What Does a Kidney Stent Actually Look Like?

A kidney stent, more precisely called a ureteral stent, is a thin, flexible hollow tube roughly the diameter of a drinking straw, with a small curl or coil at each end that gives it its signature shape. Doctors often call it a “double-J” or “pigtail” stent because those curled tips look like the letter J (or a pig’s curly tail) when viewed from the side. The design is deceptively simple for a device that sits entirely inside your body, running from your kidney down to your bladder, and the details of its shape, material, and engineering reveal a lot about why it works and why it sometimes causes problems.

The Basic Shape and Size

Picture a piece of cooked spaghetti about 20 to 30 centimeters long (roughly 8 to 12 inches), but hollow through the middle like a tiny drinking straw. That gives you a sense of the general proportions. The tube is usually between 4.7 and 8 French in diameter, which translates to roughly 1.5 to 2.7 millimeters across. For context, that is thinner than a standard pencil. Most adult stents fall in the 6 to 7 French range.

The feature that makes a ureteral stent instantly recognizable is the curl at each end. One curl sits in the kidney’s renal pelvis (the funnel-shaped collection area inside the kidney), and the other curl rests inside the bladder. These curls are what keep the stent from migrating up or down. Before curled-tip stents existed, earlier designs used collars, wings, flanges, and barbs to stay in place, but those approaches had high rates of stent migration. The double pigtail design, first developed in the 1970s, solved the migration problem elegantly and became the modern standard.¹

Along the length of the tube, you will notice tiny side holes spaced at intervals. These drainage holes allow urine to enter and exit the stent at multiple points, not just through the open ends. If one section of the stent gets blocked by a crystal deposit or pressed against the ureter wall, urine can still flow in through a nearby hole. The overall surface is smooth and slightly glossy, designed to minimize friction against the delicate tissue of the ureter.

What They Are Made Of

Most ureteral stents you will encounter are made from one of two polymers: polyurethane or silicone. They look slightly different. Polyurethane stents tend to be firmer and somewhat translucent or amber-tinted, while silicone stents are softer, more opaque, and often appear white or off-white. Both are flexible enough to bend and conform to the natural curves of the ureter, but they feel distinctly different in the hand. A polyurethane stent has a bit of spring to it; a silicone stent feels more limp and rubbery.

The choice between them is not just about handling characteristics. Research suggests that material should be matched to how long the stent needs to stay in place: polyurethane tends to perform better for shorter durations (under about six months), while silicone holds up better for longer-term placement.² A prospective study comparing polyurethane stents from one manufacturer against silicone stents from another evaluated outcomes across these two material types in controlled groups.³ The tradeoff is that softer silicone stents can be harder to insert and may not resist external compression as well, while stiffer polyurethane stents may cause more irritation to the bladder lining.

The String You Might See

Some stents have a thin thread attached to the bladder-end curl, left trailing out through the urethra. If you have had a stent placed and noticed a fine string coming out of your body, that is not a defect or a stray suture. It is there on purpose so the stent can be removed in a clinic visit without needing a scope inserted into the bladder. You simply (well, a doctor simply) pulls the string, and the curled end straightens out as it is drawn through.

Not all stents have strings. Whether one is attached depends on the surgeon’s preference, how long the stent will stay in, and the patient’s anatomy. Stents without strings require a cystoscope for removal, a thin camera-equipped tube inserted into the bladder to grab the lower curl with tiny grasping forceps. Both flexible cystoscopes and rigid instruments are used for this, and both approaches typically require only topical anesthesia rather than general sedation.⁴

Metallic Stents Look Very Different

While most stents you will see in photos or informational brochures are the soft polymer type, there is a distinct-looking alternative: the all-metal stent. The Cook Resonance stent, for example, is made entirely of a nickel-cobalt-chromium-molybdenum alloy. It looks nothing like the floppy plastic tube most people picture. Instead, it resembles a tightly wound coil of fine wire, similar in appearance to a tiny spring or a coiled telephone cord. It is shiny and rigid-looking, though it does flex.

