Abstract
Background
Saccular abdominal aortic aneurysms (AAAs) are considered to be at higher risk of rupture than fusiform AAAs, but not much is known about the extent of this risk. Therefore, this study aimed to compare the rupture presentation between fusiform and saccular AAAs.
Methods and Results
This is a retrospective cohort study on 27 290 patients who underwent primary endovascular repair for a degenerative AAA between 2016 and 2019, and who were registered in the National Clinical Database in Japan. At operation for nonruptured case, the aneurysm diameter was significantly smaller in saccular AAAs than in fusiform AAAs (median, 44.0 versus 51.0 mm; P<0.001). Similarly, aneurysm diameter at rupture was significantly smaller in saccular AAAs than in fusiform AAAs (median, 55.6 versus 68.0 mm; P<0.001). The likelihood of repair for rupture was significantly higher in saccular AAAs than in fusiform AAAs in the 40‐ to 54‐mm diameter range, in which saccular morphology was found to be an independent risk factor for rupture against fusiform morphology by adjusting for sex and aneurysm diameter (odds ratio, 2.54 [95% CI, 1.75–3.69]). In addition, receiver‐operating characteristic curve analysis revealed that the cutoff diameter to predict rupture was smaller in saccular AAAs than in fusiform AAAs (50.5 and 59.5 mm, respectively) based on the Youden index.
Conclusions
Saccular AAAs presented at smaller diameters than fusiform AAAs in patients with ruptured AAAs treated with endovascular aortic repair, which supports the idea that saccular AAAs should be treated at smaller diameters.
Keywords: abdominal aortic aneurysm, National Clinical Database, rupture, saccular aneurysm, size threshold
Subject Categories: Cardiovascular Surgery, Treatment, Aneurysm
Nonstandard Abbreviations and Acronyms
- EVAR
endovascular aortic repair
- NCD
National Clinical Database
Clinical Perspective.
What Is New?
In patients who underwent endovascular aortic repair for abdominal aortic aneurysms (AAAs), aneurysm diameter at rupture was significantly smaller in saccular AAAs than in fusiform AAAs (median, 55.6 versus 68.0 mm; P<0.001), and receiver‐operating characteristic curve analysis revealed that the cutoff diameter to predict rupture was smaller in saccular AAAs than in fusiform AAAs (50.5 and 59.5 mm, respectively).
In AAAs with a diameter of 40 to 54 mm, saccular morphology turned out to be an independent risk factor for rupture by adjusting for sex and aneurysm diameter (odds ratio, 2.54 [95% CI, 1.75–3.69]).
What Are the Clinical Implications?
Saccular AAAs presented at smaller diameters than fusiform AAAs in patients with ruptured AAAs treated with endovascular aortic repair, which supports the current idea that saccular AAAs should be treated at smaller diameters.
Saccular abdominal aortic aneurysms (AAAs), defined as asymmetric enlargement of the aorta, account for only ≈5% of all AAAs, with most being fusiform AAAs (Figure 1). 1 , 2 , 3 It has long been believed that saccular aneurysms are more prone to rupture. 2 , 4 , 5 , 6 , 7 However, to date, not much is known about the natural history and the risk of rupture in saccular AAAs.
Figure 1. Fusiform (A) and saccular (B) abdominal aortic aneurysms (AAAs).
Current international guidelines recommend elective repair for AAAs with a diameter ≥55 mm in men and ≥50 mm in women, but this statement is limited to fusiform AAAs. 8 , 9 , 10 For saccular AAAs, however, these guidelines suggest elective repair at a smaller diameter, but fail to provide a size threshold for intervention. Thus, the optimal management of saccular AAAs is unclear, and surgeons assess the risk of rupture and determine the indications for elective repair on a case‐by‐case basis.
Cohort studies reporting on the clinical management of saccular AAA have been limited. 11 , 12 According to a recent cohort study of saccular AAAs from the Netherlands, 12 saccular AAAs were operated on at smaller diameters in the elective setting and became symptomatic/ruptured at smaller diameters than fusiform AAAs. The authors also added that a diameter of 45 mm seems to be an acceptable threshold. However, the number of symptomatic/ruptured cases of saccular AAAs with a diameter <45 mm in this study was insufficient for a powerful statistical analysis.
The current study conducted a retrospective review of patients with fusiform and saccular AAAs who were treated with endovascular aortic repair (EVAR), using data registered in the National Clinical Database (NCD) in Japan. The aim of this study was to compare the rupture presentation between fusiform and saccular AAAs and analyze the contribution of aneurysm morphology and aneurysm diameter to rupture in AAAs.
METHODS
The authors declare that all supporting data are available within the article and its supplemental material. This study was approved by the Institutional Review Board at The Jikei University School of Medicine (33‐189[10806]). Informed consent was waived for this study. The study protocol was registered with the University Hospital Medical Information Network Clinical Trials Registry (UMIN000050383).
