Device-specific aneurysm sac morphology after endovascular aneurysm repair: Evaluation of contemporary graft materials

Article Outline

• Abstract
• Methods
  • Study design
  • Patient selection
  • Operative technique
  • Data collection
  • Statistical analysis
• Results
• Discussion
• Conclusion
• Author contributions
• References
• Copyright

Objective

This study analyzed device-specific aneurysm sac morphology after endovascular aneurysm repair (EVAR) with low-permeability devices.

Methods

Between September 2004 and May 2006, 122 patients were treated with EVAR. Three different devices were implanted: 47 Zenith (Cook, Indianapolis, Ind), 46 AneuRx (Medtronic, Santa Rosa, Calif) with Resilient Dacron Graft Material, and 29 Excluder (W. L. Gore & Associates, Flagstaff, Ariz) with low-porosity polytetrafluoroethylene (PTFE). Patients were followed up at 1, 6, and 12 months and then biannually with computed tomography (CT) angiography. Standard axial two-dimensional CT measurements were obtained and compared with preoperative imaging. The preoperative scan served as a baseline, and the minor axis diameter, measured at the largest axial cut of the abdominal aortic aneurysm, was compared with the same measurement at follow-up.

Results

Patient age, sex, and preoperative aneurysm morphology were similar among groups. Patients receiving the Zenith endograft had a significantly larger neck diameter; however, there was no difference in the neck length between groups. The rate of type II endoleaks was similar for the Zenith (17%), AneuRx (17%), and Excluder (14%). At 1, 6, 12, and 18 months, all three grafts were associated with sac shrinkage. The resulting decreases in mean aneurysm size at 18 months and corresponding shrinkage were Zenith, 11%, 6.4 ± 1.8 mm; AneuRx, 18.9%, 12.7 ± 2.7 mm; and the Excluder, 5.5%, 3.3 ± 0.9 (P < .05). The sac size in the 19 patients with a type II endoleak decreased 8.06% compared with a 15.43% decrease in sac size in patients without endoleak at 6 months. No significant sac expansion ≥5 mm has been observed among any of the groups to date.

Conclusions

Sac regression with all devices appears to have been favorably influenced by the new generation of graft materials and is improved compared with published reports of older generation graft materials for the AneuRx and Excluder. However, there is a trend toward greater sac regression with devices using Dacron vs PTFE. The relationship of aneurysm morphology and long-term effects on aortic stent grafts is yet to be determined.

Since the introduction of endovascular aneurysm repair (EVAR) in 1991, as many as 10 grafts have been designed and implanted in clinical trials. Current endografts vary in type of fixation (suprarenal vs infrarenal, presence of hooks/barbs), graft material (polytetrafluoroethylene [PTFE] vs Dacron), and delivery system. Device-specific outcomes have been examined for differences in the rates of endoleak, graft migration, and changes in sac morphology after EVAR. Several trials have now suggested that sac regression or enlargement after EVAR may be device specific. The original Excluder (W. L. Gore & Associates, Flagstaff, Ariz) has been associated with sac enlargement, whereas the AneuRx (Medtronic, Santa Rosa, Calif) has been associated with a stable aneurysm size after repair. Longer-body stent grafts, including the Talent (Medtronic) and Zenith (Cook, Indianapolis, Ind), have been associated with increased sac regression and reduced incidence of type II endoleaks.¹, ², ³, ⁴, ⁵, ⁶, ⁷

Although stable sac size has not been associated with aneurysm rupture, regression of aneurysm sac size is currently believed to be a marker for successful repair. Sac regression remains a reassuring sign of aneurysm exclusion and depressurization of the aneurysm sac, the primary goals of EVAR.⁸ Sac enlargement after EVAR, however, is associated with continued or intermittent pressurization of the sac, secondary to endoleak or endotension, and has been associated with late aneurysm rupture.⁶, ⁹ Sac enlargement after EVAR, without evidence of endoleak, has been attributed largely to material porosity; the manufacturers of the Excluder and AneuRx devices have modified their graft material since 2004 and now use fabric with reduced permeability.¹, ⁴, ¹⁰, ¹¹, ¹² The Zenith endograft, originally designed with a low-permeability Dacron, has often been used as a basis for comparison for other graft types because it is associated with the highest reported rates of sac regression.¹, ², ³, ⁵

The purpose of this study was to analyze device-specific aneurysm sac morphology after EVAR with low-permeability devices. We monitored morphology changes in the aneurysm sac after EVAR with the Excluder, Zenith, and AneuRx devices.

