Follow-Up of Aneurysm Neck Diameter after Endovascular Repair of Abdominal Aortic Aneurysms
Article Outline
We assessed aneurysm neck diameter change after endovascular abdominal aortic aneurysm repair (EVAR) and its relationship to stent-graft diameter. Ninety-eight patients with abdominal aortic aneurysm who underwent EVAR were studied with multislice computed tomography following a standardized protocol. A preoperative study and immediate postoperative, 6-month, 1-year, and 2-year follow-up studies were performed. The aneurysm neck was measured from adventitia to adventitia, 6 mm below the lowermost renal artery, in planar images performed perpendicular to the vessel axis (real axial section). Baseline and follow-up neck diameters were compared with stent-graft diameters. For statistical analysis, a one-way analysis of variance with repeated measures was used. Pearson's correlation coefficient was used to examine the correlation between the change in neck diameter and stent-graft diameter. The average neck diameter was 22.38 mm (range 16-32.5) on the preoperative study and 23.35 mm (17-33.9) on the immediate postoperative, 24.35 mm (18.2-34.5) on the 6-month, 24.36 mm (18-34.5) on the 1-year, and 24.39 mm (17.8-35.7) on the 2-year follow-up. The mean device diameter was 24.08 mm (20-32). A significant increase in average neck diameter was found between the preoperative, immediate postoperative control, and 6-month control. There was no significant increase in the average neck diameter between the 6-month, 1-year, and 2-year follow-up. Baseline mean stent-graft oversizing was 1.7 mm, which decreased to -0.31 mm at latest follow-up. Dilation of the neck did not significantly exceed the endograft diameter in 83 cases (87.36%). An enlargement of the infrarenal aneurysm neck occurred during the first 6 months after EVAR. No significant variation in neck diameter occurred between the 6-month and 2-year follow-up visits. In the majority of cases, dilation of the aneurysm neck does not significantly exceed stent-graft diameter and, therefore, is possibly related to the presence of the endograft.
Introduction
Surgical repair by exposure of an aortic abdominal aneurysm (AAA) from the anterior approach is the traditional treatment. Nevertheless, in 1991 Parodi et al.1 described the endovascular repair of an infrarenal AAA (EVAR) for the first time. In the beginning, this percutaneous technique was only performed in patients with high surgical risk, but currently it is increasingly becoming a less invasive alternative to open surgery.2, 3 EVAR has been demonstrated to be as successful as conventional open surgical repair in the exclusion of AAAs from arterial pressure. The mortality derived from both techniques is also similar, even if we take into account that these patients have a higher associated comorbidity, which is usually the main reason for the use of this form of treatment instead of open surgery.4
However, the perfect endovascular graft has yet to be designed. Careful case selection before endovascular repair is fundamental to the success of the procedure and is dependent on several factors, mainly the anatomy and dimensions of the aneurysm neck. The issue of aneurysm neck enlargement and potential proximal endoleak after endovascular repair has been a concern of vascular specialists deploying these devices. Dilatation of the proximal neck may lead to treatment failure by device migration or the recurrence of an endoleak with resulting aneurysm expansion.
After EVAR, the aneurysm sac diameter reduces in 33-74% of cases5, 6 but appears to stop doing so after 18 months.7 However, evidence of dilatation at the level of the infrarenal aortic neck is still controversial,8, 9 as are its pathogenesis and etiology.
The purpose of this study was to asses aneurysm neck diameter change after EVAR and its relationship to stent-graft diameter.
Materials and Methods
Since June 2003, 491 patients have been treated with EVAR at our institution. We performed a study on 98 consecutive patients with infrarenal AAA who underwent endovascular treatment in our institution. The procedure was performed in a surgical suite, by interventional radiologists, after appropriate patient selection on the basis of clinical, anatomical, and anesthesiological factors.
Thoracic aneurysms, thoracoabdominal aneurysms, and urgent endovascularly repaired aneurysms, as well as those patients from any other institutions who had previous computed tomographic (CT) angiography, were excluded.
There were six female and 92 male patients, with a mean age of 67 years (interquartile range 62-72), and all of them had an aneurysm >4.5 cm in diameter.
We used a device size approximately 10% greater than the axis of the proximal aortic neck.
All patients were studied with contrast-enhanced multislice CT following a standardized protocol. A preoperative study and immediate postoperative, 6-month, 1-year, and 2-year follow-up studies were performed.
Dual-Slice Helical CT Protocol
CT scans were performed with a commercially available CT scanner (Asteion; Toshiba, Tokyo, Japan) equipped with a double array of detectors. Z-axis coverage ranged from the level of the adrenal gland region to the bifurcation of the femoral arteries.
