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Dear stent friends,

Once again, a newsletter from our ureteral stents series. On this occasion, I have selected three very recent papers.

The first study may be understood as a revival of silicone as a polymer for long-term stent placement. Several studies have already shown that silicone has an important role in a targeted cohort of patients. This encourages the careful selection of biomaterials, allowing a reduction in stent-related effects by taking advantage of the features of each polymer for specific patient profiles.

The second paper is a retrospective study. The authors developed a predictive model that analyzes the influence of spontaneous passage of ureteral stones in stented patients. The PASS score offers a simple, evidence-based tool for estimating the probability of spontaneous stone passage in these patients. This predictive model could support bedside decision-making and optimize treatment planning for individual cases.

Finally, a scientific paper prospectively evaluated the adherence of Ca²⁺, Mg²⁺, and bacteria to three different ureteral stents after urinary stone treatment. The stents assessed were designed with coatings intended to inhibit fouling on their surface, always with the aim of preventing ureteral stent encrustation. This study explored the role of stent coatings in minimizing such complications.

I hope you enjoy this selection.

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Although silicone was practically the first biomaterial to be used in ureteral stents, its use was reduced years ago by new synthetic polymers. Mainly because of their poor mechanical properties and unfavorable friction coefficient that increased their migration rate and reduced the resistance to radial compression. However in the last decade, the scientific literature has been providing new evidence that the new silicone stents have an important role in certain cases and patients. Especially when a ureteral stent must be removed due to encrustation problems or patients pain and when the stenting time is long term.

In this case, we highlight this paper which evaluates a silicone ureteral stent in the long-term setting and includes not only patients with lithiasis but also ureteral obstruction due to extrinsic and parietal causes.

The purpose of this manuscript was to analyze in a retrospective way outcomes from long-term ureteral stenting using silicone double-j stents (maximal dwell time of 12 months). The primary endpoint was “stent failure”, defined as unplanned premature re-intervention. There were 105 patients in the retrospective study, in 10% the cause of ureteral obstruction was extrinsic, in 30% parietal and in 60% intra-luminal. Interestingly, they use 6 or 7 Fr diameter stents to provide ureteral extra prosthetic drainage.

Results showed that tumor stent failure-free survival estimates at 1, 3, 6, 9 and 12 months were 99%, 98%, 88%, 82% and 69%, respectively. Estimated mean failure-free survival was 307 days. Causes for stent failure, hence reintervention, were UTI in 8% patients, acute kidney insufficiency or hydronephrosis in 7% and flank pain in 1%. Spontaneous migration was noted in only one patient and stent encrustation was observed in 12% of the patients (61% were stented for urolithiasis).

Of all baseline and perioperative parameters, three variables were found as independent and significant predictors of stent failure in the multivariable analysis: increasing patient weight, increasing preoperative urinary leukocytes count and UTI at the time of stent insertion. Additionally, extrinsic ureteral obstruction was found as a significant predictor of stent failure compared to the intra-luminal group, while parietal causes of obstruction showed no significant difference with intra-luminal causes.

This study reveals a remarkably low long-term stent failure rate using silicone double-j ureteral stents. Almost 70% of patients are free of stent failure at 12 months after ureteral stenting. One issue to consider is 6-month replacement in patients with UTI at the time of stent insertion.

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In contemporary cohorts, more than 10% of patients are pre-stented due to manifold reasons such as obstructed UTI or pain complaints. An side effect associated with ureteral stents used in the treatment of ureteral lithiasis is the ability of the stents to cause cessation of ureteral peristalsis and consequently cause passive dilatation of the ureter, which can lead to a near threefold increase in the ureteral lumen. In some cases, patients with indwelling stents experience spontaneous and often unnoticed stone passage.

In this retrospective study, the authors develop a predictive model that may influence the spontaneous passage of ureteral stones in stented patients. Before ureteroscopy, patients underwent low-dose CT to assess for stone persistence. Spontaneous stone passage was defined as the absence of a ureteral stone on follow-up CT or ureteroscopy.

Authors included the following predictive variables: age, sex, stone location in relation to the iliac vessel crossing, ureteral location, stone diameter, stone density (HU) and shape. It is also important to highlight that stone composition was analyzed, and a sensitivity analysis to assess the impact of stent duration on spontaneous stone passage was performed.

