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J ENDOVASC THER 2014;21:728–734
^ CLINICAL
INVESTIGATION ——————————————————————————
^
Endograft Conformability and Aortoiliac Tortuosity in Endovascular Abdominal Aortic Aneurysm Repair Kevin Lee, MD; Erik Leci, BSc; Thomas Forbes, MD; Luc Dubois, MD, MSc; Guy DeRose, MD; and Adam Power, MD, MPhil Division of Vascular Surgery, Western University, London, Ontario, Canada. ^
^
Purpose: To determine conformability of stent-grafts in endovascular aneurysm repair (EVAR) using centerline of flow measurements and to compare conformability in patients with severe aortoiliac tortuosity. Methods: From 2012 to 2013, 111 consecutive patients (98 men; mean age 75.467.7 years) underwent endovascular aneurysm repair with Endurant I and II and Zenith Flex, LP, and Spiral Z stent-grafts; their pre- and post-EVAR computed tomography (CT) studies were retrospectively analyzed using quantitative 3-dimensional imaging software. The length between the lowest renal artery and the iliac bifurcation was measured using centerline of flow and was defined as the treatment length (TL). The difference in TLs pre and post EVAR were compared as a surrogate to evaluate endograft conformability. Results: A total of 203 pre and post EVAR aortoiliac TLs were measured (99 Endurant I, 20 Endurant II, 32 Flex, 6 LP, and 42 Spiral Z). Overall, there was a mean difference of 5.067.3 mm or 2.6%63.9% between the pre- and post-EVAR TLs (p,0.001). No statistically significant difference in TLs was observed among the various stent-grafts (p¼0.115). In 40 patients with severe aortoiliac tortuosity, the post-EVAR TL was 16.265.5 mm or 8.0%62.7% shorter than the pre-EVAR TL (p,0.001); again, there was no difference in TLs among the various devices implanted (p¼0.737). Conclusion: Overall, there was no difference in treatment lengths before and after EVAR among different stent-grafts, suggesting similar conformability. Interestingly, patients with severe aortoiliac tortuosity were found to have significantly shorter post-EVAR treatment lengths compared to before EVAR, which should be considered when planning EVAR. J Endovasc Ther. 2014;21:728–734 Key words: endograft, stent-graft, conformability, aortoiliac vessels, tortuosity, abdominal aortic aneurysm, aorta, aortic bifurcation, common iliac artery bifurcation, angle, length ^ ^
Little is known about stent-graft conformability in endovascular aneurysm repair (EVAR). Depending on the characteristics of the stentgraft, the native aorta and iliac arteries accordingly remodel once a stent-graft is deployed. 1,2 First-generation stent-grafts, such as Vanguard and AneuRx, were known to shorten more than 10 mm in up to 56% of
cases when deployed because of their relative rigidity.3 In 14% of cases, additional extension stent-grafts were required for type Ib endoleaks.3 Current generation stent-grafts have a tendency to shorten in tortuous aortoiliac anatomy but have much improved conformability in non-tortuous aortoiliac vessels.1,2 Severe common iliac artery (CIA) tortuosity
Adam Power received an honorarium from Cook Medical Inc for a conference presentation. The other authors declare no association with any individual, company, or organization having a vested interest in the subject matter/products mentioned in this article. Corresponding author: Adam Power, Division of Vascular Surgery, Western University, London, Ontario, Canada. Email: [email protected] Q 2014 INTERNATIONAL SOCIETY
OF
ENDOVASCULAR SPECIALISTS
doi:10.1583/14-4663MR.1
Available at www.jevt.org
J ENDOVASC THER 2014;21:728–734
ENDOGRAFT CONFORMABILITY IN EVAR Lee et al.
^
^ TABLE 1 Baseline Characteristics of the 111 Study Patients Age, y Men AAA diameter, cm Repair configuration Bifurcated Aortouni-iliac Stent-graft Endurant Zenith Anaconda Severe aortoiliac tortuosity (n¼44) Infrarenal neck angle Aortic bifurcation angle Common iliac angle
75.467.7 (88.3%) 6.060.8 96 (86.5%) 15 (13.5%) 68 (61.3%) 41 (36.9%) 2 (1.8%) 2 (4.5%) 14 (31.8%) 28 (63.6%)
^
^ Continuous data are presented as the means 6 standard deviation; categorical data are given as the counts (percentage).
can lead to unsuccessful deployment, kinking, or thrombosis of the stent-graft.4 This study’s primary objective was to compare the conformability of current generation stent-grafts using centerline of flow measurements.2,5,6 Our secondary objective was to determine stent-graft conformability in patients with severe aortoiliac tortuosity.
METHODS Patient Sample A total of 111 consecutive patients (98 men; mean age 75.467.7 years) underwent elective EVAR between 2012 and 2013 and were included in this retrospective imaging study. The mean diameter of the abdominal aortic aneurysm (AAA) was 6.060.8 cm. The majority of repairs (96, 86.5%) employed bifurcated stent-grafts; 15 (13.5%) were aortouni-iliac configurations (Table 1). The stent-grafts were predominately Endurant I or II [68 (61.3%); Medtronic Cardiovascular, Santa Rosa, CA, USA] compared to 41 (36.9%) Zenith models (Flex, LP, and Spiral Z; Cook Medical, Bloomington, IN, USA) and 2 (1.8%) Anaconda stentgrafts (Vascutek, a Terumo company, Inchinnan, Scotland).
