| | Mechanical failure of prosthetic human implants: A 10-year experience with aortic stent graft devices☆☆☆★Presented at the Fiftieth Annual Meeting of the American Association for Vascular Surgery, Boston, Mass, Jun 9-12, 2002. Received 18 June 2002; accepted 3 September 2002. Abstract Objective: The first endovascular stent graft was implanted to treat an abdominal aortic aneurysm more than a decade ago. This technique has evolved dramatically with the growing understanding of metallurgic and fabric sciences and improved device designs. However the potential for stent graft material failure remains. This investigation describes the incidence of material failure, potential modes of device fatigue, and the clinical significance of these failures. Methods: Six hundred eighty-six endovascular stent grafts were used to treat patients with aortic aneurysms. Device fatigue in the form of stent, suture fracture, or graft wear was identified with an analysis of follow-up radiographs and explanted stent grafts. A review of patient clinical histories, spiral computed tomographic scan studies, scanning electron microscopy, and energy dispersion spectroscopy of explanted devices was conducted to evaluate the modes and consequences of failure. Results: Sixty patients were identified with device fatigue, 49 of whom had abdominal endovascular repairs and 11 of whom had thoracic repairs. Of the 60 patients with stent graft fatigue, 43 patients had metallic stent fractures, 14 had suture disruptions, and three had graft holes. These material failures occurred within seven distinct stent graft designs. The average time to the recognition of failure was 19 months, with a mean follow-up period of 8 months since the event was identified. Eleven patients died, and one was lost to follow-up 2 years after identification of a stent fracture. The remaining patients are presently being followed eoyj physical examination, plain film radiograph, and computed tomographic scans for clinical sequelae of device fatigue. Conclusion: Endovascular stent graft fatigue has been recognized in numerous devices after aortic implantation. Fatigue may take the form of stent, graft, or suture failure, with certain modes unique to specific stent graft devices. The clinical significance of stent graft material failure remains uncertain. (J Vasc Surg 2003;37:16-26).
The first endovascular stent graft repair to treat an abdominal aortic aneurysm was performed by Parodi, Palmaz, and Barone1 more than a decade ago. An estimated greater than 25,000 aortic stent grafts have since been deployed worldwide, and although preliminary results have been promising, problems with deployment, stent graft migration, endoleak, material failure, and aneurysm rupture have all been reported.2, 3, 4, 5, 6, 7 Many of these problems were seen with first generation stent grafts. Fundamental mechanical and technical device problems have been addressed, and individual implants have been improved. However, as midterm results of second-generation endovascular grafts are reported, new problems are discovered.
Device fatigue remains one of the most concerning modes for potential procedure failure, encompassing the breakdown of the intrinsic mechanical parts of the stent graft. It is often difficult to identify device fatigue because patients are typically asymptomatic at the time of presentation. This difficulty has made a true understanding of the magnitude of the problem challenging. Moreover, the clinical significance of many identified material failures is unknown. The purpose of this study was to analyze a large single experience of endovascular aortic repairs to identify the incidence of device fatigue and the potential modes of fatigue and to try and determine the clinical significance of these material failures.