Metal stents are attractive for specific situations because they resist external compression (useful when a tumor is pressing on the ureter) and can go longer between exchanges than polymer stents.⁵ However, they are not the default choice for routine cases. Their appearance alone can be startling if a patient sees one on an X-ray without context: the bright, clearly metallic coil stands out sharply against soft tissue, looking almost like a piece of hardware that wandered in from an engineering shop.

Magnetic-Tip Stents and Other Design Twists

A newer variation adds a small magnetic cube to the bladder-end curl of an otherwise standard-looking polymer stent. The magnet is attached through a short string to the loop at the bottom of the stent. To remove the stent, a catheter-like retrieval device with its own magnetic tip is passed into the bladder. The two magnets lock together, and the stent is pulled out without needing a cystoscope or any kind of camera guidance.⁶ This approach has been studied in both adults and children, where avoiding cystoscopy is especially desirable.⁷

Visually, a magnetic-tip stent looks like a regular double-J stent with a small metallic bead dangling from one end. On an X-ray, the magnet appears as a distinct bright spot at the bladder end of the stent. The rest of the stent may or may not be visible on X-ray depending on its material; most polymer stents contain radiopaque markers (tiny embedded lines or dots of barium or bismuth) so they can be seen on imaging. Without those markers, a plastic stent would be essentially invisible on a standard X-ray.

What Stents Look Like After They Have Been Inside You

A freshly removed stent rarely looks as clean as the one that went in. The most visually dramatic change is encrustation, where mineral deposits build up on the stent’s surface. These deposits are mostly calcium oxalate salts that crystallize out of urine.⁸ A mildly encrusted stent looks like it has been lightly frosted with gritty white or yellowish mineral powder. A heavily encrusted one can be so coated that it barely resembles a tube anymore; the curled tips may be buried under lumps of stone-like material, and the side holes can be completely blocked.

The process behind encrustation involves multiple factors including how long the stent stays in, the patient’s individual urine chemistry, and the formation of biological films on the surface.⁹ Two pathways drive the buildup: a thin conditioning film that forms almost immediately when the stent contacts urine, and bacterial biofilms that develop on top of that initial layer.¹⁰ The longer the stent stays in, the worse the encrustation tends to be. Stents left beyond their intended dwell time can become so encrusted that they are difficult to remove and may require additional procedures to break up the deposits first.

The Invisible Problem on the Surface

Even a stent that looks relatively clean to the naked eye is rarely sterile. Bacterial biofilms, which are colonies of bacteria embedded in a protective slimy matrix, form on stent surfaces remarkably quickly. One study examining 30 ureteral stents that had been in place for five to 128 days found that 90% had bacteria adhering to them. In laboratory experiments, common urinary tract pathogens were able to attach and begin forming biofilms on stent material within 24 hours.¹¹

This is worth knowing because it explains something that confuses many patients: you can have a biofilm-colonized stent and a clean urine culture at the same time. The bacteria living in the biofilm are physically stuck to the stent surface and sheltered by the matrix they produce. They do not necessarily shed into the urine in high enough numbers to trigger a positive culture, yet they can still contribute to symptoms and drive mineral encrustation.¹² Different coatings and biomaterials are being developed to reduce bacterial adhesion and crystal deposition on stent surfaces, but no current solution eliminates the problem entirely.

Why the Curled End Causes Discomfort

If you have a stent and are wondering why your bladder feels like it is being poked, the answer is literally visible in the design. The lower curl of the stent sits inside the bladder, and every time the bladder contracts or you move in certain ways, that coiled plastic tip rubs against or presses into the bladder wall. This mechanical irritation is thought to be the primary driver of the pain, urgency, and frequency that stent patients commonly experience.¹³

There is also a second mechanism at play. The stent holds the ureter open as an unobstructed channel between the kidney and bladder. When the bladder pressure spikes during urination or even just filling, urine can reflux backward up through the stent into the kidney. That backflow transmits high pressure to the renal pelvis, producing flank pain that some patients describe as similar to a kidney stone. The exact cause of stent discomfort remains an active area of research, but the physical characteristics of the lower curl, particularly its length and stiffness, are clearly involved. Softer materials and shorter bladder curls tend to cause less irritation, which is one reason material choice matters beyond just durability.