A total of 120-150 mL of nonionic 30% contrast material was administered i.v. through an antecubital vein, with an automated injector at a rate of 3-5 mL/sec. Acquisition was automatically performed with the sure-start technique, and the threshold to start scanning was set at 140 UH in the proximal abdominal aorta. Craniocaudal helical CT was performed in a single breath-hold with the following parameters: 120 kVp, 200 mAs, 3 mm (x2) collimation, table speed of 12 mm per rotation, and pitch of 4. Scanning time ranged 18-23 sec and the length of the acquisition ranged 250-400 mm. Images obtained were in a range of 150-250 and were reconstructed using an independent workstation (Vitrea, Vital Images Inc., Minnetonka, MN). Multiplanar reformations, three-dimensional, and maximum-intensity-projection reconstructions were performed for each case, using for the purpose of this study, in some cases, only the oblique reconstructions. In those cases of an important anterior or lateral aortic angulation, the measurement was performed in an oblique or double oblique reconstruction in order to obtain the true axial section (Fig. 1).
Fig. 1
A–C Double oblique reconstruction to obtain the true axial section.
Data Analysis
For CT angiographic image analysis, one experienced radiologist measured the transverse proximal neck diameter in the true axial sections, with a scale of 1:150. The aneurysm neck was measured from adventitia to adventitia (outer perimeter of the aortic neck wall), 6 mm below the lowermost renal artery, in planar images performed perpendicular to the vessel axis in order to obtain the real axial section. Accessory renal arteries were not considered (Fig. 2).
Fig. 2
Measurement of proximal neck diameter.
Statistical Analysis
Data were analyzed with the statistical software program SPSS 9.0 (SPSS Inc., Chicago, IL). Qualitative data are presented as absolute frequencies and relative percentages, and quantitative values are mean, median, and standard deviation (minimum, maximum) depending on the distribution of the values.
Changes in the diameter of the proximal neck were referenced to the measurement on the preoperative study. The variation of the proximal neck diameter was calculated in millimeters as the difference between the diameter calculated in the preoperative study (reference value) and diameters in the immediate postoperative study and at 6 months, 1 year, and 2 years.
For the statistical analysis, a one-way analysis of variance with repeated measures was used to assess the significance of postoperative and follow-up size changes. p < 0.05 was considered statistically significant. Pearson's correlation coefficient was used to examine correlations between the change in neck diameter and stent-graft diameter.
Results
Measurements in millimeters of the proximal neck diameters are shown in Table I. The average neck diameter was 22.38 mm (range 16-32.5) in the preoperative study, 23.35 mm (17-33.9) in the immediate postoperative, 24.35 mm (18.2-34.5) in the 6-month, 24.36 mm (18-34.5) in the 1-year, and 24.39 mm (17.8-35.7) in the 2-year studies.
Table I. Aortic neck diameters
| Mean ± SD (range) | Median | Coefficient of variation | |
|---|---|---|---|
| Pre (n = 98) | 22.38 ± 3.1 (16-32.5) | 22.28 | 12.3 |
| Post (n = 98) | 23.35 ± 3.3 (17-33.9) | 23.24 | 12.7 |
| 6 months (n = 97) | 24.35 ± 3.5 (18.2-34.5) | 24.3 | 13.2 |
| 12 months (n = 93) | 24.36 ± 3.2 (18-34.5) | 24.2 | 12.8 |
| 24 months (n = 57) | 24.39 ± 3.6 (17.8-35.7) | 24.4 | 12.9 |
The mean device diameter was 24.08 mm (range 20-32). Mean neck diameters increased from 22.38 mm in the preoperative study to 23.35 mm (1 ± 0.8 mm increase, p < 0.001) in the immediate postoperative study. At 6-month control the mean neck diameter increased by 1.9 ± 1.2 mm over baseline (p < 0.001) (Fig. 3).
Fig. 3
Follow-up of aneurysm neck diameter and its relationship to stent-graft diameter.
A statistically significant increase in the average neck diameter between the preoperative and immediate postoperative control (p < 0.001) as well as the 6-month follow-up (p < 0.001) was observed.
The study shows an average increase of the aneurysm neck of 1.97 mm after the 6-month mark.
There was no significant increase in the average neck diameter between the 6-month, 1-year, and 2-year follow-up studies.
Baseline mean stent-graft oversizing was 1.7 mm, which decreased to -0.31 mm at latest follow-up.
Dilation of the neck did not significantly exceed the endograft diameter in 83 cases (87.36%).
