, USA). The stent has a wall thickness of 0.5 mm, a length of 28.5mm (excluding coils), an internal diameter of 1.5 mm, and a total number of 42 side-holes that are placed at intervals of three to four with an average spacing between side-holes of about 7.5 mm. (b) Model of the urinary system, including kidney, ureter, and bladder compartments. The geometrical properties required to construct this model were taken from Ref. . The image also shows the modeled ureteric obstruction (highlighted in red). It was positioned at a distance of approximately 50mm from the UVJ between hole 11 and hole 12. The unobstructed model has the same geometrical characteristics, except for the obstruction. (c) Symmetric view of the stented urinary system (obstructed), including its wireframe view and the corresponding reference coordinate system. (d) Zoomed-in view of the tetrahedral meshing performed on the obstructed model. The number of mesh elements for both unobstructed and obstructed models is also reported.

FIG. 8.

(a) Photograph of the in vitro ureter model. A more detailed description of the model is provided in Ref. . The red dashed box indicates the location of the tape used for labeling positions along the model. (b) Experimental setup adopted in this study to carry out in vitro particle-accumulation tests. It included a reservoir containing supersaturated artificial urine (AU), a peristaltic pump (Minipuls3, Gilson), a funnel, a hot plate stirrer (Stuart Hot Plate Stirrer), the ureter model (with the stent shown in green), and a camera (EOS 600D, Canon, Japan). A photograph of the overall setup is also reported in (c).