Database
The NCD in Japan, which was launched in 2010 and commenced patient registration in 2011, is a nationwide prospective registry that can collect data on surgical procedures from >5000 institutions throughout Japan and has high coverage because of its link with the surgeon/hospital certification system. 13 In addition, previous studies have verified the data quality of the NCD. 14 , 15 , 16 For EVAR procedures for AAAs, the Japanese Committee for Stentgraft Management, established in December 2006 to ensure safe and appropriate use of commercial stent grafts, has started a nationwide EVAR registry from 2007, using a web‐based case‐registry form. 17 Participating institutions were obligated to register detailed data, including preoperative findings on AAAs, operative findings, and postoperative outcomes of EVAR. Since 2016, through the collaboration between the Japanese Committee for Stentgraft Management and the NCD, the data registration is now done on the NCD website.
Inclusion and Exclusion Criteria for Data
Patients undergoing primary EVAR for a degenerative AAA in Japan between January 2016 and December 2019 were included in the study. Cases of AAA with concomitant iliac artery aneurysm, dissecting/inflammatory/mycotic AAA, or AAA with vasculitis/connective tissue disease were not included in the study. Additionally, cases were not included if the AAA was treated with snorkel/chimney, fenestrated/branched, or debranching EVAR, because such AAA is not classified as standard, infrarenal AAA. Patients with AAA who underwent open surgical repair were not included because it was not required to register AAA morphology in the NCD for such cases. Those with an aneurysm diameter <25 mm were also excluded because the suggested reporting standard in the guidelines states that the definition of AAA, which is ≥30 mm in diameter in men, should be lower in women and in the Asian population and therefore suggests an exceptional situation. 9
Collected Data
Data registered into the NCD for each patient included age, sex, comorbidities, and the cause, anatomic factors, and clinical status of the AAA. Comorbidities registered included hypertension, diabetes, coronary artery disease, cerebrovascular disease, renal dysfunction (estimated glomerular filtration rate <60 mL/min per 1.73 m2), and respiratory disorder. Anatomic factors included the shape of the AAA (fusiform or saccular) and aneurysm diameter; maximum minor‐axis diameter was chosen if fusiform, and maximum transaortic diameter was chosen if saccular. How the AAA diameter is measured in fusiform and saccular AAAs is defined and clearly annotated on the NCD website, although to measure it in the axial plane or using the perpendicular plane to the centerline of the aorta was not standardized. Status of the AAA was described according to the existence of rupture (nonruptured or ruptured). The determination of AAA shape and measurement of diameter was made by each surgeon, and then each surgeon was required to input all of these data in the database on the NCD website.
Outcomes
The primary outcome was the aneurysm diameter at which saccular AAAs were operated on by EVAR in the nonruptured and ruptured cases. The secondary outcome was rupture of AAA.
Statistical Analysis
Patients were stratified according to the shape of the AAA (fusiform versus saccular) and according to the clinical status (nonruptured versus ruptured). Data were obtained on the aneurysm diameter at which saccular AAAs were operated on by EVAR in the nonruptured and ruptured cases. Then, the likelihood of repair for rupture was compared by aneurysm diameters of fusiform and saccular AAAs using categorical variables that were created in 5‐mm increments beginning at 30 mm and ending with 69 mm. The likelihood of repair for rupture was defined as the number of ruptured cases over the total number of cases. The odds ratio (OR) for rupture was determined by adjusting for all variables included in the guidelines as indications for repair (namely, sex, aneurysm diameter, and aneurysm morphology), except for growth rate, which was not captured in the NCD. Furthermore, receiver‐operating characteristic (ROC) curve analysis was performed to evaluate the predicting power of rupture for saccular AAAs to rupture as well as that for fusiform AAAs using the Youden index.
The normality of the distribution of the data was tested using the Shapiro‐Wilk test. Categorical variables are presented as numbers and percentages, and continuous variables are presented as means and SDs or medians and interquartile ranges. Categorical variables were compared with a χ2 test, and continuous variables were compared using a t test or Mann‐Whitney U test when appropriate. To compare the risk of rupture between fusiform and saccular AAAs, an OR was determined per diameter category using logistic regression analysis. Because the effect of aneurysm diameter category on rupture may affect the effect of female sex or saccular morphology on rupture, interaction between these variables was evaluated using logistic regression analysis. All statistical analyses were performed using SPSS Statistics, version 27 (IBM, Armonk, NY), and P<0.05 was considered statistically significant.
RESULTS
From January 2016 to December 2019, all 27 418 patients who underwent primary standard EVAR for degenerative AAAs were registered in the NCD. Those with AAA diameter <25 mm (n=128) were excluded from the study. Finally, a total of 27 290 patients were included in the study (Figure 2).
Figure 2. Patient flowchart.
