Palmaz Stent: How It Works, Medical Uses, and MRI Safety

The Palmaz stent, invented by Argentine-born vascular radiologist Julio Palmaz in the mid-1980s, was the first balloon-expandable stent to receive FDA approval for use in human arteries. Its slotted-tube design, laser-cut from a single piece of stainless steel, set the template for nearly every coronary and peripheral stent that followed. Though newer stent generations have largely replaced it in day-to-day clinical use, the Palmaz stent’s engineering principles and clinical track record remain foundational to interventional medicine.

How the Palmaz Stent Works

Unlike self-expanding stents made from shape-memory alloys, the Palmaz stent starts as a small, tightly crimped tube threaded over a deflated balloon catheter. Once positioned inside a narrowed artery, the balloon is inflated, forcing the stent’s stainless-steel mesh to expand outward against the vessel wall. The stent plastically deforms, meaning it stays open permanently after the balloon is deflated and removed. Finite-element modeling of this process has mapped the stress and strain fields in the stent wall during expansion, revealing details like how much the stent shortens lengthwise as it widens and where structural weak points develop.¹

This balloon-expandable design gives operators direct control over the final diameter of the stent. In bench testing with human arterial tissue, balloon-expandable stents achieved their nominal maximum diameter over 70% of the time, while self-expanding stents never reached their rated size and showed much greater variation in the lumen they created.² That precision matters in tight, calcified lesions where the operator needs to know exactly how wide the stent will be once deployed. The tradeoff is that balloon-expandable stents are rigid once placed and do not adapt if the vessel changes shape afterward, which is one reason self-expanding nitinol stents eventually became preferred in arteries that flex and bend, like those in the legs.

Iliac and Leg Artery Results

The Palmaz stent built its clinical reputation in the iliac arteries, the large vessels that supply the pelvis and legs. Peripheral artery disease in this location causes claudication and, when severe, threatens the limb. Balloon angioplasty alone often fell short because the artery would spring back closed or develop a tear that re-narrowed the vessel. Placing a Palmaz stent after angioplasty kept the vessel propped open.

A multicenter trial reported sustained clinical benefit in about 91% of patients at one year, 84% at two years, and roughly 69% at 43 months, with an angiographic patency rate of 92%.³ A separate follow-up study showed clinical success of about 99% immediately after the procedure and 86% at four years, with only about 5% of arteries developing significant restenosis.⁴ These numbers were strong enough to make iliac stenting a standard treatment.

Performance dropped the further down the leg you went. In a study of 310 patients followed for two to four years, iliac artery lesions showed a restenosis rate of just 0.5% at six months and a four-year primary patency of about 86%. The superficial femoral artery, which runs through the thigh, had an 11% restenosis rate at six months and four-year patency around 65%. Popliteal artery stents behind the knee fared worst, with 20% restenosis at six months and only about 50% patency at four years.⁵ The pattern makes intuitive sense: arteries in the thigh and knee undergo constant bending and compression during walking, which stresses a rigid stainless-steel scaffold in ways that the relatively stable iliac artery does not.

The Coronary Palmaz-Schatz Stent

The version designed specifically for coronary arteries was the Palmaz-Schatz stent, a shorter device with an articulation point in the middle to help it navigate the curves of the heart’s blood vessels. It became the first coronary stent approved by the FDA in 1994 and changed how heart attacks and angina were treated. Before coronary stenting, balloon angioplasty had a frustrating restenosis rate, and the only surgical alternative was bypass grafting. The Palmaz-Schatz stent cut the rate of emergency vessel closure and reduced the need for repeat procedures.

One of the lingering challenges was figuring out what medications patients needed after a stent was placed. Early protocols used full anticoagulation with drugs like warfarin and heparin, which prevented clotting inside the stent but caused significant bleeding complications. A landmark randomized trial in the New England Journal of Medicine compared antiplatelet therapy (aspirin plus ticlopidine) against traditional anticoagulation after coronary stenting. The antiplatelet approach was dramatically better: stent vessel occlusion occurred in only about 0.8% of the antiplatelet group versus 5.4% in the anticoagulation group.⁶ That study changed practice worldwide and is the reason dual antiplatelet therapy remains the backbone of post-stent management today.