Methods  Study population Six-hundred eighty-six patients underwent endovascular aortic aneurysm repair over a 10-year period, 39 of which were implanted by the senior author while attending at the Montefiore Medical Center and the remaining 647 performed at the Mount Sinai Medical Center. Four hundred four of these patients had a full compliment of follow-up analyses for review and form the study set for this investigation. A total of 60 patients (15%) of this subset had stent graft fatigue as identified with radiographic studies or the analysis of an explanted stent graft device. Patients were followed at 1, 3, 6, and 12 months and annually thereafter, in accordance with the specific US Food and Drug Administration protocols for each device, with physical examination, plain film radiographs, duplex ultrasonography, and spiral computed tomographic (CT) scans. The median follow-up period was 30 months, with a range from 1 to 52 months. The information obtained from these visits and the results of the radiologic studies were analyzed for evidence of device fatigue. Device fatigue was defined as an actual change in the mechanical nature of the device, such as suture disruption, metal fracture, or fabric erosion. Procedure failure was defined as a repressurization of the aneurysm sac or rupture of the aneurysm. Techniques for endovascular repair The three techniques used for aortic aneurysm repair included: tubular (aortoaortic), aortouniiliac, bifurcated-modular, and bifurcated-unibody. All of these techniques have been demonstrated in detail previously.8, 9, 10, 11, 12, 13 Devices for abdominal aortic aneurysm and thoracic aortic aneurysm repair Seven different devices were used to treat patients with abdominal aortic aneurysms, and two for thoracic aortic aneurysms. Extensive descriptions have been provided in the literature of the nine devices used: Talent,14, 15 Vanguard,14, 16 Parodi-Modified Endovascular stent graft,13 EVT,10, 17 AneuRx,18 Gore Excluder,19 Cordis Quantum LP,20 and the Talent and Gore thoracic devices.21 Stent graft analysis Radiographic studies Plain film radiographs and CT scans were performed at 1, 6, and 12 months and annually thereafter. CT scanning consisted of a noncontrast examination followed by a dynamic CT angiogram to evaluate for endoleaks or changes in aneurysm size. Plain films were used to analyze the stent components of the stent graft. Initially two views, anteroposterior and lateral, were used; however, over the past 2 years, right posterior oblique and left anterior oblique views have been added to improve visualization of the structural components of the device Explant analysis Five grafts with device fatigue were available for explant analysis after open surgical conversion. The device was first inspected macroscopically for evidence of stent fracture, fabric tears, or suture breaks, and then representative regions of the metal stent were studied with scanning electron microscopy (SEM). Preparation of specimens for SEM Explanted stent grafts were rinsed of remaining debris and fixed in buffered formaldehyde. Each specimen was rinsed in distilled water and air dried for 24 hours. Representative regions of the metallic stent were ultrasonicated in a mild cleaning agent for 10 minutes, rinsed in distilled water, and ultrasonicated in absolute alcohol for another 10 minutes. The specimens were mounted on SEM stubs with silver paint. A Hitachi S530 SEM was used to evaluate the metal surface for defects and to analyze the site of fracture for evidence of fatigue. Energy dispersion spectroscopy (EDS) analysis was used at 15 kV on selected specimens to determine the surface composition of the metal and to assess for metal leaching. Results Sixty patients (49 men and 11 women) were identified with device fatigue, 55 on radiographic analysis and five on explantation. Forty-nine of the devices with fatigue were seen in abdominal aortic aneurysm stent grafts, and 11 were found in thoracic aortic stent grafts. The 60 fatigued stent grafts in this study were distributed among seven different devices (Table I).
| | |  | Device for aortic aneurysm repair | Total implanted | Radiographs reviewed | Total fatigue/fracture | Average time to fracture/fatigue (mo; range) | Average follow-up since fracture/fatigue (mo; range)† | |
|---|
| Abdominal | | | | | | |
|  Vanguard | 26 | 22 (85%) | 16 (72%) | 26* (3-48) | 13 (1-39) | |
| Talent | 337 | 232 (69%) | 24 (10%) | 13 (1-31) | 5 (1-12) | |
| Modified Parodi | 164 | 24 (15%) | 5 (21%) | 38 (33-48) | 6 (1-8) | |
| EVT/Ancure | 9/20 | 7/6 | 1/0 (14%) | 8 | 24 and then lost to follow-up | |
| AneuRx | 39 | 33 (85%) | 3 (10%) | 10 (1-24) | 3 (1-6) | |
| Gore | 18 | 18 (100%) | 0 | | | |
| Teramed | 10 | 10 (100%) | 0 | | | |
| Thoracic | | | | | | |
| Gore TAG | 22 | 19 (86%) | 7 (37%) | 24 (3-38) | 12 (1-42) | |
| Talent | 41‡ | 33 (80%) | 4 (12%) | 9.5 (1-24) | 4 (2-7) | |
| Total | 686 | 404 | 60 | 19 | 8 | |
| *Excluding patient with acute conversion. †Excluding those patients who underwent open conversion and stent graft explanation. ‡Including emergent use not part of clinical study. | | | | |
The average time to fatigue for all devices combined was 19 months (range, 1 to 48 months), with an average follow-up period since fatigue identification of 8 months. A marked variation in the time to failure was seen depending on the type of fatigue (ie, suture fracture, graft holes, or stent fractures; Table II).
