Journal of Vascular and Endovascular Therapy

Keywords

EVAR; Zenith; Stent graft; Surveillance; Follow-up; Complication; Re-intervention; survival

Introduction

The number of endovascular aneurysm repairs (EVARs) has continuously increased since its introduction over twenty years ago [1-3]. Successful short-term results have been reported, but the systematic long-term results are still limited. This concerns especially device-specific results.

The EVAR Trial 1 was the first randomized controlled trial comparing EVAR to OR with long-term results now available [4]. The short-term data showed significantly lower mortality in EVAR group during the first six months, but beyond eight years mortality rates were conversely [4,5]. The difference in late mortality was predominantly caused by the late aneurysm ruptures in EVAR group [4]. Also, the rate of re-interventions was higher among EVAR patients compared to OR at all followup time points. The trial was started in 1999 and some of the stent grafts implanted at that time are no longer available thus making the utilization of the results into current practise difficult. The device specific short-term data showed fewer reinterventions and lowest all-cause mortality in patients treated with the Zenith (Cook Inc, Bloomington, Ind.) stent graft, but the highest graft occlusion rate compared to other stent grafts [6].

The Zenith is the only stent graft still in use since its introduction in 1997 and approval by U.S Food and Drug Administration (FDA) in 2003. The structural design of the device has remained basically the same since its introduction over twenty years ago. To date, three long-term studies analysing over ten-year followup results of the Zenith stent graft have been published [4,7,8]. These studies show low aneurysm rupture, conversion and reintervention rate up to sixteen-years, but also highlights the need for lifelong surveillance. The objective of this review is to summarise the long-term results of the Zenith stent graft.

Long-Term Survival

Currently EVAR is the method of choice for treating abdominal aortic aneurysm (AAA), although it was originally developed to offer a less-invasive treatment option to OR for patients with multiple co-morbidities. Therefore, it is understandable that in papers reporting long-term results of Zenith stent graft approximately only half of the patients are still alive at five years [7,9]. On the other hand, one quarter of the patients are still alive fourteen years after the initial EVAR procedure and every tenth survived up to sixteen years. As EVAR has almost replaced OR in AAA treatment we need to be able to offer the patients devices that are durable over a long period of time.

During the long-term follow-up approximately 22% of the patients under systematic surveillance require additional procedure for graft related complications [7]. The re-intervention-free survivals are high at 5, 10 and 16 years with the Zenith stent graft (80- 88%, 73-76% and 73%) [7,9]. The most common complication is an endoleak type II, but eventually only one-third of them require treatment [7]. The recent guideline of European Society for Vascular Surgery (ESVS) for the treatment of AAA recommends even less frequently need for treatment [10]. The complications that were often seen in other early stent grafts, such as endoleak type III, stent fracture and migration, are uncommon with the Zenith stent graft even in the long-term surveillance [11,12]. Reported rates for migration and for stent fracture are 0.3-2% and 0.7%, respectively [4,7,8]. Two long-term studies of the Zenith stent graft report type III endoleak to be the most common form of late stent graft failure, however with low rates of 0.7-1.5% [8,9]. This finding emphasizes the meaning of durability of stent grafts as it implies an unstable stent graft structure or position.

Despite its many positive features, the Zenith stent graft has been criticized for the increased risk of limb occlusion. It has been suggested that the underlying reason for occlusion is the segmental long stents in skeleton [6,13,14]. In general, the reported occlusion rates vary between 0-8.8% in short- and midterm studies [15,16]. The corresponding figures especially for the Zenith device are 2.7-5.6% with most of the occlusions occurring during the early years after EVAR [9,17,18]. The current long-term studies confirm this finding as in up to sixteen-year surveillance the occlusion was detected in 3.4-5% of the patients [7,9].

In our long-term study with the Zenith stent graft, despite the regular surveillance and active treatment of complications, the annual aneurysm rupture rate remains at 0.49% [7]. These ruptures (N=10) were mainly secondary due to late type I endoleak. Some of them could have been avoided by open conversion, but the patients were considered too sick for OR. Nine out of ten ruptures occurred among patients under regular surveillance. Interestingly, early type II endoleak was found to be a significant risk factor for late re-intervention and even aneurysm rupture despite the fact that almost half of them seal spontaneously and only minority of them resulted in sac enlargement in early surveillance. The finding is in line with the EVAR Trial 1 [4]. Most of the ruptures are seen in early years after EVAR, but one rupture as late as fourteen years after uncomplicated early survival occurred in our long-term study [7,8]. In spite of all the challenges with the endovascular technique, the aneurysm rupture-free survival after EVAR is high with the Zenith stent graft; 96-98% at ten years [7-9].

Surveillance after EVAR

Many centres fail in regular EVAR surveillance and considerable number of patients are lost to follow-up [19,20]. These patients seem to have the worse survival outcomes [21]. In EVAR Trial 1 many patients were discharged from surveillance after several years. The secondary aneurysm rupture rate was higher in EVAR group compared to OR, but owing to the insufficient surveillance, some patients may have lost the option of planned re-intervention preventing the rupture [9]. Despite the systematic surveillance, the aneurysm rupture is not totally avoidable, and the aneurysm rupture risk remains [7].

New complications keep appearing years after EVAR and the lifelong surveillance is mandatory to detect all potential reintervention requiring complications. Most of the patients are re-intervention-free survivors (78%) and clearly not all patients need annual life-long surveillance after EVAR [7]. There are recommendations for surveillance after EVAR which significantly reduce the burden of follow-up visits [10]. EVAR Trail 1 showed that re-interventions occur in long-term surveillance also in patients who were free from re-interventions at two and five years. Therefore, we must avoid too early modified selected surveillance after EVAR [4].

Increased cancer mortality was detected among EVAR patients in EVAR Trial 1, but the long-term study of Zenith didn’t confirm that finding as the cancer mortality in the study population and the age-matched general population were similar [4,7]. In modern computed tomography (CT) the radiation exposure is lower than early years and most of the surveillance is carried out by ultrasonography nowadays so the currently treated patients will probably never exceed the amount of radiation that their counterparts were exposed in early years of EVAR. Indeed, the focus should be instead in treatment providers repeated radiation exposure. The recent guidelines recommend ultrasonography (US) based surveillance reducing the cumulative radiation dose of the patient in long-term surveillance and many centres have shift the surveillance from CT to US years ago [10,22].

Conclusion

The Zenith stent graft provides an effective treatment option for AAA patients. It shows high re-intervention- and aneurysm rupture-free survival in long-term follow-up. Despite the systematic surveillance aneurysm rupture is not totally avoidable. Most of the patients have uncomplicated surveillance and in future, we need studies to clarify, how to safely maximize the surveillance intervals but still find the life-threatening complications early enough. The endovascular treatment and imaging technology have improved during the last decades, but we need safe, systematic, easy and cost-effective surveillance after EVAR.

The summary of EVAR Trials states, that there is no survival difference between EVAR and OR in long-term and EVAR is more costly over a patient's lifetime. With currently used devices, modern imaging modalities and personalized surveillance the advantages of EVAR might, however, be achieved. The good longterm results with Zenith stent graft support the use of EVAR for AAA treatment also in the future.

References

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