An alternative approach tested at the time was a heparin-coated version of the Palmaz-Schatz stent. In a pilot study of 207 patients with stable angina, implantation succeeded in 98% and stent thrombosis did not occur in any phase of the trial, yielding an overall clinical success rate at discharge of 99%.⁷ Later phases of the study progressively shortened post-procedure heparin infusions and eventually replaced anticoagulation with ticlopidine and aspirin, simplifying the recovery process. A separate study using the new antiplatelet-only regimen reported no major cardiac events in the first month and a six-month restenosis rate of about 22%.⁸

Renal Artery Stenting

The Palmaz stent found another important niche in renal artery stenosis, the narrowing of the arteries that feed the kidneys. This condition drives hard-to-control high blood pressure and, if left untreated, kidney failure. Balloon angioplasty alone had poor results at the ostium, the point where the renal artery branches off the aorta, because the stiff aortic wall would cause the artery to spring back shut.

The largest registry tracked 1,058 patients who received Palmaz-Schatz stents for renal artery stenosis. At four years, systolic blood pressure had dropped from an average of about 168 to 147 mmHg, and patients needed fewer blood pressure medications. Kidney function also improved, with serum creatinine falling from about 1.7 to 1.3 mg/dL.⁹ A preliminary multicenter report had earlier established that the stent achieved technical success in nearly all cases when conventional angioplasty failed, with hypertension cured or improved in about 64% of patients at six months.¹⁰ A European multicenter study confirmed a 98% technical success rate.¹¹

These results made Palmaz stenting the go-to procedure for atherosclerotic renal artery stenosis for years. Worth noting is that subsequent large randomized trials of renal stenting generally have cast doubt on whether the procedure benefits all patients with renal artery disease, but the Palmaz-era data established the feasibility and safety of the technique for the subset of patients with severe, symptomatic disease.

Pediatric and Congenital Heart Uses

One of the more creative applications of the Palmaz stent has been in children and young adults with congenital heart defects. Coarctation of the aorta, a narrowing of the body’s main artery, and branch pulmonary artery stenosis, a narrowing of the vessels leading to the lungs, are two conditions where catheter-based stenting can spare patients from open-heart surgery. The Palmaz stent’s ability to be precisely dilated to a specific diameter made it well suited for these anatomically demanding situations.

A study of 21 patients with native or recurrent aortic coarctation demonstrated that Palmaz stents could successfully treat both types.¹² An important advantage in pediatric patients is that these stents can be redilatated as the child grows. In a follow-up study, Palmaz stents previously placed for coarctation were successfully re-expanded during a second catheterization, increasing the median lumen diameter from about 9.2 mm to 11.7 mm and dropping the pressure gradient across the narrowing from around 10.5 mmHg to nearly zero.¹³ That redilatabillity is critical because a stent implanted in a five-year-old will be too small by the time that child is a teenager.

Use Outside Blood Vessels

Though designed for arteries, the Palmaz stent was also adapted for non-vascular obstructions. One such application was in the airways. In patients with inoperable tracheobronchial tumors or strictures that left them struggling to breathe, Palmaz stents were placed to hold open the trachea or major bronchi. In a series of 22 patients, 21 reported immediate improvement in breathing, and mean survival was about 12 months, with two patients tolerating the stent for over three years.¹⁴ The stent was used as a palliative tool in these cases, not a cure, but the relief from suffocating airway obstruction made a real difference in quality of life. Biliary and urologic applications followed similar logic, using the stent’s strong radial force to hold open tubular structures that disease had squeezed shut.

How Arteries Respond to the Stent

Placing a metal scaffold inside a living artery triggers a biological response. The vessel wall recognizes the stent as a foreign body, and smooth muscle cells migrate inward and proliferate, forming a new inner lining called neointima. When that lining grows too thick, it narrows the vessel again, a process called in-stent restenosis. Understanding this biology has been central to improving stent technology.

Animal studies directly compared how vessels reacted to Palmaz stents versus other designs. In canine arteries, the degree of neointimal thickening over Palmaz stents was modest and comparable to that seen with Wallstents, but significantly less than over Strecker tantalum stents. The neointima over Palmaz stents also matured faster, shifting sooner from an active, cellular tissue to a more stable, scar-like lining.¹⁵ Neointimal buildup was generally more pronounced in the femoral arteries than in the iliac arteries, consistent with the clinical observation that leg stents fare worse over time.

Efforts to reduce restenosis through stent coatings met with mixed results. Covering the Palmaz stent with a thin layer of PTFE (the same material as non-stick cookware coating) showed promise in human iliac arteries, with the covered portions maintaining a significantly wider lumen than the uncovered portions.¹⁶ However, heparin polymer coatings, which were expected to prevent clotting and therefore reduce narrowing, backfired in animal testing. Both types of heparin-coated Palmaz stents tested in pig arteries produced more narrowing than uncoated stents, driven by an unexpectedly intense inflammatory reaction to the polymer, not the heparin itself.¹⁷ The lesson was that the coating material’s biocompatibility matters as much as or more than the drug it carries.