Eleven of the 60 patients followed with device fatigue have died, and one patient was lost to follow-up 2 years after fatigue was identified. Of the 11 deaths, three were device related, and the remaining eight patients died of causes unrelated to their aortic aneurysms. Excluding one early postoperative death, the average time to death was 29 months (range, 17 to 52 months) after implantation and 13 months (range, 2 to 26 months) after fatigue was first identified. The remaining patients continue to be followed for clinical sequelae of their stent graft fatigue. | | | | Device | No. fatigued | Location | No. | Average time to fatigue (mo; range) | |
|---|
| AAA | 49 | | | | |
| Talent | 24 | Graft hole | 1 | 16 | |
| | | Metal stent fracture | 23 | 13 (1-31) | |
| | | Longitudinal bar | 14 | | |
| | | Aortic body: proximal | 7 | | |
| | | Aortic body: distal | 1 | | |
| | | Iliac | 6 | | |
| | | Proximal transrenal stent | 5 | | |
| | | Z-shaped body stent | 4 | | |
| Vanguard | 16 | Suture disruption | 14 | 25 (3-48) | |
| | | Row separation | 5 | | |
| | | Body separation | 9 | | |
| | | Graft hole | 2 | Intraoperative & 36 | |
| EVT | 1 | Hooks and shanks | 1 | 8 | |
| Modified Parodi | 5 | Metal stent fracture | 5 | 38 (33-48) | |
| | | Top row of stent | 4 | | |
| | | Second row | 1 | | |
| AneuRx | 3 | Metal stent fracture | 3 | 10 (1-24) | |
| TAA | 11 | | | | |
| Gore | 7 | Metal stent fracture | 7 | 24 (3-38) | |
| | | Isolated longitudinal bar | 3 | | |
| | | Longitudinal bar and z-stent | 2 | | |
| | | Z-stent | 2 | | |
| Talent | 4 | Metal stent fracture | 4 | 9.5 (1-24) | |
| | | Z-stent | 3 | | |
| | | Longitudinal bar | 1 | | |
| | | | | |
Stent graft fatigue analyzed by device Abdominal aorta stent grafts Twenty-four of the 60 patients with device fatigue had Talent grafts inserted (Table II). Fatigue was recognized within several different regions of the stent graft. One patient had a wear hole detected in an explanted prosthesis at the site of graft to stent fixation (Fig 1, A).
The remaining Talent endografts had fractured stents. Fourteen fractures occurred along the longitudinal bar of the graft ( Fig 1, B to D), and nine in the serpentine shaped nitinol ( Fig 1, E, F). Device fatigue was identified in 16 Vanguard abdominal aortic aneurysm stent grafts; 14 had evidence of suture disruption, and two had fabric wear holes (Fig 2, A to D).
The remaining nine fatigues were metal fractures: one EVT device (Fig 3), five modified Parodi endovascular stent grafts (Fig 4), and three AneuRx devices (Fig 5).
Metal fatigue within AneuRx grafts was difficult to conclusively identify on plain film examination because of overlapping metal densities of the stent and the relatively close diamond pattern. Thoracic aorta stent grafts Eleven of the 60 cases with device fatigue were metal fractures in thoracic aortic stent graft devices: seven Gore TAG and four Talent. Although the seven TAG devices had fractures in either the longitudinal bar, the z-shaped stent, or both (Fig 6), the four Talent thoracic aortic aneurysm devices had fractures in the z-shaped stents only.
Explants Five of the six patients who underwent open conversion had the stent grafts explanted. One patient underwent conversion at the time of the primary stent graft procedure for an inadvertent iliac artery rupture with insertion of a second device to seal a type III endoleak. The remaining four patients had a new endoleak and an enlarging aortic aneurysm. The average time to explantation was 28 months (range, 15 to 36 months). SEM and EDS analysis SEM analysis was conducted on all five explanted prostheses. Special attention was given to regions of the stent graft where corrosion would be expected to form (ie, crimped area of the ends of the nitinol wire stent in the Talent device, the platinum-nitinol interface of the Vanguard stent graft, and the high stressed angled regions of zig-zag nitinol stents; Fig 7, A to C).