AAA indicates abdominal aortic aneurysm; EVAR, endovascular aortic repair; and NCD, National Clinical Database.
Patient Characteristics
Patient characteristics, including the morphology and the clinical status of AAA at treatment, are shown in Table 1. Among the 27 290 cases that were included in this study, 7.8% (n=2142) of AAAs were saccular and the remaining 92.2% (n=25 148) were fusiform, while 5.3% (n=1443) were ruptured and the remaining 94.7% (n=25 847) were nonruptured. Specifically, 4.3% (n=92) of saccular AAAs and 5.4% (n=1351) of fusiform AAAs were ruptured. At operation for nonruptured case, the aneurysm diameter was significantly smaller in saccular AAAs than in fusiform AAAs (median, 44.0 versus 51.0 mm; P<0.001). Similarly, aneurysm diameter at rupture was significantly smaller in saccular AAAs than in fusiform AAAs (median, 55.6 versus 68.0 mm; P<0.001). Comparing the nonruptured and ruptured cases, sex, coronary artery disease, renal dysfunction, and respiratory disorder were significantly different in the fusiform AAAs, whereas renal dysfunction and respiratory disorder were significantly different in the saccular AAAs. Ratio of women tended to be higher in ruptured cases than in nonruptured cases in fusiform AAAs, but not in saccular AAAs.
Table 1.
Patient Characteristics and Data of AAAs
| Variable | Fusiform AAA (N=25 148) | Saccular AAA (N=2142) | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Nonruptured (N=23 797) | Ruptured (N=1351) | P value | Nonruptured (N=2050) | Ruptured (N=92) | P value | |||||
| N | % | N | % | N | % | N | % | |||
| Age, median (IQR), y | 78 (72–83) | 78 (70–85) | 0.440 | 76 (71–82) | 77.5 (69.5–86.0) | 0.285 | ||||
| Female sex | 4483 | 18.8 | 352 | 26.1 | <0.001 | 369 | 18.0 | 18 | 19.6 | 0.703 |
| Respiratory disorder | 3680 | 15.5 | 157 | 11.6 | <0.001 | 306 | 14.9 | 25 | 27.2 | 0.001 |
| Stroke | 2891 | 12.1 | 160 | 11.8 | 0.738 | 300 | 14.6 | 17 | 18.5 | 0.310 |
| Coronary artery disease | 5234 | 22 | 145 | 10.7 | <0.001 | 370 | 18.0 | 15 | 16.3 | 0.670 |
| Hypertension | 15 883 | 66.7 | 873 | 64.6 | 0.107 | 1450 | 70.7 | 64 | 69.6 | 0.810 |
| Diabetes | 3478 | 14.6 | 168 | 12.4 | 0.027 | 342 | 16.7 | 9 | 9.8 | 0.080 |
| Chronic kidney disease | 12 928 | 54.3 | 1011 | 74.8 | <0.001 | 1086 | 53.0 | 68 | 73.9 | <0.001 |
| Dialysis dependence | 952 | 4.0 | 60 | 4.4 | 0.423 | 98 | 4.8 | 8 | 8.7 | 0.090 |
| Aneurysm diameter, median (IQR), mm | 51.0 (47.0–56.0) | 68.0 (58.0–80.0) | <0.001 | 44.0 (37.0–50.0) | 55.6 (45.0–71.0) | <0.001 | ||||
| Aneurysm diameter categories, mm | ||||||||||
| <30 | 52 | 0.2 | 3 | 0.2 | <0.001 | 113 | 5.5 | 1 | 1.1 | <0.001 |
| 30–34 | 243 | 1.0 | 4 | 0.3 | 245 | 12.0 | 3 | 3.3 | ||
| 35–39 | 413 | 1.7 | 11 | 0.8 | 332 | 16.2 | 5 | 5.4 | ||
| 40–44 | 1972 | 8.3 | 26 | 1.9 | 379 | 18.5 | 13 | 14.1 | ||
| 45–49 | 5784 | 24.3 | 66 | 4.9 | 381 | 18.6 | 11 | 12.0 | ||
| 50–54 | 7942 | 33.4 | 122 | 9.0 | 311 | 15.2 | 11 | 12.0 | ||
| 55–59 | 3242 | 13.6 | 129 | 9.5 | 119 | 5.8 | 6 | 6.5 | ||
| 60–64 | 1906 | 8.0 | 184 | 13.6 | 78 | 3.8 | 9 | 9.8 | ||
| 65–69 | 885 | 3.7 | 156 | 11.5 | 31 | 1.5 | 7 | 7.6 | ||
| 70–74 | 640 | 2.7 | 185 | 13.7 | 25 | 1.2 | 10 | 10.9 | ||
| 75–79 | 302 | 1.3 | 118 | 8.7 | 13 | 0.6 | 3 | 3.3 | ||
| ≧80 | 416 | 1.7 | 347 | 25.7 | 23 | 1.1 | 13 | 14.1 | ||
AAA indicates abdominal aortic aneurysm; and IQR, interquartile range.