MRI Safety and Imaging Artifacts

The original Palmaz stent is made from 316L stainless steel, an alloy that is only weakly ferromagnetic but still enough to cause problems in an MRI scanner. Early testing showed that stainless-steel stents created severe “black-hole” artifacts on MRI images, making it impossible to see what was happening inside or around the stent. Devices made from 304 stainless steel were worse, adding visible image distortion to the artifact problem. As a rule, the more ferromagnetic the device, the worse the MRI artifact.¹⁸

A practical concern for patients was whether it was safe to undergo MRI at all after receiving a Palmaz stent. Case reports of patients who had MRI within two weeks of Palmaz stent implantation in the pulmonary arteries and superior vena cava showed no acute adverse outcomes or long-term problems.¹⁹ Modern guidelines generally consider the 316L Palmaz stent “MR conditional,” meaning it can be safely scanned under specified conditions, though the imaging artifact means the area immediately around the stent will remain unreadable. Newer stent platforms made from cobalt-chromium or platinum-chromium alloys produce less artifact, and nitinol self-expanding stents are nearly MRI-invisible, which is one of several reasons stainless steel has largely been supplanted.

Material Durability Compared to Later Alloys

Stainless steel was the dominant stent material in the 1990s, but it was not the only option, and long-term durability has become an important consideration as patients live decades with implanted stents. Accelerated corrosion testing of 28 different peripheral stents found that nitinol showed the lowest susceptibility to corrosion and the longest time before damage appeared. Cobalt-chromium-nickel alloys performed worst, showing corrosion damage roughly ten times sooner than nitinol. When stents did eventually fail, nitinol stents tended to shorten, while stainless-steel and tantalum stents fractured.²⁰ Stent fracture is a real-world concern in areas of the body that flex repeatedly, and the shift away from stainless steel to nitinol and newer alloys for peripheral arteries was driven partly by this fracture risk.

From Bare Metal to Drug-Eluting Stents

The Palmaz and Palmaz-Schatz stents are classified as bare-metal stents, meaning they are simply metal scaffolds without any drug coating. The persistent problem of in-stent restenosis, which affected roughly one in five coronary stent patients within six months, drove the development of drug-eluting stents that release antiproliferative medications to suppress the growth of neointimal tissue. The first widely used drug-eluting stent, the Cypher sirolimus-eluting stent, arrived in 2003.

A pooled analysis of 14 randomized trials comparing sirolimus-eluting stents against bare-metal stents found no significant difference in the risk of death or heart attack between the two types. The major advantage of the drug-eluting stent was a large reduction in the combined risk of death, heart attack, or need for a repeat procedure.²¹ In other words, drug-eluting stents did not make people live longer, but they did spare many of them from having a second procedure. The rate of stent thrombosis, the feared complication where a blood clot suddenly blocks the stent, was statistically similar between the two types.

What caught researchers off guard was a phenomenon called neoatherosclerosis, the development of new atherosclerotic plaques inside the layer of tissue that grows over the stent. Autopsy studies found that neoatherosclerosis appeared much earlier in drug-eluting stents than in bare-metal stents, with a median onset of roughly 420 days in drug-eluting stents versus about 2,160 days in bare-metal stents.²² The drugs that suppressed smooth muscle cell growth also appeared to alter the healing environment in ways that accelerated a different kind of disease. For bare-metal stents, disruption of these neoatherosclerotic plaques became a recognized driver of very late stent thrombosis, the kind occurring more than three years after implantation.²³

This finding reframed the conversation about stent durability. The Palmaz-era assumption was that once a bare-metal stent healed over and the neointima stabilized, the job was done. Neoatherosclerosis showed that the job is never truly done: the stented segment of artery remains a site of ongoing biological activity for the rest of the patient’s life, susceptible to a new round of the same disease process that narrowed it in the first place. Current-generation drug-eluting stents have thinner struts, more biocompatible polymers, and some use biodegradable coatings that dissolve after releasing their drug payload, all refinements that trace their lineage back to the lessons learned from the original bare-metal Palmaz platform.

Where the Palmaz Stent Still Fits

Despite being a product of 1980s engineering, the Palmaz stent and its direct descendants have not entirely vanished from clinical practice. Large-diameter Palmaz stents remain useful in situations that call for high radial force and precise placement, particularly in large vessels like the iliac arteries, the aorta in children with coarctation, and in certain non-vascular applications. The Palmaz Genesis and XL variants updated the original platform with improved deliverability and a wider range of sizes while retaining the core balloon-expandable, stainless-steel architecture. In congenital heart disease, where patients may need multiple redilatations over a lifetime, the Palmaz platform’s track record of safe re-expansion gives it an ongoing role that newer designs have not fully displaced.