24.2% of patients experienced spontaneous stone passage with a median stone diameter of 4.4 mm and median stone density of 480 HU. The median follow-up duration was 26 days. Indications for initial ureteral stenting in this study were stone diameter ≥ 5 mm in 26% of patients, unmanageable pain in 26%, UTI in 25%, suspicion of fornix ruptura in 6% etc. The proportion of patients with spontaneous stone passage related to stone diameters of ≤ 5 mm, 5-7 mm and > 7 mm was 43.4%, 16.4% and 9.1%, respectively. Stones with low density(< 1000 HU) had an spontaneous stone passage rate of 32.7%, compared to 3.4% for high-density stones.

In the final multivariable model, the following predictors were independently associated with spontaneous stone passage: stone diameter ≤ 5 mm, mid to distal ureter location, location at the ureterovesical junction and low density. Surprisingly,the use of alpha-blockers, antibiotics, or analgesic medication was not associated with spontaneous stone passage in the univariable analysis. Although the opposite is to be expected, particularly with the use of of alpha-blockers.

This study demonstrated that nearly a quarter of ureteral stones passed spontaneously within 4 weeks of stent placement. The sum of points for stone density, location and maximum stone size results in the PASS score. The PASS score provides a simple, evidence-based tool for estimating spontaneous stone passage probability in stented patients. Therefore, the score facilitates individualized treatment planning.

Clinical implications of this score: Patients with a high PASS score may benefit from a confirmatory CT scan prior to ureteroscopy to ensure stone persistence. In contrast, a low PASS score may justify omitting CT, reducing radiation exposure and costs. These patients may benefit from timely secondary ureteroscopy to avoid prolonged stenting and its associated complications.

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Despite their clinical benefits, ureteral stents are prone to encrustation, which can lead to severe complications such as infection, stent blockage, and patient discomfort.

Encrustation is influenced by several factors, including the duration of stent placement, the chemical composition of urine, and patient-specific variables like metabolic conditions. Studies have identified bacterial colonization and mineral adherence, particularly of Ca²⁺ and Mg²⁺, as primary contributors to stent encrustation. Research has demonstrated that bacterial biofilms develop on the stent surface, serving as critical nucleation sites for mineral deposition. However, it is also known that encrustation can occur in the absence of infection.

As I have already mentioned in previous newsletters, new biomaterial designs and coatings are being intensively researched to overcome these drawbacks. Other strategies related to the use of crystallisation inhibitors and urinary pH modification are giving positive outcomes.

Authors of this scientific paper prospectively evaluated the adherence of Ca²⁺, Mg²⁺, and bacteria to three different ureteral stents after endoscopic treatment for urinary calculi. This study was a single-center, randomized, single-blind clinical trial comparing encrustation of three different ureteral stents: Percuflex stent with a Hydroplus coating; Tria stent with a Percushield coating; InLayOptima stent with a pHreeCoat coating after flexible-ureteroscopy and ECIRS techniques. Therefore, the stents evaluated are designed with coatings that should inhibit fouling on their surface due to their specific coatings. This is what makes this study interesting, unlike previous studies evaluating standard ureteral stents without anti-fouling coatings.

In total, 61 patients were enrolled, one month after stent placement. There were no significant differences in preoperative factors regarding patients and stone characteristics in the three groups.

The data of biomineral attachment to ureteral stents showed that Ca²⁺ adhered Percuflex with a Hydroplus coating stents were significantly higher than that of the other groups. The median amount of Mg in Percuflex stents was significantly higher. The median number of bacteria was also highest in Percuflex group, however, there were no significant differences in bacterial adhesion. The incidence of stone encrustation was also higher in Percuflex group. As for the relative USSQ domain, the postoperative urinary symptom score on stent removal day was significantly lower in Tria stent with a Percushield coating and InLayOptima stent. The univariate analysis showed that patients with higher pH, small residual fragments, and stent type were the risk factors of Ca²⁺ encrustation.

The authors' results revealed that stents with Percushield and pHreeCoat coatings significantly outperformed Hydroplus-coated stents in minimizing Ca²⁺ and Mg²⁺ adherence. The stent coating material appears to play a crucial role in encrustation, potentially impacting patients’ quality of life after ureteral stent insertion.

In a nutshell, only in selected cases, especially in recurrent stone formers patients, the use of stents with antifouling coatings reduces stent-related complications.