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Study Measurements Contrast-enhanced computed tomography (CT) images were processed in Digital Imaging and Communications in Medicine (DICOM) format and imported to an Aquarius 3D Workstation (TeraRecon, San Mateo, CA, USA) to generate reconstructed images. A center lumen line (CLL) was then created from the supraceliac aorta to the external iliac artery automatically (or manually when the automatic function did not create an adequate CLL). For bifurcated stent-grafts, the distances from the lowest renal artery to the right and left internal iliac arteries were measured. For aortouni-iliac (AUI) configurations, only one treatment length was measured from the lowest renal artery to the appropriate internal iliac artery. These measurements were determined both before and after EVAR (Fig. 1). In addition to centerline of flow measurements, the angles of the infrarenal aorta, aortic bifurcation, and CIA were also determined using the 3D reconstruction of the aortic lumen. The infrarenal angle was defined as the angle between the centerline in the infrarenal neck and the line in the aneurysm sac. Similarly, the aortic bifurcation angle was defined as the angle between the centerline in the aneurysm sac and in the CIA, while the CIA angle was the maximum angulation of the CIA (Fig. 2). Severe aortoiliac artery tortuosity was defined as at least one angle .908 among the three measured. Contrary to other studies, this definition allows for an easy and simple method for selecting patients with severe aortoiliac tortuosity when planning EVAR.7 Following the same methods described above, two independent observers (A.P. and K.L.), who were blinded to the endografts used, made the measurements. Any differences between observers were averaged; any major differences (.10 mm) were discussed, and a consensus measurement was reached.
Statistical Analysis A chi-squared test was performed for analysis of categorical variables. Analysis of variance (ANOVA) was used to compare continuous variables. Comparison of treat-
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Figure 1 ^ Treatment length (A) pre EVAR and (B) post EVAR in a patient with severe aortoiliac tortuosity.
ment lengths before and after EVAR was analyzed using paired t tests. ANOVA was performed to analyze the difference in preand post-EVAR treatment length change between the stent-grafts and for subgroup analyses in patients with and without severe aortoiliac tortuosity. Univariate analysis was done to determine variables influencing change in treatment length, and multiple regression analysis was performed using significant variables from the univariate anal-
ysis. P,0.05 was considered statistically significant. Statistical analysis was performed with IBM SPSS Statistics (version 20.0; IBM Corporation, Armonk, NY, USA).
RESULTS A total of 207 treatment lengths were measured, but the 2 Anaconda grafts (4 measurements) were excluded, leaving 203 lengths for analysis. The stent-graft model information
Figure 2 ^ Examples of severe aortoiliac tortuosity: (A) right aortic bifurcation angle .908 and (B) left common iliac angle .908.
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^
^ TABLE 2 Comparison of Pre- and Post-EVAR Length Differences Among the Tested Stent-Graft Models Length Difference, mm Endurant I
Endurant II
Zenith Flex
Spiral Z
Zenith LP
p
Overall 3.866.2 (n¼99) 4.167.3 (n¼20) 6.268.7 (n¼32) 7.268.1 (n¼42) 4.369.4 (n¼6) 0.115 Severe aortoiliac 15.365.7 (n¼13) 14.462.4 (n¼5) 17.465.6 (n¼7) 16.866.5 (n¼14) * 0.737 tortuosity No severe aortoiliac 2.164.1 (n¼87) 0.764.5 (n¼15) 1.764.3 (n¼23) 2.863.5 (n¼29) 1.266.0 (n¼5) 0.550 tortuosity
^
^ * Only one patient; not included in analysis.
was missing in 4 (2%) of the treatment lengths. There were 44 angles in 40 patients that met the criteria for severe aortoiliac tortuosity: 2 infrarenal neck angles, 14 aortic bifurcation angles, and 28 CIAs (Table 1). The patients with severe aortoiliac tortuosity were older (mean age 78.568.0 vs. 74.767.3 years, p¼0.014), had larger mean aneurysm diameters (6.561.0 vs. 6.060.7 cm, p¼0.003), and longer pre-EVAR treatment lengths (204.1624.1 vs. 184.7616.8 mm, p,0.001). The two groups were similar in sex distribution (p¼0.246). Forty treatment lengths were measured in the presence of severe aortoiliac tortuosity compared to 159 without. Post-EVAR CT studies were performed at a mean of 53.6624.7 days. The mean pre- vs. post-EVAR treatment length difference was found to be 5.067.3 mm (mean change 2.6%63.9%) for the entire cohort. The mean difference was greater in the 40 patients with severe aortoiliac tortuosity [16.265.5 (mean change 8.0%62.7%)] compared to those without [2.164.4 mm (mean change 1.1%62.4%), p,0.001]. As shown in Table 2, there was no statistically significant difference among Endurant I and II and Zenith Flex, LP, and Spiral Z stentgrafts in pre- and post-EVAR treatment lengths (p¼0.115). In patients with severe aortoiliac tortuosity, there was no statistically significant difference among stent-grafts (p¼0.737) vs. patients without aortoiliac tortuosity (p¼0.550). The proportion of patients with severe aortoiliac tortuosity did not differ among stent-grafts (p¼0.106). Severe aortoiliac tortuosity and aneurysm diameter were found to be significant vari-
ables on univariate analysis (p,0.001), but on multiple regression analysis, only the presence of severe aortoiliac tortuosity was found to be significant (p,0.001).