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Methods 

Study design 

From September 2004 to May 2006, 122 consecutive patients underwent EVAR at two institutions in Texas. Three different endoprosthesis were implanted: 47 Zenith, 46 AneuRx with Resilient Dacron Graft Material, and 29 Excluder with low-porosity PTFE.

Patient selection 

All patients underwent a preoperative evaluation with computed tomographic angiography (CTA). Preoperative patient comorbidities captured included coronary artery disease, chronic obstructive pulmonary disease, chronic renal insufficiency, peripheral vascular disease, and cerebrovascular disease, and the overall number for each patient are summarized in Table I. Aneurysms >5.5 cm in men or 5.0 cm in women were offered repair as well as aneurysms twice the diameter of the normal infrarenal aorta.

Table I. Demographics of patients

Characteristica	Zenith (n = 47)	AneuRx (n = 46)	Excluder (n = 29)	P
Age, years	74.9	73.2	73.2	NS
Comorbidities, No.	2.3	2.4	2.3	NS
AAA size, mm	57.3	56.8	57.1	NS
Neck diameter, mm	28.6±1.1	25.4±0.9	25.2±0.8	<.05
Neck length, mm	17.4±0.6	17.6±0.8	17.9±0.7	NS
Renal aortic bifurcation length, mm	105.3±15.1	106.6±12.4	103±13.5	NS
Right CIA diameter, mm	15.4±4.1	15.5±4.6	14.9±3.7	NS
Left CIA diameter, mm	15.3±3.9	15.6±4.2	15.0±3.6	NS
Renal–right iliac bifurcation, mm	155.3±19.1	158.0±17.9	154.5±18.0	NS
Renal–left iliac bifurcation, mm	156.5±18.5	157.7±17.0	154.8±16.9	NS

AAA, Abdominal aortic aneurysm; CIA, common iliac artery.

Device selection was based on instructions for use for each device. All three devices required a proximal neck of ≥15 mm, and the AneuRx and Excluder required proximal neck diameter of ≤26 mm. The Zenith required a proximal neck diameter of <32 mm; therefore, all necks >28 mm were treated with the Zenith. Aneurysms with an infrarenal neck of <26 mm were treated with any of the three devices according to surgeon discretion and preoperative aneurysm morphology.

Operative technique 

All patients underwent EVAR with one of three contemporary low-permeability devices as follows: 47 Zenith, 46 AneuRx with Resilient Graft Material, and 29 Excluder with low-porosity PTFE. The new AneuRx Resilient Graft Material was improved by increasing the twist per inch from 6 to 12 compared with the older reduced-porosity material. The raw material of the graft was unchanged. The increased twist per inch results in a 52% greater density of the graft and a 50% reduction in permeability. The Excluder graft now consists of an expanded PTFE (ePTFE) graft, a low-permeability film layer, and an ePTFE reinforcing film to reduce graft permeability compared with the older Excluder device without the ePTFE reinforcing film.

Angiography was performed to localize the renal arteries with the main device in place. The main body was deployed under fluoroscopic imaging. Completion angiograms were used to assess for graft patency and endoleak, and intravascular ultrasound (IVUS) was used to assess gate cannulation during the procedure as well as the stent graft to vessel wall apposition and length.

Data collection 

Patients were followed up prospectively with CTA at 1-, 3-, 6-, and 12-month intervals, and then biannually. Data for all patients were captured prospectively in a vascular database and retrospectively reviewed. Standard axial two-dimensional CT measurements were obtained and compared with preoperative imaging. Changes in sac size, aneurysmal morphology, and the presence of endoleak were compared among the different devices.

