“Magic” Bioabsorbable Metal Stents: The Role of the “Houdini stent” in the disappearance of CLI
In 1892, Ehrich Weiss (aka Harry Houdini) with his brother, and later his beloved wife, Bess, burst upon the vaudeville scene by performing the first of over 11,000 performances of their famous disappearing act “Metamorphosis.” Harry was tied up, shackled with irons and locks, and put in a locked trunk in full view of the audience. A small curtain was placed over the trunk by Bess, who immediately stepped out behind one side of the curtain while Harry stepped from the other side. The curtain was immediately raised and the trunk opened, only to find Bess tied and locked within the trunk, all within 10 seconds, completely mystifying the audience. Harry Houdini went on to become the world's greatest magician and escape artist, and I could not help but to think of him when I read this report on the Magic (Biotronik Inc., Berlin, Germany) absorbable metal stent (AMS). Is this technology “hocus-pocus,” or the real thing, as we are certainly in great need of weapons to fight the war against critical limb ischemia (CLI).
CLI is rapidly becoming a “global epidemic” and is responsible for an estimated 220,000-240,000 major and minor amputations yearly in the United States and Europe.1-2 The incidence of CLI and diabetes is increasing yearly and one out of four diabetics will experience CLI in their lifetime, with a 7-40 times greater risk for an amputation than the non-diabetic.3 The clinical costs of CLI and amputations are high, with the 30-day perioperative mortality and morbidity ranging from 4-30% and 20-37%, respectively, in the highest risk patient populations, and overall less than 50% of amputees achieving full mobility.1-3 The 2- to 5-year survival for the amputee significantly decreases versus the non-amputee, and those patients who achieved limb salvage in a recent report by Kalra et al were found to have a 60% 5-year survival rate after revascularization versus only a 26% survival rate after an amputation.4
The economic costs to the patient, family and our health care system have likewise been found to be extremely high, with the estimated total U.S. costs of treating CLI between $10-20 billion yearly. There is an estimated annual cost of $49,000 to maintain one amputee at home and between $70,000-$100,000 for the 15-20% of amputees who will now require permanent nursing home placement and professional nursing care.2,4-5 Clearly, the clinical and economic costs of CLI are staggering, and any new “metamorphosis” or treatment strategy, such as a “disappearing stent,” that could facilitate a “great escape” from the crippling grip of CLI would be as revolutionary or “disruptive” as the balloon of the 1970’s and the bare metal stent (BMS) of the 1980’s.
BMS were revolutionary as an excellent solution for elastic vessel recoil and dissections after balloon angioplasty, and have become a primary platform for the treatment of most cardiovascular diseases. BMS, however, have unfortunately created their own “diseases,” including intimal hyperplasia, subacute thrombosis, toxicity, inflammation, and stent fracture. Many of these “diseases” are being creatively treated, but certainly a safe and effective short-term nonpermanent BMS solution (
Stent fractures have been reported in almost every vascular territory, including coronary artery stents.6-7 Several recent reports have identified nitinol stent fractures in infrainguinal arteries to be common and associated with adverse clinical outcomes.8-9 Scheinert et al analyzed 261 SFA stents in 121 legs and reported a 37.2% incidence of stent fractures in the treated legs and 64 of 261 stents (24.5%) with a significant decrease in 12-month stent patency in those patients suffering stent fractures (41.1% vs. 84.3%, p 9
One may theorize that the incidence of stent fractures in infrapopliteal vessels should be less than in the SFA, but much is still left to be learned regarding the incidence and mechanisms of stent crush and fracture. I suspect the report by Sianos et al of several drug-eluting coronary stent fractures all presenting with thrombosis should alert us to the potential for infrapopliteal stent fractures.7 Clearly, many clinicians have lowered their threshold for utilizing recent novel non-stent plaque debulking technologies, such as excimer laser atherectomy (Spectranetics Corporation, Colorado Springs, CO), plaque excision atherectomy (SilverHawk, FoxHollow Technologies, Inc., Redwood City, CA) and non-stent cryoplasty (Polarcath, Boston Scientific Corporation, Maple Grove, MA) when treating CLI, indicating a potential interest in a non-BMS or a bioabsorbable stent in this clinical scenario.
These authors report a first-in-man or near first-in-man report using the Magic AMS in 20 CLI patients with 12-month Kaplan Meier clinical results for mortality, primary patency, secondary patency, and limb salvage of 85.0%, 73.3%, 78.9%, and 94.7%, respectively, in Rutherford class 4 and 5 patients. Immediate technical success was high, utilizing intravascular ultrasound (IVUS)-directed stent deployment with clinical follow-up being provided by duplex ultrasound (DU). This safety and efficacy study was somewhat limited to short lesions (
It would be interesting to know the exact comparison of radial stent strength changes over time between the Magic AMS versus known BMS, especially if this technology is to be expanded to other peripheral vessels exposed to significant vascular recoil forces (renals, celiacs, etc). Previous biodegradable or absorbable stents have been prone to significant inflammatory responses and unwanted clinical sequelae. The authors note magnesium (more than 90%) and rare earth elements as an alloy that possess ideal properties as an AMS platform. After reviewing their reference, I must admit this alloy sounds promising and certainly deserves to be tested further in this “metamorphosis” involving stent technology and design.
One may ask, do we need dedicated AMS infrapopliteal stents at all? I would vote “yes,” especially when considering the rather poor results reported thus far with BMS in this area, and the explosion of multi-channel CTA in the diagnosis and follow-up of cardiovascular diseases. Simply to have imaging-friendly and “disappearing stents” available only for when most-needed has far-reaching clinical implications. What a needed relief if we find that AMS do not require long-term clopidogrel (Plavix, Sanofi-Synthelabo, Inc., New York City, NY) therapy, especially for the elderly patients on a fixed income. In PCI, mean luminal diameters (MLD) have always equated to improved outcomes and stent patency, but this has not been considered as important in the interventional treatment of PVD, primarily because of larger vessel size. Often, when interventionally treating infrapopliteal vessels, clinicians are forced to accept improved but not maximum flow or maximum MLD for fear of creating dissections and not having the same “bail-out” stents available as in PCI. Many clinicians may completely avoid PTA in infrapopliteal vessels for similar reasoning and again accept improved, but perhaps not optimal, pedal flow and potentially accept a less-than-optimal revascularization result with current technology. These vessels are oftentimes heavily calcified and chronic total occlusions (CTOs) occur in approximately 50% of CLI patients, further underscoring the significant potential for vascular recoil and suboptimal MLD achieved and accepted with our current infrapopliteal revascularization strategies. A safe and effective tibial AMS could allow for more aggressive plaque debulking or PTA with resulting optimal MLD. Just as in PCI, the potentials for clinical benefits are further magnified when we consider the potential for utilizing AMS in even smaller tibial-pedal vessels (
In conclusion, the potential impact of this novel “disruptive” technology is unlimited when considering its potential use in every vessel, in every size from head-to-toe and everything “in-between.” CLI is a devastating disease with a huge global impact. Our tools to treat this disease have been limited in the past, but several emerging technologies have expanded our CLI “tool box” and the Biotronik Magic AMS appears to be yet another very promising tool. The results of future trials will be anxiously anticipated and I personally hope soon be able to add this tool to my arsenal of weapons to fight CLI. Unlike most other recent technologies, it is of particular interest that significant resources are being allocated to develop this exciting new technology primarily for peripheral tibial vessels and not only for coronary arteries and PCI. As Robert Ripley might have said, “We’ve now got a disappearing stent to treat CLI, believe it or not!”