DISCUSSION Measuring treatment lengths in EVAR patients from the lowest renal artery to the internal iliac artery can be used as a surrogate for graft conformability by comparing pre- and postoperative lengths. Centerline of flow measurement was chosen as the way to measure treatment lengths since it provides more accurate measurements than marker catheter angiography, which often underestimates treatment lengths.8 Furthermore, 3D CT reconstruction software used for centerline of flow measurements has been previously shown to provide the highest intraobserver consistency and interobserver reliability when assessing diameter and length measurements.9 We have defined severe aortoiliac artery tortuosity as having at least one angle .908 at the infrarenal neck, aortic bifurcation, or CIA bifurcation. This definition provides a more widely applicable method to identify patients with severe aortoiliac tortuosity. Other measures, such as the iliac tortuosity index, were designed to recognize factors affecting implantation and are not necessarily used to assess graft conformability.7 For instance, the iliac tortuosity index accounts for only the most acute angle from the common femoral artery to the aortic bifurcation, but our method includes two angles (aortic bifurcation and CIA), both of which we feel specifi-
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cally affect endograft conformability. Severe aortic bifurcation angulation accounted for 32% of patients in our study with severe aortoiliac tortuosity, which would not have been captured using the tortuosity index. The 5.0-mm mean difference between preand post-EVAR treatment lengths in our study is near the range reported by others (3.2 to 4 mm).1,2 Change in treatment lengths before and after EVAR occurs due to a combination of remodeling of the native aorta, stent-graft conformability, and stiffness of guidewires and delivery systems during EVAR. Therefore, a completely conformable endograft would result in identical pre- and post-treatment lengths. Our study showed significant improvement in current-generation stent-graft conformability compared to first-generation stent-grafts.3 In our subgroup analysis of patients without severe aortoiliac tortuosity, change in treatment length before and after EVAR was only 1.1%, which provides compelling data that there is improved conformability of stentgrafts during EVAR in patients without severe aortoiliac tortuosity. Investigators1,2 have previously identified that tortuous anatomy in EVAR is associated with stent-graft path shortening of 10 mm or more. In our study, severe aortoiliac tortuosity was identified in 20% of treatment lengths measured. These patients tended to be older, had greater AAA diameter, and longer preEVAR treatment lengths than patients without severe aortoiliac tortuosity. In patients with severe aortoiliac tortuosity, the treatment length after EVAR averaged 8.0% shorter than the pre-EVAR treatment length, which is a considerable change from the 1.1% for patients without severe aortoiliac tortuosity. Our group has previously shown women to have more hostile aneurysm neck anatomy but similar technical and clinical outcomes compared to men.10 In patients with severe aortoiliac tortuosity, the proportion of women in our study was higher than in the total study population (16% vs. 9%), but it did not reach statistical significance. Whittaker et al.1 reported a similar 10-mm shortening among Excluder stent-grafts in 11% of iliac limbs. Similar to our study, renal to internal iliac artery tortuosity was the only
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significant factor that predicted this shortening effect. Abbruzzese et al.11 have shown equivalent clinical outcomes at 5 years between Zenith, Excluder, and AneuRx stentgrafts when aneurysmal morphology was correlated to device-specific Instructions for Use (IFU). In their study, neck diameter, neck length, neck angle, and aneurysm sac angulation were measured and compared to the IFU, but iliac anatomy was not evaluated. Despite the lack of IFU criteria for aortoiliac or iliac artery angulation, it is important to consider these angles during EVAR planning. Our results suggest that the conformability of stent-grafts examined in this study is similar. The proportion of patients with severe aortoiliac tortuosity in each stent-graft model did not differ. We did not expect to see any difference in graft-specific conformability between Endurant I and II since both models have identical designs with nitinol stents. Interestingly, among the Zenith stent-grafts that do have a difference in composition and design, there were also no differences in conformability. The Zenith Flex is designed with stainless steel stents compared to Spiral Z and LP, which use nitinol stents. The Spiral Z also has a continuous spiral stent configuration that is different from the discontinuous stent configuration of Zenith Flex and LP. We observed substantial shortening of treatment lengths in patients with severe aortoiliac tortuosity regardless of the manufacturer or stent model. This shortening effect could be due to a change in length of the stent-graft, a change in native vessels during deployment, or a combination of both. Deployment of a stent-graft into a tortuous aorta may cause overlap of fabric between the metal stents at points of severe angulation, which in turn forces the native aorta to take a straighter and shorter path. On the other hand, the aorta may remodel to become straighter and therefore shorter secondary to the stiff wire and delivery system before deployment. Thus, the stent-graft itself may not shorten but instead conforms to a remodeled aorta. Shortening of the stent-graft in patients with severe aortoiliac tortuosity can have deleterious effects. It could lead to inadequate coverage of the distal sealing zone, especially
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considering that the greatest shortening effect in our study was as much as 32 mm. Inadequate distal sealing can potentiate a type Ib endoleak, requiring additional extensions. Velazquez et al.12 demonstrated that improved accuracy of treatment length measurements by centerline of flow led to an average of 1.6 fewer iliac extensions required per EVAR when compared to marker catheter arteriography. However, as seen in our study, centerline of flow measurements cannot accurately predict the treatment length in patients with severe aortoiliac tortuosity. If, on the other hand, the native aorta shortens after stiff wire and delivery device placement, then the chosen stent-graft may be too long and inadvertently cover the internal iliac artery. Inadvertent internal iliac artery coverage occurs in 3% to 10% of cases and can lead to buttock claudication, buttock necrosis, colon ischemia, and endoleak from a patent internal iliac artery.13 In our study, inadvertent internal iliac artery coverage did not occur in the studied population. It is unclear from our study and previous reports the exact mechanism for shortening of the treatment length, and further studies are required.