The preoperative scan served as a baseline. Preoperative measurements recorded included neck diameter, neck length, aneurysm size, renal aortic bifurcation length, common iliac artery diameter, and distance from the lowest renal to the iliac bifurcation. The minor axis diameter, measured at the largest axial cut of the abdominal aortic aneurysm, was compared with the same measurement at follow-up. The minor axis was used to avoid overestimation of aneurysm size at points of angulation. To avoid interobserver variability and possible bias, all sac measurements were obtained retrospectively by a single observer who was not directly involved in patient care. Diameter changes ≥5 mm were considered significant for sac regression or expansion based on the Ad Hoc Committee for Standardized Reporting practices in Vascular Surgery of The Society for Vascular Surgery/American Association for Vascular Surgery.⁸

Statistical analysis 

Patient demographics were reported. Changes in aneurysm sac size are presented as mean ± standard deviation and as a percentage increase or decrease from original size. Analysis of variance was used to analyze differences between patients implanted with the three graft types. A value of P ≤ .05 was considered statistically significant.

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Results 

Between September 2004 and May 2006, 122 patients underwent EVAR at two institutions in Texas, during which 47 Zenith, 46 AneuRx with Resilient Graft Material, and 29 Excluder with low-porosity PTFE were implanted in the infrarenal aorta. The mean follow-up was 22.7 ± 3.9 months for all patients, with no significant differences in the length of follow-up between the groups. There was 100% follow-up at 1 month for all patients. At 6 months there were 40 Zenith (85%), 38 AneuRx (83%), and 25 Excluder (86%) available for follow-up studies; at 12 months there were 35 Zenith (75%), 33 AneuRx (72%), and 20 Excluder (69%) available; and at 18 months there were 29 Zenith (62%), 27 AneuRx (57%), and 17 Excluder (58%) available. Late deaths not related to the aneurysm occurred in four patients receiving the Zenith, three receiving the AneuRx, and two receiving the Excluder.

Patient demographics, including age, sex, comorbidities, preoperative aneurysm morphology, and mean maximum aneurysm diameter at baseline, were similar among the three groups (Table I). Technical success was 100%, with no intraoperative conversions. No deaths occurred ≤30 days. No stent fractures, late surgical conversions, or aneurysm ruptures occurred during follow-up. There have been two graft migrations with the Zenith requiring placement of proximal cuffs for type I endoleaks, one migration not requiring intervention and one limb thrombosis requiring femorofemoral bypass with the AneuRx, and one migration of an Excluder did not require intervention.

The overall rate of endoleak was not significantly different; specifically, the rate of type II endoleak at 6 months was Zenith, 17%; AneuRx, 17%; and Excluder, 14%. During this period of implantation with the contemporary graft materials, no secondary interventions were performed for type II endoleaks. The senior authors (C. J. B., and F. R. A.) treat type II endoleaks conservatively, requiring a >5-mm increase in aneurysm size from the first postoperative CTA before treatment.

Device-specific sac morphology is given in Table II. At 1 year, sac size decreased by 10.4 ± 2.6 mm with the AneuRx, 6.7 ± 1.9 mm with the Zenith, and 3.4 ± 0.9 mm with the Excluder (P < .05). Sac regression was maintained out to 18 months with the three devices as well, with Dacron stent grafts having significantly greater sac regression than the Excluder graft. At 18 months, sac size decreased by 12.7 ± 2.7 mm with the AneuRx, 6.4 ± 1.8 mm with the Zenith, and 3.3 ± 0.9 mm with the Excluder (P < .05). The percentage of sac regression during our follow-up is shown in Fig 1, and absolute sac regression is demonstrated in Fig 2. At 1 year, 65% of patients with the AneuRx, 63% with the Zenith, and 34% with Excluder stent grafts had aneurysm sac shrinkage of ≥5 mm. The aneurysm in one patient with a type II end leak and an AneuRx stent graft increased from 57 to 63 mm during a 3-year span, but no treatment was initiated because he was recently diagnosed with metastatic cancer. Among all graft types, 19 patients (8.06%) with a type II endoleak experienced a smaller decrease in sac size compared with the 84 patients (15.43%) without endoleak at the 6-month follow-up (Fig 3).

