Keywords

1 Introduction

Considering the impact on the quality of life of patients caused by double-J stents (DJS), different stent designs have been developed focusing mainly on the decrease or suppression of vesicoureteral reflux (VUR) and the reduction of bladder trigone irritation, for the improvement of patient comfort [1,2,3,4]. Many of these designs are based on changes at the distal end, such as the attachment of antireflux membranes and valves, their replacement by less voluminous designs or the complete removal of this section to create intraureteral stents [5,6,7,8,9,10].

2 Antireflux Membranes and Valves

Antireflux membranes and valves are devices incorporated to the distal end of a standard DJS design, with the purpose of preventing intraluminal reflux through the internal channel of the stent. There are two variants, the antireflux-membrane valve and the polymeric flap valve [5, 6, 11].

The antireflux membrane valve consists of a transparent silicone membrane in the shape of a pouch, that is attached at the vesical end of a DJS, wrapped around the outlet of the internal channel and the lateral orifices [5, 11, 12] (Fig. 1). This design is currently available for clinical use and its antireflux mechanism works as a one-way valve, automatically collapses as the bladder pressure increases thus preventing intraluminal VUR [12]. This membrane valve allows only antegrade urine movement with minimal impact on pressure and flow resistance but has the limitation of preventing antegrade insertion of the stent [5, 12].

Fig. 1
figure 1

Double-J ureteral stent with the antireflux-membrane valve. Reprinted from: Ecke et al. [5], with per-mission from Elsevier and T. H. Ecke as right holders of the image

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In terms of its clinical evaluation, a significant decrease in VUR and suprapubic and flank pain during urination were observed, as well as a reduction in hydronephrosis degree and rate of stent exchange [5]. A significant improvement in patient comfort, compared to current DJS, is described, suggesting that it may be due to less damage to the bladder urothelium provided by the antireflux-membrane valve [5, 13]. Nevertheless, Ritter et al. [14], by means of the Ureteral Stent Symptom Questionnaire (USSQ), do not show significant differences against a DJS, in the symptoms or in the quality of life of the patients, although it does deliver a significant reduction of the VUR. Thus, although this antireflux membrane has shown a significant reduction in VUR, a direct relationship between this trend and the improvement of patient symptomatology cannot be made.

The polymeric flap valve, developed by Park et al. [15], consists of a polymeric device that attaches to the vesical edge of the DJS, shaped as two lip-like membranes and an inner cavity [15]. Manufactured by 3D printing, flexible Tango-Plus FLX980 is used as the material of this flap valve [15]. The mechanism of this device is based on the difference between ureteral and bladder pressures. When the intravesical pressure rises, the valve occludes, preventing the retrograde flow of urine [15]. The efficacy of the valve was analysed in vivo by Kim et al. [6] in the porcine model. By means of a simulated voiding cystourethrography, the study revealed significant lower grades of VUR. However, low grade VUR was still present with a rate of 18% since this flap valve does not prevent extraluminal VUR either. This is a 24 h acute study, therefore potential long-term complications, such as possible valve obstruction due to encrustation remain uncertain [16, 17].

In the end, these designs block urine reflux through the internal channel of the DJS, but fail to prevent VUR in its entirety. In stented ureters, VUR occurs both through the lumen of the stent and around it. The extraluminal reflux will prevail as long as there is a stent reaching the ureterovesical junction (UVJ) through the ureteral orifice [18].

3 Distal End Modifications

Changes in the stents’ distal ends pursue a common objective, the reduction of material at the level of the UVJ and bladder trigone in order to mitigate the discomfort in stented patients [7, 8, 10, 11].

The modification of the distal end of the Tail Stent (Boston Scientific® Corporation, USA) consists of a progressive narrowing of its diameter, from 7 to 3 Fr, in a distal direction [7]. Unlike a standard DJS, this vesical end is not a pigtail, but straight, with the aim of reducing the volume of the stent at the ureteral orifices and bladder [7]. Regarding its assessment in the clinical setting, it causes 21% less lower urinary tract symptoms (LUTS) than a DJS, but provides no significant improvement in stent-related pain and urothelial inflammation [7].

On the other hand, the Buoy stent (Cook® Medical, USA), a stent with the features of a tail stent, except for its proximal largest diameter of 10 Fr, was analysed in the porcine model [19]. This Buoy stent provides effective urine drainage, adequate ureteral healing after endoureterotomy and causes less histologic damage of the UVJ, when compared to a standard 7 Fr CDJ and to a Endopyelotomy stent (Cook® Medical, USA) [19]. Nevertheless, the potential of this design to improve patient comfort remains unknown.