Limitations The retrospective nature of our study may have led to a selection bias. In terms of centerline measurements, we did not dissociate aortic and iliac segments to determine where exactly the shortening effect occurred, and we did not study the effect of aneurysm thrombus load or presence of artery calcification that may have affected graft conformability. In addition, our study did not specifically address rotation of the stent-graft to facilitate contralateral cannulation, which can alter the stent-graft path.1 There were only two cases in which the iliac limbs were crossed, so a separate analysis was not performed. Our study did not show any difference in conformability among stentgrafts in patients with severe aortoiliac tortuosity, but the number of patients in this group was small. Finally, only the first post-EVAR CT scans were evaluated, so we were not able to determine if further remodeling occurred in longer follow-up or if long-term clinical out-
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come is influenced by severe aortoiliac tortuosity.
Conclusion Our study showed no difference in treatment length change among several different models of stent-grafts, which suggests similar conformability. We did, however, find that significant shortening of treatment length occurs in patients with severe aortoiliac tortuosity. This shortening effect could negatively impact distal sealing zones during EVAR. Therefore, developing an accurate method to predict shortening of treatment lengths in patients with severe aortoiliac tortuosity using centerline of flow measurement would be helpful.
REFERENCES 1. Whittaker DR, Dwyer J, Fillinger MF. Prediction of altered endograft path during endovascular abdominal aortic aneurysm repair with the Gore Excluder. J Vasc Surg. 2005;41:575–583. 2. Higashiura W, Kichikawa K, Sakaguchi S, et al. Accuracy of centerline of flow measurement for sizing of the Zenith AAA endovascular graft and predictive factor for risk of inadequate sizing. Cardiovasc Intervent Radiol. 2009;32:441–448. 3. White GH, May J, Waugh RC, et al. Shortening of endografts during deployment in endovascular AAA repair. J Endovasc Surg. 1999;6:4– 10. 4. Wyss TR, Dick F, Brown LC, et al. The influence of thrombus, calcification, angulation, and tortuosity of attachment sites on the time to the first graft-related complication after endovascular aneurysm repair. J Vasc Surg. 2011; 54:965–971. 5. Sobocinski J, Chenorhokian H, Maurel B, et al. The benefits of EVAR planning using a 3D workstation. Eur J Vasc Endovasc Surg. 2013; 46:418–423. 6. Aziz I, Lee J, Lee JT, et al. Accuracy of threedimensional simulation in the sizing of aortic endoluminal devices. Ann Vasc Surg. 2003;17: 129–136. 7. Chaikof EL, Fillinger MF, Matsumura JS, et al. Identifying and grading factors that modify the outcome of endovascular aortic aneurysm repair. J Vasc Surg. 2002;35:1061–1066. 8. Wyers MC, Fillinger MF, Schermerhorn ML, et al. Endovascular repair of abdominal aortic
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aneurysm without preoperative arteriography. J Vasc Surg. 2003;38:730–738. 9. Parker MV, O’Donnell SD, Chang AS, et al. What imaging studies are necessary for abdominal aortic endograft sizing? A prospective blinded study using conventional computed tomography, aortography, and three-dimensional computed tomography. J Vasc Surg. 2005;41:199–205. 10. Dubois L, Novick TV, Harris JR, et al. Outcomes after endovascular abdominal aortic aneurysm repair are equivalent between genders despite anatomic differences in women. J Vasc Surg. 2013;57:382,389.e1. 11. Abbruzzese TA, Kwolek CJ, Brewster DC, et al. Outcomes following endovascular abdominal
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aortic aneurysm repair (EVAR): an anatomic and device-specific analysis. J Vasc Surg. 2008; 48:19–28. 12. Velazquez OC, Woo EY, Carpenter JP, et al. Decreased use of iliac extensions and reduced graft junctions with software-assisted centerline measurements in selection of endograft components for endovascular aneurysm repair. J Vasc Surg. 2004;40:222–227. 13. Wyers MC, Schermerhorn ML, Fillinger MF, et al. Internal iliac occlusion without coil embolization during endovascular abdominal aortic aneurysm repair. J Vasc Surg. 2002;36:1138– 1146.