Table II. Aneurysm diameter changes after endovascular aneurysm repair

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• Fig 1. 

  Device-specific percentage of sac regression.

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• Fig 2. 

  Device-specific sac regression.

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• Fig 3. 

  Effect of endoleak on sac regression at 6-months.

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Discussion 

Since its advent, EVAR has become widely accepted for the treatment of patients with abdominal aortic aneurysms; however, long-term imaging surveillance is indicated secondary to concerns of long-term durability. Imaging is intended to identify endograft failures resulting from stent fractures, graft migration, and persistent endoleak that lead to sac enlargement. Sac enlargement after EVAR is indicative of continued sac pressurization with failure to fully exclude the aneurysm from circulation and is therefore concerning for continued risk of rupture.

Multiple trials have now lent support to indicate that rates of sac expansion or regression after EVAR are strongly correlated with the graft type implanted.¹, ², ³, ⁴, ⁵, ⁶ The original Excluder device has the highest reported rates of sac enlargement, demonstrated to occur in up to 37% of patients at 4 years.⁸ Upon review of surveillance CT scans of those patients in the original Gore Excluder Pivotal Trial, Fillinger¹³ demonstrated that up to 74% of enlarging sacs after EVAR could be attributable to endotension, which is the continued pressurization of the sac with subsequent sac enlargement in the absence of apparent endoleak. It is generally thought that the development of endotension with this graft has been attributable to the permeability of the graft fabric allowing serous transudate to pass through the material contributing to continued sac pressurization.

Aneurysms treated with early generation AneuRx grafts often had a stable sac size after repair, without evidence of regression or expansion; however, these have been associated with a higher incidence of microleaks, or persistent transgraft blood flow, occurring through the thin graft material. Although these type IV endoleaks often spontaneously thrombose and do not usually require secondary intervention,⁷, ¹¹, ¹² they are likely the cause of the reported lower rates of sac regression seen with this graft.

The Zenith stent graft, designed with a low-permeability Dacron fabric, has been associated with the highest rates of sac regression and has often served as a basis of comparison for trials reviewing sac size change after EVAR with the Gore Excluder and Medtronic AneuRx grafts. Trials have estimated that up to 77% of aneurysms treated with the Zenith endografts experience sac regression and only 0.6% experience sac enlargement.¹⁴ Trials comparing these three grafts before 2004, before Gore and Medtronic updated their graft material, consistently demonstrated lower rates of sac enlargement with the Zenith endografts. Sternbergh et al² demonstrated a sac regression of 7.6 mm with the Zenith and 3.5 mm with the AneuRx at 12 months. Greenberg et al³ demonstrated less regression with the original Excluder compared with the Zenith. They further demonstrated that even in the presence of a small endoleak, the aneurysms treated with Zenith endografts would continue to decrease in volume whereas the sac size of those treated with the Excluder would increase. Ouriel et al⁷ further demonstrated that sac shrinkage was most common with the Zenith endograft compared with pre-2004 Talent, AneuRx, and Excluder devices.⁷

Since the aforementioned studies were conducted, however, both Gore and Medtronic have released updated devices with material adjustments to the Excluder and AneuRx, respectively. In 2004, Gore released an updated version of the Excluder endograft that now has a low-permeability layer in attempt to reduce flow across the PTFE graft fabric. Haider et al¹ have since reported that sac shrinkage rates after EVAR with the Gore Excluder low-permeability device are significantly higher than the rates with the original Gore Excluder (63.9% vs 25%, P < .001) and similar rates compared with the Zenith endograft (65.3%). Goodney et al¹⁰ further demonstrated that aneurysms treated with the original Excluder could be relined with the low-permeability updated Gore Excluder, with resultant stabilization and even sac regression in several patients. Also in 2004, Medtronic updated the graft material used in the AneuRx graft to the Resilient Graft Material made of reduced-porosity Dacron. Rates of sac regression since the changes in this material have not been directly examined.