There was minimal evidence of corrosion on all the specimens examined. The stent wire was covered with a relatively uniform oxide surface layer ( Fig 7, D) except for an occasional pit. These pits were analyzed with EDS, and an apparent decrease in the nickel concentration on the metal surface was noted. The expected balanced ratio of titanium to nickel (51%:49%) was commonly seen in the area immediately subjacent to the regions with defects (Fig 8, A, B) One explanted TAG device with a metal fracture was analyzed; the fracture surface showed cleavage lines within the radius of the stent typical of stress fatigue ( Fig 9).
Discussion Since the first endovascular stent graft was introduced by Parodi, Palmaz, and Barone1 over a decade ago, advancements in stent graft materials, designs, and deployment techniques have all contributed to the rapid growth and utility of these devices. Despite enhanced understanding of the metallurgic and fabric properties of endovascular stent grafts, material failure continues to be a potential problem. The inherent properties of the materials (strength and corrosion resistance) combined with extrinsic forces contribute significantly to the risk of device fatigue. Before deployment, and while still in the delivery sheath, the metallic stent may experience increased risk for failure based on damage during loading and subsequent confinement in the delivery catheter. Once implanted, the device is then subjected to additional extrinsic forces imposed by the geometry of the tortuous aorta and the impact of continuous, high-pressure blood flow. In this study of 60 patients with stent graft fatigue, 43 of the patients had metallic stent fractures, 14 had suture-stent disruptions, and three had graft holes. It is useful to analyze these three failure modes before speculating on the overall clinical significance of stent graft fatigue. Metallic fracture The 43 metallic stent fractures analyzed in this study occurred in devices fabricated by five different manufacturers. Thirty-seven fractures were documented in superelastic nitinol (nickel and titanium) stents, five fractures in stainless steel (316L) devices, and one in an elgiloy (cobalt-chromium, nickel) stent. Elgiloy fractures were initially reported in the hooks of first generation EVT devices during the early phase of the EVT trial.17 Even though the hooks were remodeled to a more gradual angle to decrease the stress applied across the hook, Najibi et al22 reported two cases of hook fractures identified 36 months after implantation of the remodeled Ancure device. Although the EVT device in this study had similar fractures to the ones reported in the literature, there was no evidence of fractures in the remodeled Ancure system. However, the mean follow-up for patients with Ancure devices in this study was only 13 months. Stress fatigue is just one of the causes postulated for metal fractures in these devices. Metal corrosion has been identified on SEM studies of explanted Stentor grafts, with more severe corrosive irregularities detected in those devices implanted for longer period of time.23, 24 However, SEM failed to show significant signs of corrosion in the nitinol stents of this study and, in fact, revealed a relatively uniform oxide layer on the surface of the metal stents, which may provide resistance to corrosion. The lack of corrosion in this study compared with the results of the earlier Stentor models may reflect the improved understanding of nitinol processing and the importance of surface treatments, such as electropolishing,25 titanium nitride annealing,26 heat treatments, and nitric acid passivation27 to create a thin, uniform oxide layer to help prevent corrosion and increase biocompatibility and long-term durability of the metal.28 Overall structural integrity of these devices can be affected by morphologic characteristics of the recipient vessel, changes in the aneurysm shape over time, or cyclic loading from aortic pulsations.29, 30 Kinking of the graft can occur from these stresses and has been associated with limb occlusion and device or modular component migration in the Stentor/Vanguard model29 and with metal stent fracture of the longitudinal bars in the Talent device.30 In this report, 18 fractures occurred in the longitudinal nitinol bars of Talent or Gore stent grafts. These events were associated with tortuosity of the implant vessels with presumed increased stress across the nitinol wire. The remaining metal stent fractures occurred in Palmaz stainless steel stents (n = 5) and high stressed angled regions of nitinol z-shaped stents (n = 19). The cyclic load from the aortic pulsations can generate enough stress to cause microcracks, material irregularities on the surface of metals produced during laser cutting, to propagate and eventually fracture. Unfortunately, limited