The Polaris™ Loop® design, developed by Boston Scientific® (Boston Scientific® Corporation, USA), consists of a 6 Fr single pigtail stent with a double loop at its distal end, whose diameter is equal or inferior to 3 Fr, reducing almost 70% of material at the distal level, with regard to a standard DJS [10] (Fig. 2). Thus the interaction of the stent with the bladder urothelium and the intramural ureter is restricted, reducing discomfort of patients [10]. Most of the clinical improvements induced by this design are not significant compared to DJS’s commercial designs [10, 20]. Many comparative studies with standard DJS show that despite there is a reduction in pain, filling symptoms, and analgesic consumption provided by the Polaris™ Loop®, these results are not significant [10, 20].

Fig. 2
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Distal end of the Polaris™ Loop® ureteral stent, Boston Scientific (Boston Scientific Corporation, USA)

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Another upgrade of ureteral stent designs is the suture stent, a single pigtail stent whose distal end has been replaced by one or two suture threads [8, 11]. A preliminary version of this design was firstly described in 1993 by Hübner et al. [21], although the most representative devices with this design are JFil® and MiniJFil®, developed by Vogt et al. [8, 22,23,24,25]. They claim that the intact areas of the ureter do not require the urinary drainage provided by a DJS and therefore, in proximal ureteral obstructions, the material of distal sections of the DJS can be replaced by a narrower component, such as a suture thread [8]. The JFil® is constituted by 50% of polymeric DJS, arranged in a proximal position and the remaining 50% composed by two suture threads, whereas the MiniJFil® has only the proximal pigtail, to which the suture threads have been attached. In both cases, the suture is double, made of 5-0 polypropylene, presenting a total diameter of 0.6 Fr [8]. Clinical studies evidenced an effective urinary drainage capacity, as well as a significant reduction in urinary pain and symptoms [8]. Besides, MiniJFil® has demonstrated its clinical safety and efficacy used after ESWL and ureteroscopic lithotripsy [22]. Nevertheless, this design is not exempt from complications, since up to 20% migration is detected, which in the case of being proximal, represents an endourological challenge, in addition to the potential risk of disrupting urine drainage [8].

As for the significant reduction of urinary symptoms and pain provided by these designs, the authors suggest its potential to limit the occurrence of VUR [8]. However, VUR incidence has not been analysed in patients and, similarly to the aforementioned antireflux designs, the presence of the suture crossing the ureteral orifice could again prevent the complete eradication of this adverse effect [8, 22]. All these designs have in common, that the potential inhibition of VUR may be partial. Regardless of calibre, the stent in all cases traverses the ureteral orifice preventing its closure when intravesical pressure rises and therefore perpetuating the incompetence of the antireflux mechanism of the UVJ.

With regard to further modifications of DJS’s distal end, recently B. Vogt has published two clinical cases presenting the treatment of malignant ureteral obstructions with a new polymeric ureteral stent design [26, 27]. The main feature of this innovative design is the suppression of the bladder pigtail and the incorporation of a silicone piece with an antireflux function. Instead of being located on the bladder, this distal structure remains at the ureteral orifice, avoiding the interaction of the material with the urothelium of the vesical trigone. These two patients have proven the feasibility of stent placement both in single and in tandem and the safety and effectiveness of the device [26, 27]. However, distal migration of a stent with such characteristics in the bladder may aggravate substantially urinary symptoms in patients [27].

Finally, in an effort to avoid cystoscopic removal of DJS, a ureteral stent has been developed with a magnetic system consisting of a cylinder shaped magnet fixed through a string on the vesical pigtail of the stent [28]. For its extraction, a retrieval device with a magnetic tip is introduced; which attaches to the magnet of the stent, enabling the extraction of the DJS by pulling out the catheter [28]. Clinical evaluations of the Black-Star® stent (Urotech®, Germany) using USSQ and visual analogue scales have revealed a lower incidence of pain and discomfort during removal with the use of this system, especially in men [28, 29]. On the other hand, the likelihood of the onset of encrustations on urinary stents should be considered, since it might disable the magnetic extraction system of this device [29].

4 Intraureteral Stents

Provided that a device at the UVJ disrupts its antireflux mechanism and triggers urinary symptoms and pain, the next step appeared to be to develop ureteral stents that spare the whole distal end, becoming intraureteral stents. Under this rationale, Soria et al. have developed an antireflux intraureteral ureteral stent registered as BraidStent® [9]. This intraureteral stent is a self-retaining design comprising a proximal pigtail, a central braided body of 3 Fr lacking internal channel, and a double helix as the distal end [9] (Fig. 3). The development of this intraureteral design is based on the principle that the way to prevent both intraluminal and extraluminal reflux is to preserve the UVJ intact [18, 30].