Our results at 12 months demonstrated a significant sac size reduction for the AneuRx and Zenith Dacron stent grafts, defined as >5 mm by The Society of Vascular Surgery.⁸ Aneurysms treated with the updated AneuRx device experienced a sac regression significantly greater than the Zenith stent graft, a finding contrasting with prior studies with earlier AneuRx graft materials. Although our data show a lower rate of sac regression with the Zenith endograft compared with rates reported in the literature, this maybe attributed to our tendency to use this graft in more complex aneurysm anatomy with larger neck diameters. Intuitively, in the absence of significant endoleaks and with successful aneurysm exclusion, sac regression should be equivalent. However, intermittent or undiagnosed problems with proximal and distal seal may lead to repressurization of the sac and a decrease in the percentage of aneurysms that experience regression when treated with this graft.

Although the Gore Excluder did not meet the 5-mm requirement for regression at 12 months, a trend toward decreasing sac size was observed. Recent series have demonstrated equivalent rates of sac shrinkage seen after use of the new Excluder and the Zenith endografts.³ The Excluder, however, demonstrated a significantly lower absolute sac regression compared with the AneuRx and Zenith grafts at the 1-, 6-, and 12-month time intervals in our series. Furthermore, although all contemporary grafts appear to successfully exclude the aneurysm and prevent sac enlargement during a 12-month follow-up, those grafts designed with thicker Dacron graft material experienced significantly greater sac regression than those designed with thin-walled PTFE.

Contrasting evidence regarding the device-specific incidence of endoleaks has been reported. Ouriel et al,⁷ has suggested an increased rate of type II endoleaks with the Excluder (original material) possibly accounting for increased incidence of sac growth. Sheenan et al,¹⁵ however, demonstrated only slight variation in the early incidence of type II endoleak but concluded that the long-term prevalence is difficult to assess secondary to events over time, including spontaneous resolution of endoleaks, development of new endoleaks, and initial aneurysm size. Although the rate of type II endoleak among the varying graft types in our series was similar, we noted that the rate of sac regression was smaller in those with endoleak compared with those without. This finding is consistent with prior investigations and certainly not unexpected as the mechanism of endoleak leads to intermittent or continuous sac pressurization, albeit often less than full systemic pressure.

One potential weakness of our trial lies in the short-term follow-up. Several trials have shown early sac regression followed by reexpansion after 12 to 36 months in the original Excluder device.⁹, ¹⁶ Furthermore, aortic calcification has been implicated as a causative factor in the failure of aneurysm sac regression after EVAR. Postulated mechanisms suggest a decrease in wall compliance and a resultant increase in endotension.⁴ Rhee et al¹⁷ suggest that aortic calcification should be considered a mitigating factor in failed sac regression when there is 75% to 100% circumferential involvement of the aorta. A second limitation of this study, therefore, is that we did not compare the degree of aortic calcification between the grafts, thereby leaving a bias against any graft used in aortas with heavier calcification. It is unlikely, however, that the difference among the groups would be statistically significant.

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Conclusion 

Although device-specific results and sac regression have been studied in the past, to our knowledge this is the first study to date to evaluate device specific sac regression with both the contemporary Excluder and AneuRx devices. The introduction of new generation graft materials appears to have favorably influenced sac regression with all devices. These are improved compared with published reports of older generation graft materials of the AneuRx and Excluder. However, there is a trend toward greater sac regression with devices made with Dacron compared with PTFE. The relationship of aneurysm morphology and long-term effects on aortic stent grafts is yet to be determined. It will be important to follow-up these patients to determine durability.

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Author contributions 

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References 

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