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^ CLINICAL
INVESTIGATION ——————————————————————————
^
Endograft Conformability and Aortoiliac Tortuosity in Endovascular Abdominal Aortic Aneurysm Repair Kevin Lee, MD; Erik Leci, BSc; Thomas Forbes, MD; Luc Dubois, MD, MSc; Guy DeRose, MD; and Adam Power, MD, MPhil Division of Vascular Surgery, Western University, London, Ontario, Canada. ^
^
Purpose: To determine conformability of stent-grafts in endovascular aneurysm repair (EVAR) using centerline of flow measurements and to compare conformability in patients with severe aortoiliac tortuosity. Methods: From 2012 to 2013, 111 consecutive patients (98 men; mean age 75.467.7 years) underwent endovascular aneurysm repair with Endurant I and II and Zenith Flex, LP, and Spiral Z stent-grafts; their pre- and post-EVAR computed tomography (CT) studies were retrospectively analyzed using quantitative 3-dimensional imaging software. The length between the lowest renal artery and the iliac bifurcation was measured using centerline of flow and was defined as the treatment length (TL). The difference in TLs pre and post EVAR were compared as a surrogate to evaluate endograft conformability. Results: A total of 203 pre and post EVAR aortoiliac TLs were measured (99 Endurant I, 20 Endurant II, 32 Flex, 6 LP, and 42 Spiral Z). Overall, there was a mean difference of 5.067.3 mm or 2.6%63.9% between the pre- and post-EVAR TLs (p,0.001). No statistically significant difference in TLs was observed among the various stent-grafts (p¼0.115). In 40 patients with severe aortoiliac tortuosity, the post-EVAR TL was 16.265.5 mm or 8.0%62.7% shorter than the pre-EVAR TL (p,0.001); again, there was no difference in TLs among the various devices implanted (p¼0.737). Conclusion: Overall, there was no difference in treatment lengths before and after EVAR among different stent-grafts, suggesting similar conformability. Interestingly, patients with severe aortoiliac tortuosity were found to have significantly shorter post-EVAR treatment lengths compared to before EVAR, which should be considered when planning EVAR. J Endovasc Ther. 2014;21:728–734 Key words: endograft, stent-graft, conformability, aortoiliac vessels, tortuosity, abdominal aortic aneurysm, aorta, aortic bifurcation, common iliac artery bifurcation, angle, length ^ ^
Little is known about stent-graft conformability in endovascular aneurysm repair (EVAR). Depending on the characteristics of the stentgraft, the native aorta and iliac arteries accordingly remodel once a stent-graft is deployed. 1,2 First-generation stent-grafts, such as Vanguard and AneuRx, were known to shorten more than 10 mm in up to 56% of
cases when deployed because of their relative rigidity.3 In 14% of cases, additional extension stent-grafts were required for type Ib endoleaks.3 Current generation stent-grafts have a tendency to shorten in tortuous aortoiliac anatomy but have much improved conformability in non-tortuous aortoiliac vessels.1,2 Severe common iliac artery (CIA) tortuosity
Adam Power received an honorarium from Cook Medical Inc for a conference presentation. The other authors declare no association with any individual, company, or organization having a vested interest in the subject matter/products mentioned in this article. Corresponding author: Adam Power, Division of Vascular Surgery, Western University, London, Ontario, Canada. Email: [email protected] Q 2014 INTERNATIONAL SOCIETY
OF
ENDOVASCULAR SPECIALISTS
doi:10.1583/14-4663MR.1
Available at www.jevt.org
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^
^ TABLE 1 Baseline Characteristics of the 111 Study Patients Age, y Men AAA diameter, cm Repair configuration Bifurcated Aortouni-iliac Stent-graft Endurant Zenith Anaconda Severe aortoiliac tortuosity (n¼44) Infrarenal neck angle Aortic bifurcation angle Common iliac angle
75.467.7 (88.3%) 6.060.8 96 (86.5%) 15 (13.5%) 68 (61.3%) 41 (36.9%) 2 (1.8%) 2 (4.5%) 14 (31.8%) 28 (63.6%)
^
^ Continuous data are presented as the means 6 standard deviation; categorical data are given as the counts (percentage).
can lead to unsuccessful deployment, kinking, or thrombosis of the stent-graft.4 This study’s primary objective was to compare the conformability of current generation stent-grafts using centerline of flow measurements.2,5,6 Our secondary objective was to determine stent-graft conformability in patients with severe aortoiliac tortuosity.
METHODS Patient Sample A total of 111 consecutive patients (98 men; mean age 75.467.7 years) underwent elective EVAR between 2012 and 2013 and were included in this retrospective imaging study. The mean diameter of the abdominal aortic aneurysm (AAA) was 6.060.8 cm. The majority of repairs (96, 86.5%) employed bifurcated stent-grafts; 15 (13.5%) were aortouni-iliac configurations (Table 1). The stent-grafts were predominately Endurant I or II [68 (61.3%); Medtronic Cardiovascular, Santa Rosa, CA, USA] compared to 41 (36.9%) Zenith models (Flex, LP, and Spiral Z; Cook Medical, Bloomington, IN, USA) and 2 (1.8%) Anaconda stentgrafts (Vascutek, a Terumo company, Inchinnan, Scotland).