Fig. 3
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BraidStent®, self-retaining, antireflux ureteral stent developed by Soria et al.

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The validation of the BraidStent® by Soria et al. [9] in the swine model showed that this design meets the requirements of a DJS for passive ureteral dilation, completely avoiding VUR and significantly decreasing macroscopic and histologic damage in UVJ, which will probably reduce discomfort in stented patients [31, 32]. In addition, the effect of the stent on ureteral healing has been evaluated experimentally, showing that selective intubation of the affected area provides surgical success rates of over 85%, suggesting ureteral surgery as one of the indications that may benefit from this intraureteral design [33]. However, since the endoscopic removal of an intraureteral stents involves certain difficulty, a biodegradable BraidStent® has been developed and has undergone experimental assessment, showing a safe, controlled and predictable degradation rate [34, 35]. The biodegradable BraidStent® and its counterpart coated with heparin, the BraidStent®-H, maintain the characteristics proven in previous studies that tested the biostable BraidStent® [9, 32, 36,37,38].

The benefits arisen from the suppression of stent material at the UVJ on patients’ quality of life are substantiated in the randomized study by Yoshida et al. [39]. In which the insertion of an intraureteral stent after ureteroscopic lithotripsy causes significantly less pain and urinary symptoms, as well as an also significant reduction in the consumption of analgesic drugs [39]. However, despite being designated as an intraureteral stent, a thread attached to the stent reaches the bladder to enable cystoscopic removal, supporting the idea that an intraureteral device would benefit from biodegradable properties. Once more, it is uncertain whether this vesical thread may interfere on the onset of VUR, since the study did not assess this parameter [39].

It is of note that indications for any intraureteral stent are going to be more limited than those of a standard DJS. Circumstances requiring the dilatation of the UVJ will not be amenable to treatment with intraureteral designs, but with rather other stent catheterizing the ureteral orifice. Such designs will therefore offer the possibility of avoiding the adverse effects related to the distal end of DJS to a certain proportion of patients requiring stenting, excluding those with distal ureteral lesions close to the UVJ, those who require prestenting for ureteroscopic treatment, or with lithiasic fragments after lithotripsy [9, 33, 34].

5 Modifications of the Core Architecture of a DJS

With different features to those presented above, the Percuflex™ Helical ureteral stent (Boston Scientific® Corporation, USA), is a spiral cut flexible ureteral stent, which maintains the morphology of a standard DJS at the distal ends. This device is commercially available, whose spiral conformation has been developed to adapt to the shape of the ureter and to better accommodate patient movement [40, 41]. This design, under experimental conditions, drains urine in a comparable way to a DJS Percuflex™ Plus (Boston Scientific® Corporation, USA) [40]. In a comparative clinical study, an improvement in patient comfort is described by a significant reduction of the need for analgesics, although it does not report a significant decrease in pain intensity [41].

6 Dual-Lumen Ureteral Stents

Dual-lumen ureteral stent (Gyrus ACMI Corporation, USA) is a device that has been designed to improve urine drainage in extrinsic compressions. This design consists of two DJS attached to each other to provide two internal drainage pathways. In its ex vivo evaluation, this prototype provides significantly more extra and intraluminal drainage under extrinsic compression conditions, compared to a standard DJS. This feature may potentially improve the quality of life of patients with extrinsic ureteral obstructions [42].

7 Conclusions

Nowadays, the exploration of new ureteral stent designs is one of the main pathways, along with the development of materials and coatings, to improve the performance of current DJS. So far, it seems that these new designs mainly tend to modify the standard double pigtail design by progressively reducing and eliminating the presence of stent material at the level of the UVJ. In the context of suture stents and intraureteral stents, they have shown promising results in terms of improving patients’ quality of life. However, indications of these devices differ from those of standard DJS, not being suitable for all patients that require ureteral stenting. The shortcoming of stents with modified distal ends is that in the event of complications or proximal migrations, their removal is technically more challenging than the removal of a DJS which may involve a potential risk for the patient.

Ultimately, design improvements aim at diversification, towards the development of more specific devices to adapt to different circumstances, so that the adverse effects resulting from the generalized use of standard DJS can be avoided. For the development of new designs, it is desirable that simultaneous modifications are made to the materials to enhance their performance, being of particular interest the ability to degrade safely.