in vivo information exists on fatigue crack propagation for endovascular stents. Studies to date have focused on orthopedic or heart valve implants, which are larger devices and have different loading patterns.31 Although the metals used today have high crack propagation thresholds, the strut widths are thin, approximating 250 μm, thereby leaving a small distance necessary for a microcrack to travel before fracture.32 Fabric fatigue All five explanted stent grafts had evidence of fabric fatigue. Although degradation of conventional polyester grafts has been reported, it usually occurs 10 to 20 years after implantation.33 The fabric wear seen in endovascular stent grafts occurs much earlier. The pulsatile flow of the aorta and the configuration of the stent graft allows for micromotion of the individual metal stents against the fabric leading to eventual breakdown and graft wear. Fabric fatigue was one of the first reported causes of aneurysm rupture and stent graft failure in the earlier Stentor graft.2 External abrasions of the fabric on the metal stent and frank graft holes have been described.24, 33, 34 In the Stentor and Vanguard devices, the stent is attached to the graft at the proximal and distal ends only allowing for continuous motion between the stent and the graft. Fewer examples of graft fenestrations have been reported with the Talent design, whose stent is completely fixed to the graft with sutures, thereby reducing the movement between the graft and the nitinol stent. In the one patient whose device was explanted immediately on open conversion and in which fabric fatigue was seen, micromotion of the Vanguard device is unlikely to have caused the failure. Fatigue had to have occurred during manufacturing or packaging of the device into the deployment catheter. Theoretically, if the stent graft is drawn into the catheter from the opposite direction from which it will be deployed, the angled points of the stents can press firmly against the graft fabric, potentially causing a small tear or hole. Suture breakage Polypropylene sutures that are used to assemble the Stentor/Vanguard device may also be subject to fatigue. Micromotion of the individual nitinol stents causes friction and wear of the sutures, with ultimate suture fracture and stent row separation. As of January 2001, there has been a 21% prevalence rate of row separation reported in the Vanguard model.35 Of the 14 suture disruptions that were identified in this report, five were row separations and nine occurred in the body; this finding is in support of Riepe's hypothesis that there is more motion in the larger frames of the body stents.3 Besides the intrinsic properties of the materials used for stent graft fabrication and the extrinsic hostile environment of the native aorta, other factors could play a role in device fatigue. Twenty-four of the 60 patients (40%) in this report were diagnosed with endoleaks during the follow-up period. The precise time of device failure and endoleak formation is often impossible to determine; however, only five patients were found to have metallic fractures before the onset of the endoleak. The remaining 19 patients had an endoleak develop followed by metallic fracture or had both events simultaneously discovered. Endoleaks have been shown to maintain pressure and turbulent flow in the aneurysm sac, leading to eventual enlargement of the aneurysm and risk for rupture.6, 37 The residual aneurysm sac pressure may be communicated to the stent graft itself in the form of increased device pulsatility that can lead to metallic fracture, suture disruption, and fabric wear, resulting in stent graft device fatigue. Although most authors agree in the significance of a type I or type III endoleak contributing to aneurysm dilatation and risk of eventual rupture, the role a type II endoleak has is less clear.38, 39, 40 In this study, there were seven type II endoleaks, six of which were diagnosed before the stent graft fracture. All six of these endoleaks were observed on average 20 months (range, 6 to 32 months) before diagnosis of the stent graft fracture. One can hypothesize that although a type II endoleak may not generate enough pressure to cause aneurysm dilatation and rupture, that there is enough pulsatile flow to expose the stent graft to excess micromotion, which in turn could lead to device fatigue. Clinical significance Although fractures were found in 60 patients with endovascular stent grafts, most of the patients has been asymptomatic and have not as of yet needed interventions for device fatigue. Although three patients had symptomatic aneurysms develop in the setting of stent fractures, it is unclear whether these events were related to the fatigued stents. One patient with an immediate type III leak (graft hole) after implantation died after open conversion as a