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Study Measurements Contrast-enhanced computed tomography (CT) images were processed in Digital Imaging and Communications in Medicine (DICOM) format and imported to an Aquarius 3D Workstation (TeraRecon, San Mateo, CA, USA) to generate reconstructed images. A center lumen line (CLL) was then created from the supraceliac aorta to the external iliac artery automatically (or manually when the automatic function did not create an adequate CLL). For bifurcated stent-grafts, the distances from the lowest renal artery to the right and left internal iliac arteries were measured. For aortouni-iliac (AUI) configurations, only one treatment length was measured from the lowest renal artery to the appropriate internal iliac artery. These measurements were determined both before and after EVAR (Fig. 1). In addition to centerline of flow measurements, the angles of the infrarenal aorta, aortic bifurcation, and CIA were also determined using the 3D reconstruction of the aortic lumen. The infrarenal angle was defined as the angle between the centerline in the infrarenal neck and the line in the aneurysm sac. Similarly, the aortic bifurcation angle was defined as the angle between the centerline in the aneurysm sac and in the CIA, while the CIA angle was the maximum angulation of the CIA (Fig. 2). Severe aortoiliac artery tortuosity was defined as at least one angle .908 among the three measured. Contrary to other studies, this definition allows for an easy and simple method for selecting patients with severe aortoiliac tortuosity when planning EVAR.7 Following the same methods described above, two independent observers (A.P. and K.L.), who were blinded to the endografts used, made the measurements. Any differences between observers were averaged; any major differences (.10 mm) were discussed, and a consensus measurement was reached.
Statistical Analysis A chi-squared test was performed for analysis of categorical variables. Analysis of variance (ANOVA) was used to compare continuous variables. Comparison of treat-
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J ENDOVASC THER 2014;21:728–734
Figure 1 ^ Treatment length (A) pre EVAR and (B) post EVAR in a patient with severe aortoiliac tortuosity.
ment lengths before and after EVAR was analyzed using paired t tests. ANOVA was performed to analyze the difference in preand post-EVAR treatment length change between the stent-grafts and for subgroup analyses in patients with and without severe aortoiliac tortuosity. Univariate analysis was done to determine variables influencing change in treatment length, and multiple regression analysis was performed using significant variables from the univariate anal-
ysis. P,0.05 was considered statistically significant. Statistical analysis was performed with IBM SPSS Statistics (version 20.0; IBM Corporation, Armonk, NY, USA).
RESULTS A total of 207 treatment lengths were measured, but the 2 Anaconda grafts (4 measurements) were excluded, leaving 203 lengths for analysis. The stent-graft model information
Figure 2 ^ Examples of severe aortoiliac tortuosity: (A) right aortic bifurcation angle .908 and (B) left common iliac angle .908.
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^
^ TABLE 2 Comparison of Pre- and Post-EVAR Length Differences Among the Tested Stent-Graft Models Length Difference, mm Endurant I
Endurant II
Zenith Flex
Spiral Z
Zenith LP
p
Overall 3.866.2 (n¼99) 4.167.3 (n¼20) 6.268.7 (n¼32) 7.268.1 (n¼42) 4.369.4 (n¼6) 0.115 Severe aortoiliac 15.365.7 (n¼13) 14.462.4 (n¼5) 17.465.6 (n¼7) 16.866.5 (n¼14) * 0.737 tortuosity No severe aortoiliac 2.164.1 (n¼87) 0.764.5 (n¼15) 1.764.3 (n¼23) 2.863.5 (n¼29) 1.266.0 (n¼5) 0.550 tortuosity
^
^ * Only one patient; not included in analysis.
was missing in 4 (2%) of the treatment lengths. There were 44 angles in 40 patients that met the criteria for severe aortoiliac tortuosity: 2 infrarenal neck angles, 14 aortic bifurcation angles, and 28 CIAs (Table 1). The patients with severe aortoiliac tortuosity were older (mean age 78.568.0 vs. 74.767.3 years, p¼0.014), had larger mean aneurysm diameters (6.561.0 vs. 6.060.7 cm, p¼0.003), and longer pre-EVAR treatment lengths (204.1624.1 vs. 184.7616.8 mm, p,0.001). The two groups were similar in sex distribution (p¼0.246). Forty treatment lengths were measured in the presence of severe aortoiliac tortuosity compared to 159 without. Post-EVAR CT studies were performed at a mean of 53.6624.7 days. The mean pre- vs. post-EVAR treatment length difference was found to be 5.067.3 mm (mean change 2.6%63.9%) for the entire cohort. The mean difference was greater in the 40 patients with severe aortoiliac tortuosity [16.265.5 (mean change 8.0%62.7%)] compared to those without [2.164.4 mm (mean change 1.1%62.4%), p,0.001]. As shown in Table 2, there was no statistically significant difference among Endurant I and II and Zenith Flex, LP, and Spiral Z stentgrafts in pre- and post-EVAR treatment lengths (p¼0.115). In patients with severe aortoiliac tortuosity, there was no statistically significant difference among stent-grafts (p¼0.737) vs. patients without aortoiliac tortuosity (p¼0.550). The proportion of patients with severe aortoiliac tortuosity did not differ among stent-grafts (p¼0.106). Severe aortoiliac tortuosity and aneurysm diameter were found to be significant vari-
ables on univariate analysis (p,0.001), but on multiple regression analysis, only the presence of severe aortoiliac tortuosity was found to be significant (p,0.001).