Sizing Stents for Children

Pediatric ureteral stents are visually similar to adult versions but scaled down. Children’s ureters are shorter and narrower, so the stent length must be adjusted. One approach that emerged from studying over 150 pediatric cases found a reliable relationship: the appropriate stent length in centimeters tracks closely with a child’s age in years plus ten.¹⁴ A five-year-old, for instance, would typically get a 15-centimeter stent. This relationship held regardless of gender or which side the stent was placed on. Infant and toddler stents can be startlingly small, barely longer than an adult’s finger, with proportionally tinier curls at each end.

Pediatric stent design also tends to favor narrower gauges (smaller French sizes) and softer materials to reduce trauma to smaller, more delicate anatomy. The magnetic-tip design described earlier is particularly appealing in pediatric patients, since removing a stent from a child without cystoscopy means avoiding sedation or anesthesia that a scope-based removal would typically require.

Biodegradable Stents on the Horizon

One of the most interesting directions in stent design is the development of biodegradable ureteral stents. These are designed to dissolve inside the body after they have done their job, eliminating the need for a removal procedure entirely. The concept is driven by a straightforward problem: every conventional stent that goes in has to come out, and that removal adds a medical visit, cost, and discomfort. For patients who forget or miss their removal appointment, a retained stent can become heavily encrusted and lead to serious complications.¹⁵

Researchers have experimented with stents made from natural polysaccharides like alginate and gellan gum, sometimes blended with gelatin. These stents look similar to conventional ones when freshly made, though they tend to be more opaque and have a slightly different texture. The dissolution timeline can be tuned based on the material composition, with complete breakdown occurring somewhere between 14 and 60 days depending on the formulation.¹⁶ The challenge is ensuring the stent holds its shape and keeps the ureter open for long enough, then dissolves predictably without releasing fragments that could cause blockages. This technology is not yet in routine clinical use but represents one of the more active areas of ureteral stent research.

How the Design Evolved to Get Here

The earliest ureteral stents were not the self-contained internal devices we know today. They were essentially catheters that ran from the kidney out through the body, with an external end that had to be managed and was prone to infection. The first report of a polyethylene stent used inside a human ureter came in 1952. Polyethylene was chosen because it was durable and water-repellent, but it turned out to be susceptible to the same problems that still plague stents today: migration, encrustation, and infection.¹

Silicone stents followed in the 1960s, offering better drainage and longer service life but still migrating out of position. Engineers tried adding various mechanical retention features, collars and barbs and flanges, to anchor the stent. None worked reliably until the double pigtail design arrived in the 1970s. That simple idea, curling each end so the stent would anchor itself by coiling in the kidney and bladder, turned out to be the lasting solution. The basic silhouette of today’s ureteral stent has not fundamentally changed since then. What has changed is the materials science, the coatings, and the addition of features like magnetic tips and drug-eluting surfaces that try to address the complications the original design could not prevent.

What You See on Imaging

If you are looking at your own X-ray or CT scan trying to spot a stent, here is what to expect. Most polymer stents are not directly visible on X-ray because plastic does not block X-rays. What you will see instead is a thin line running from the kidney area down to the bladder, formed by the radiopaque markers embedded in the stent wall. These markers are typically thin stripes or segments of heavy-metal material (barium sulfate or bismuth compounds mixed into the polymer) that show up as bright white lines on the image. At the top and bottom of that line, you should see the characteristic curls, each forming a rough loop or J shape.

Metal stents, by contrast, are unmistakable. They appear as bright, dense coils, dramatically more visible than the faint marker lines of a polymer stent. On a CT scan, any type of stent is easier to see, and the scan can also reveal whether encrustation has developed around the stent, showing up as bright mineral deposits clinging to the tube’s outline. If you have been told your stent is “in good position,” what the radiologist is confirming is that one curl is sitting in the kidney’s collecting system and the other is in the bladder, with the shaft running a clean line between them and no kinking or displacement.