Ureteral stents were first described in 1949 and continue to be one of the most commonly used tools among urologists. However, as much as they reduce risk of obstruction after instrumentation and decompress the urinary tract, they do not come free of complications. In >80% of patients, stents have been associated with patient discomfort, infection, and encrustation.¹

Stent encrustation is a common and complex complication associated with ureteral stents. It is defined as deposition of crystals and minerals from the urine on the ureteral stent’s inner and outer surfaces. Risk of encrustation directly correlates with length of treatment, with signs of encrustation seen in 9% of patients after 6 weeks and 77% of patients after 12 weeks.^2,3 It has also been proposed that biofilm formation and bacterial adhesion have a role in the mechanism of encrustation. On placement, all stents become coated with a conditioning film formed by urinary proteins and ions that promote adhesion of bacteria onto the stent’s surface (see Figure). This, due to either the biofilm’s net positive charge attracting negatively charged crystals or calcium-binding proteins that allow crystals to directly bind to stents, is thought to stimulate stent encrustation.⁴ Other patient-specific contributing factors include history of urolithiasis, cancer, malabsorptive syndromes, and forgotten stents in patients with poor compliance or low health literacy.

Figure. Mechanism of stent encrustation. Reprinted with permission from Tomer et al, J Urol. 2021;205(1):68-77.²

Standard KUB is often sufficient to diagnose the extent of encrustation while CT should be used for surgical planning for more severely encrusted cases. Geavlete et al reported 832 cases of stent encrustation in a review of 50,000 endourological procedures over 25 years.⁵ The authors found the most common location of encrustation was the distal curl alone in 432 (52%) cases, followed by both proximal and distal curls in 235 (28%) cases, and encrustation of the entire stent in 112 (12%) cases. It was rare to find proximal curl encrustation alone (3.1%), distal curl with stent shaft (3%), or stent shaft alone (2 cases).

Grading systems have been developed to assess the severity of encrustation, like the FECal (forgotten, encrusted, calcified) and KUB systems, classifying into either mild (<5 mm, <50% stent encrustation) or severe (≥5 mm, ≥50% encrustation).² In milder cases, cystoscopic removal is usually successful. In severe cases, a multimodal, staged approach with cystolitholapaxy, extracorporeal shock wave lithotripsy, ureteroscopy, percutaneous nephrolithotomy, or pyelolithotomy is typically used to target the distal curl, stent shaft, and proximal curl stone. Nephrectomy has been performed for severe cases where kidney function has been compromised. Avoiding excessive force when extracting a stent is imperative, as stents may break from loss of tensile strength and potentially cause ureteral injuries or avulsions.

The most common stents are made of nondegradable polymeric and metallic materials such as silicone and/or polyurethane. There have been different approaches to prevent stent encrustation including different stent materials, coatings, and medical treatment to change urine’s pH and composition.

Coated Stents

• Heparin-coated stents initially showed some promise with no encrustation within 10-12 months vs 76% encrustation of noncoated conventional stents. However, these data were refuted, and heparin was found to only reduce encrustation in a sterile environment.
• Triclosan and antibiotic-coated stents were developed with the goal of reducing risk of biofilm formation and urinary tract infection. They were found to have no difference in biofilm formation, encrustation, or infection, and have been deemed unsuccessful due to antibiotic resistance.^1,2
• Hydrogel coating consists of a thin layer of hydrogel capable of absorbing water to prevent bacterial adhesion. However, studies have pointed out that due to absorption of urinary solutes, hydrogel-coated stents could have the same or even higher risk of encrustation.¹
• The Percushield ureteral stent was developed to create a nonionic, super-smooth, hydrophobic inner and outer surface that reduces the adhesion of calcium and magnesium salts.² In vitro studies using the Percushield stent showed significant reduction in encrustation in artificial sterile and infected urine. Nonetheless, Yoshida et al demonstrated, using micro-CT, that there was no statistical difference in outer or inner surface encrustation between the Percushield stent and the conventional hydrogel-coated surface.⁶
• Silver nitrate– and ofloxacin-coated copolymer stents initially showed decreased biofilm formation in rabbits but failed to yield similar results in clinical trials.²
• Oxalate-degrading enzyme coating showed promise but was never taken to market.²

Stent Design

The creation of a biodegradable stent has been in the works for the past 20 years but has been halted by manufacturing limitations. Hopefully with the help of new technologies such as 3D/4D printing these limitations might be overcome and a low-cost mass production can be achieved.¹

Medical Treatments

Medical treatments that alter the urinary chemistry could potentially prevent encrustation. Tavoosian et al found that potassium citrate can significantly reduce double-J stent encrustation in patients with urolithiasis.⁷ While this result is encouraging and could be considered as preventive treatment, it does not seem to be a common practice among urologists. Similarly, Yoshida et al found an association between high triglycerides and total cholesterol with an increase in urinary excretion of lithogenic components such as oxalate, calcium, potassium, and chloride while LDLs increased urinary excretion of protective factors such as citrate and magnesium.⁶ This was the first time this relationship was reported, and the effect of treatment of dyslipidemia in patients with expected long-term stent treatment could be an area of research moving forward.

Despite considerable advances in stent technology, the ideal stent that avoids encrustation has not yet been developed. A novel stent may look promising in vitro; however, success has not translated in the clinical setting. Will it be a new stent shape or design, a new stent coating, an effective biodegradable option, or some combination? Time will tell. Until then, focusing on changes in clinical practice and improved quality initiatives may be the best approach. Ensuring stents are removed in a timely fashion is imperative to avoid the complications associated with stent encrustation. In addition, continued efforts to identify situations where stents are not needed will be an important evolution and change in culture that will benefit the clinical care of this patient population.