DISCUSSION Measuring treatment lengths in EVAR patients from the lowest renal artery to the internal iliac artery can be used as a surrogate for graft conformability by comparing pre- and postoperative lengths. Centerline of flow measurement was chosen as the way to measure treatment lengths since it provides more accurate measurements than marker catheter angiography, which often underestimates treatment lengths.8 Furthermore, 3D CT reconstruction software used for centerline of flow measurements has been previously shown to provide the highest intraobserver consistency and interobserver reliability when assessing diameter and length measurements.9 We have defined severe aortoiliac artery tortuosity as having at least one angle .908 at the infrarenal neck, aortic bifurcation, or CIA bifurcation. This definition provides a more widely applicable method to identify patients with severe aortoiliac tortuosity. Other measures, such as the iliac tortuosity index, were designed to recognize factors affecting implantation and are not necessarily used to assess graft conformability.7 For instance, the iliac tortuosity index accounts for only the most acute angle from the common femoral artery to the aortic bifurcation, but our method includes two angles (aortic bifurcation and CIA), both of which we feel specifi-
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cally affect endograft conformability. Severe aortic bifurcation angulation accounted for 32% of patients in our study with severe aortoiliac tortuosity, which would not have been captured using the tortuosity index. The 5.0-mm mean difference between preand post-EVAR treatment lengths in our study is near the range reported by others (3.2 to 4 mm).1,2 Change in treatment lengths before and after EVAR occurs due to a combination of remodeling of the native aorta, stent-graft conformability, and stiffness of guidewires and delivery systems during EVAR. Therefore, a completely conformable endograft would result in identical pre- and post-treatment lengths. Our study showed significant improvement in current-generation stent-graft conformability compared to first-generation stent-grafts.3 In our subgroup analysis of patients without severe aortoiliac tortuosity, change in treatment length before and after EVAR was only 1.1%, which provides compelling data that there is improved conformability of stentgrafts during EVAR in patients without severe aortoiliac tortuosity. Investigators1,2 have previously identified that tortuous anatomy in EVAR is associated with stent-graft path shortening of 10 mm or more. In our study, severe aortoiliac tortuosity was identified in 20% of treatment lengths measured. These patients tended to be older, had greater AAA diameter, and longer preEVAR treatment lengths than patients without severe aortoiliac tortuosity. In patients with severe aortoiliac tortuosity, the treatment length after EVAR averaged 8.0% shorter than the pre-EVAR treatment length, which is a considerable change from the 1.1% for patients without severe aortoiliac tortuosity. Our group has previously shown women to have more hostile aneurysm neck anatomy but similar technical and clinical outcomes compared to men.10 In patients with severe aortoiliac tortuosity, the proportion of women in our study was higher than in the total study population (16% vs. 9%), but it did not reach statistical significance. Whittaker et al.1 reported a similar 10-mm shortening among Excluder stent-grafts in 11% of iliac limbs. Similar to our study, renal to internal iliac artery tortuosity was the only
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significant factor that predicted this shortening effect. Abbruzzese et al.11 have shown equivalent clinical outcomes at 5 years between Zenith, Excluder, and AneuRx stentgrafts when aneurysmal morphology was correlated to device-specific Instructions for Use (IFU). In their study, neck diameter, neck length, neck angle, and aneurysm sac angulation were measured and compared to the IFU, but iliac anatomy was not evaluated. Despite the lack of IFU criteria for aortoiliac or iliac artery angulation, it is important to consider these angles during EVAR planning. Our results suggest that the conformability of stent-grafts examined in this study is similar. The proportion of patients with severe aortoiliac tortuosity in each stent-graft model did not differ. We did not expect to see any difference in graft-specific conformability between Endurant I and II since both models have identical designs with nitinol stents. Interestingly, among the Zenith stent-grafts that do have a difference in composition and design, there were also no differences in conformability. The Zenith Flex is designed with stainless steel stents compared to Spiral Z and LP, which use nitinol stents. The Spiral Z also has a continuous spiral stent configuration that is different from the discontinuous stent configuration of Zenith Flex and LP. We observed substantial shortening of treatment lengths in patients with severe aortoiliac tortuosity regardless of the manufacturer or stent model. This shortening effect could be due to a change in length of the stent-graft, a change in native vessels during deployment, or a combination of both. Deployment of a stent-graft into a tortuous aorta may cause overlap of fabric between the metal stents at points of severe angulation, which in turn forces the native aorta to take a straighter and shorter path. On the other hand, the aorta may remodel to become straighter and therefore shorter secondary to the stiff wire and delivery system before deployment. Thus, the stent-graft itself may not shorten but instead conforms to a remodeled aorta. Shortening of the stent-graft in patients with severe aortoiliac tortuosity can have deleterious effects. It could lead to inadequate coverage of the distal sealing zone, especially
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considering that the greatest shortening effect in our study was as much as 32 mm. Inadequate distal sealing can potentiate a type Ib endoleak, requiring additional extensions. Velazquez et al.12 demonstrated that improved accuracy of treatment length measurements by centerline of flow led to an average of 1.6 fewer iliac extensions required per EVAR when compared to marker catheter arteriography. However, as seen in our study, centerline of flow measurements cannot accurately predict the treatment length in patients with severe aortoiliac tortuosity. If, on the other hand, the native aorta shortens after stiff wire and delivery device placement, then the chosen stent-graft may be too long and inadvertently cover the internal iliac artery. Inadvertent internal iliac artery coverage occurs in 3% to 10% of cases and can lead to buttock claudication, buttock necrosis, colon ischemia, and endoleak from a patent internal iliac artery.13 In our study, inadvertent internal iliac artery coverage did not occur in the studied population. It is unclear from our study and previous reports the exact mechanism for shortening of the treatment length, and further studies are required.