Discussion
Changes in neck diameter after EVAR have been described in multiple studies with different results.8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18 There is still controversy with respect to the dilation of neck diameter.
In our study the postoperative scan data demonstrated a significant mean increase of 1.97 mm of the proximal neck at 6 months compared to the preoperative neck size. A statistically significant increase in the average neck diameter between the preoperative and immediate postoperative control (p < 0.001) as well as the 6-month follow-up (p < 0.001) was observed. These findings are in agreement with other studies.
In the study of Badran et al.12 most of the change in the absolute and mean aneurysm neck diameter was in the first 2 years, with an increase in aneurysm neck diameter in excess of the intraobserver error in 36% of patients. Resch et al.7 demonstrated an increase in the neck diameter of more than 3 mm in 46% of patients examined at 18 months after EVAR. Prinssen et al.13 reported a continuous and linear rate of increase of 1 mm/year at the level of the proximal endovascular anastomosis, but they used a cross-sectional area to measure the neck and did not directly compare their results .
In Wever et al.14 there was significant dilation of the proximal neck, with a median increase of 10% at 6 months and 15.5% at 12 months. However, their follow-up did not exceed 1 year, in contrast to our study with a 2-year follow-up.
Leurs et al.15 found neck dilatation and proximal migration in 1,342 (32%) and 192 (4.5%) of 4,233 patients, respectively. They demonstrated a statistically significant correlation between proximal migration and neck dilatation and hypothesized that migration may be caused by neck dilatation.
On the other hand, in contrast to our results, there are several studies showing no changes in proximal neck diameter. Walker et al.11 showed no significant changes in proximal aortic neck diameter after EVAR, but this study included a low number of patients. According to Mahnke et al.,16 the risk of neck enlargement is very low. May et al.4 concluded that there is a high probability of no enlargement of the proximal neck of AAA for at least 5 years after endoluminal repair. They hypothesized that endografts positioned correctly immediately below the renal arteries protect the superior 2 cm of infrarenal aorta in a manner that does not occur after open repair of AAA.
Makaroun and Deaton17 found that most proximal aortic necks remain stable but approximately 20% of necks increase in diameter by 2 years and that smaller necks dilate more often than larger ones.
In our study, we used self-expanding stents like in the majority of reports, whereas in the report of Malas et al.18 there was no proximal neck dilatation with a balloon-expanding stent in comparison with the use of self-expanding stents.
We found a significant increase in the average neck diameter between the preoperative and immediate postoperative control and with the 6-month follow-up but no significant difference thereafter, which shows a progressive reduction in the diameter increase for the aortic necks as they approached the graft diameter. Our findings are in agreement with the report of Sternbergh et al.,19 with stabilization of neck diameter 6 months after implantation. However, there was no significant increase in neck diameter at the immediate postimplant measurement compared with the preoperative measurement. They found a small but significant change in aortic neck diameter at 6 months after implantation, which then stabilized at 12 and 24 months; but they did not find changes in the diameter immediately after implantation.
We found that dilation of the neck did not significantly exceed the endograft diameter in 83 cases (87.36%). The neck diameters were found to be greater than the unconstrained stent-graft diameters in only five patients of 73 in the Badran et al. report.12
Conclusion
We demonstrated an increase in neck diameter in the immediate and 6-month postdeployment CT scans, with stabilization of the neck diameter between 6 months and 2 years post-EVAR.
The clinical significance of these findings is unclear. Changes in aortic neck diameter after EVAR could have important implications in long-term durability of this treatment. Significant dilation may lead to loss of seal or to fixation of the endograft, causing a proximal type I endoleak or endograft migration.
There is still controversy in the literature, with various studies reporting different results that cannot be directly compared because of the use of different methods of measurement; it would be necessary to establish a unique method to measure the neck in order to compare all of these different results.
Two possible explanations can be given for continued growth of the infrarenal neck after EVAR, a continuation of the processes of aortic wall degeneration or an effect of the radial expansile force generated by the endovascular stent.
If the outward force is the main cause of postoperative neck dilatation, the diameter will probably not stretch beyond the maximal stent size; but if the progressive enlargement is a consequence of the aneurysmal disease, it will probably present a continued dilatation.
In our study, in the majority of cases, dilation of the aneurysm neck did not significantly exceed the stent-graft diameter; therefore, it is possibly related to the adaptation of the aortic wall to the presence of the endograft.
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PII: S0890-5096(08)00089-7
doi:10.1016/j.avsg.2008.01.009
© 2008 Annals of Vascular Surgery Inc. Published by Elsevier Inc All rights reserved.