consequence of that leak. This adverse event is assumed to be device related. Concern regarding fractured stent erosion through the fabric of stent grafts remains; however, this event has not been clearly identified in this study. At this time, we recommend that if a patient is asymptomatic and there is no evidence of aneurysm enlargement, rupture, or a type I or III endoleak, observation of the stent graft fatigue is acceptable in the setting of increased graft surveillance. Conclusion Endovascular grafts have been clinically used to treat aortic aneurysms for more than 10 years. A significant growth in the understanding of the failure modes of the materials used to fabricate these devices and a respect for the relatively harsh environment into which they must function has led to the development of improved stent graft designs. Endovascular graft devices may experience metal fracture, fabric erosion, and ultimate device fatigue as a result of these two components. Stent graft packaging into delivery catheters, sterilization, and shelf aging may all further impact the occurrence and timing of device integrity failure. Improved manufacturing practices can decrease the frequency of these fatigue events; however, clinical implications of many forms of device fatigue remains to be defined. Acknowledgements We thank Mr Norman Katz for technical assistance with the scanning electron microscopy portion of this investigation and Mr Joseph Samet for help with the photographic preparation of this manuscript.
Discussion Dr Piergiorgo Cao (Perugia, Italy). Material fatigue is a central issue in stent graft durability. I have a few questions. Did you hypothesize that the degradation of the material was dependent more from the lenghth of the follow-up done than from the structural characteristic of the endografts? So, was it more time dependent than structural dependent? Did you correlate any difference between internal and external stenting of the graft? The external nitinol can be separated from the bloodstream and therefore protected from degradation. Another important issue is the connection between the fabric and the stent. I found in your abstract that the Excluder graft, in which the stent is not sutured but embedded inside the PTFE, was more degradative and presented more mechanical failure. I wonder if you have any explanation for that. Dr Tikva S. Jacobs. In our study, we only had three explanted devices that had evidence of fabric erosion, so it is a little difficult to draw any conclusions from that. But of those three grafts that had the fabric erosion, they were both in nitinol and Dacron materials and both of them were in devices that had a nitinol endoskeleton. Dr Richard P. Cambria (Boston, Mass). Dr Jacobs, it was a wonderfully illustrated presentation, obviously with a long experience. My question may be somewhat repetitive to Dr Cao's, but it was a little difficult to follow in terms of the different types of grafts. Do your findings have implications for either graft material, polyester versus PTFE? And with particular reference to your homemade system, we saw the overall data there, but how about the comparisons between polyester and PTFE? And to reiterate the question, in your summary data, does this have design implications for whether or not an exoskeleton or an endoskeleton is preferred? Dr Jacobs. To answer your first question about Dacron versus PTFE, again, we only had five explanted devices to really analyze the material, and only three of them had evidence of fabric erosion. All three were in Dacron devices. We did not find any evidence of fabric erosion in the PTFE. In reference to the design with endoskeletons versus exoskeletons, the majority of our device fatigue was actually in metal stent fracture. Seventy-two percent of our device fatigues were metal stent fractures. And it did not really matter whether it was an endoskeleton or an exoskeleton. What we noticed is that the majority of them were either in the longitudinal bars of the metal stents or at the point, the apex, of the z-shaped stents. And it is most likely the metal stent fracture that is secondary to cyclic loading and stress. References 1.
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Division of Vascular Surgery, Department of Surgery, Mount Sinai School of Medicine. New York, NY ☆ Competition of interest: nil. ☆☆ Reprint requests: Dr Michael L. Marin, Division of Vascular Surgery, Department of Surgery, Mount Sinai Medical Center, 5 E 98th St, Box 1259, New York, NY 10029-6574 (e-mail: michael.marin@mountsinai.org). ★ 0741-5214/2003/$30.00 + 0 PII: S0741-5214(02)75199-9 doi:10.1067/mva.2003.58 © 2003 Society for Vascular Surgery and The American Association for Vascular Surgery. Published by Elsevier Inc. All rights reserved. | |
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