Limitations The retrospective nature of our study may have led to a selection bias. In terms of centerline measurements, we did not dissociate aortic and iliac segments to determine where exactly the shortening effect occurred, and we did not study the effect of aneurysm thrombus load or presence of artery calcification that may have affected graft conformability. In addition, our study did not specifically address rotation of the stent-graft to facilitate contralateral cannulation, which can alter the stent-graft path.1 There were only two cases in which the iliac limbs were crossed, so a separate analysis was not performed. Our study did not show any difference in conformability among stentgrafts in patients with severe aortoiliac tortuosity, but the number of patients in this group was small. Finally, only the first post-EVAR CT scans were evaluated, so we were not able to determine if further remodeling occurred in longer follow-up or if long-term clinical out-
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come is influenced by severe aortoiliac tortuosity.
Conclusion Our study showed no difference in treatment length change among several different models of stent-grafts, which suggests similar conformability. We did, however, find that significant shortening of treatment length occurs in patients with severe aortoiliac tortuosity. This shortening effect could negatively impact distal sealing zones during EVAR. Therefore, developing an accurate method to predict shortening of treatment lengths in patients with severe aortoiliac tortuosity using centerline of flow measurement would be helpful.
REFERENCES 1. Whittaker DR, Dwyer J, Fillinger MF. Prediction of altered endograft path during endovascular abdominal aortic aneurysm repair with the Gore Excluder. J Vasc Surg. 2005;41:575–583. 2. Higashiura W, Kichikawa K, Sakaguchi S, et al. Accuracy of centerline of flow measurement for sizing of the Zenith AAA endovascular graft and predictive factor for risk of inadequate sizing. Cardiovasc Intervent Radiol. 2009;32:441–448. 3. White GH, May J, Waugh RC, et al. Shortening of endografts during deployment in endovascular AAA repair. J Endovasc Surg. 1999;6:4– 10. 4. Wyss TR, Dick F, Brown LC, et al. The influence of thrombus, calcification, angulation, and tortuosity of attachment sites on the time to the first graft-related complication after endovascular aneurysm repair. J Vasc Surg. 2011; 54:965–971. 5. Sobocinski J, Chenorhokian H, Maurel B, et al. The benefits of EVAR planning using a 3D workstation. Eur J Vasc Endovasc Surg. 2013; 46:418–423. 6. Aziz I, Lee J, Lee JT, et al. Accuracy of threedimensional simulation in the sizing of aortic endoluminal devices. Ann Vasc Surg. 2003;17: 129–136. 7. Chaikof EL, Fillinger MF, Matsumura JS, et al. Identifying and grading factors that modify the outcome of endovascular aortic aneurysm repair. J Vasc Surg. 2002;35:1061–1066. 8. Wyers MC, Fillinger MF, Schermerhorn ML, et al. Endovascular repair of abdominal aortic
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aneurysm without preoperative arteriography. J Vasc Surg. 2003;38:730–738. 9. Parker MV, O’Donnell SD, Chang AS, et al. What imaging studies are necessary for abdominal aortic endograft sizing? A prospective blinded study using conventional computed tomography, aortography, and three-dimensional computed tomography. J Vasc Surg. 2005;41:199–205. 10. Dubois L, Novick TV, Harris JR, et al. Outcomes after endovascular abdominal aortic aneurysm repair are equivalent between genders despite anatomic differences in women. J Vasc Surg. 2013;57:382,389.e1. 11. Abbruzzese TA, Kwolek CJ, Brewster DC, et al. Outcomes following endovascular abdominal
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aortic aneurysm repair (EVAR): an anatomic and device-specific analysis. J Vasc Surg. 2008; 48:19–28. 12. Velazquez OC, Woo EY, Carpenter JP, et al. Decreased use of iliac extensions and reduced graft junctions with software-assisted centerline measurements in selection of endograft components for endovascular aneurysm repair. J Vasc Surg. 2004;40:222–227. 13. Wyers MC, Schermerhorn ML, Fillinger MF, et al. Internal iliac occlusion without coil embolization during endovascular abdominal aortic aneurysm repair. J Vasc Surg. 2002;36:1138– 1146.