Comparison of the Likelihood of Repair for Rupture Between Fusiform and Saccular AAAs
In Table 2, comparison of the likelihood of repair for rupture was made by aneurysm diameter between fusiform and saccular AAAs using categories of 5‐mm diameter increments. In AAAs with aneurysm diameter of 40 to 54 mm, each category was significantly more likely to rupture in saccular AAAs than in fusiform AAAs. On the other hand, in AAAs with aneurysm diameters of 30 to 39 and 55 to 69 mm, there was no statistically significant difference in the likelihoods of repair for rupture between fusiform and saccular AAAs.
Table 2.
Comparison of Likelihood of Repair for Rupture Between Fusiform and Saccular AAAs
| Aneurysm diameter, mm | Fusiform AAA | Saccular AAA | OR | 95% CI | P value | |||||
|---|---|---|---|---|---|---|---|---|---|---|
| Total | Rupture | % | Total | Rupture | % | Lower | Upper | |||
| 30–34 | 247 | 4 | 1.6 | 248 | 3 | 1.2 | 0.744 | 0.165 | 3.359 | 0.700 |
| 35–39 | 424 | 11 | 2.6 | 337 | 5 | 1.5 | 0.565 | 0.195 | 1.643 | 0.295 |
| 40–44 | 1998 | 26 | 1.3 | 392 | 13 | 3.3 | 2.602 | 1.325 | 5.108 | 0.005 |
| 45–49 | 5850 | 66 | 1.1 | 392 | 11 | 2.8 | 2.530 | 1.325 | 4.831 | 0.005 |
| 50–54 | 8064 | 122 | 1.5 | 322 | 11 | 3.4 | 2.303 | 1.230 | 4.312 | 0.009 |
| 55–59 | 3371 | 129 | 3.8 | 125 | 6 | 4.8 | 1.267 | 0.548 | 2.931 | 0.580 |
| 60–64 | 2090 | 184 | 8.8 | 87 | 9 | 10.3 | 1.195 | 0.590 | 2.422 | 0.621 |
| 65–69 | 1041 | 156 | 15.0 | 38 | 7 | 18.4 | 1.281 | 0.554 | 2.960 | 0.562 |
AAA indicates abdominal aortic aneurysm; and OR, odds ratio.
In addition, focusing on ruptured cases, Figure 3 shows the distribution of ruptured cases by diameter category in fusiform and saccular AAAs, suggesting that saccular AAAs may rupture at smaller diameters than fusiform AAAs.
Figure 3. Distribution of ruptured aneurysms by diameter category in fusiform and saccular abdominal aortic aneurysms (AAAs).
The numbers next to the bars represent the percentages of rupture cases in each diameter category among all rupture cases of fusiform or saccular AAAs.
As shown in Table 3, risk analysis of rupture was performed for small (30–39 mm in diameter), medium (40–54 mm in diameter), and large (55–69 mm in diameter) AAAs, adjusted for sex, aneurysm shape, and diameter category. There was no significant interaction between aneurysm diameter and aneurysm morphology and between aneurysm diameter and sex (Tables S1–S4). As a result, saccular shape turned out to be an independent risk factor for rupture in medium AAAs (OR, 2.54 [95% CI, 1.75–3.69]), but not in small and large AAAs (OR, 0.62 [95% CI, 0.26–1.47]; and OR, 1.28 [95% CI, 0.81–2.02], respectively). In addition, female sex was identified as an independent risk factor for rupture in all AAAs except small AAAs, and diameter category was identified as an independent risk factor for rupture only in large AAAs.
Table 3.
Adjusted OR for Rupture in Small, Medium, and Large AAAs
| Variable | OR | 95% CI | P value | |
|---|---|---|---|---|
| Lower | Upper | |||
| Small AAA (30–39 mm) (N=1256) | ||||
| Female sex | 0.942 | 0.317 | 2.798 | 0.914 |
| Aneurysm morphology | ||||
| Fusiform (reference) | ||||
| Saccular | 0.619 | 0.260 | 1.472 | 0.278 |
| Aneurysm diameter, mm | ||||
| 30–34 (Reference) | ||||
| 35–39 | 1.458 | 0.595 | 3.576 | 0.410 |
| Medium AAA (40–54 mm) (N=17 018) | ||||
| Female sex | 2.155 | 1.650 | 2.816 | <0.001 |
| Aneurysm morphology | ||||
| Fusiform (reference) | ||||
| Saccular | 2.538 | 1.747 | 3.686 | <0.001 |
| Aneurysm diameter, mm | ||||
| 40–44 (Reference) | ||||
| 45–49 | 0.881 | 0.594 | 1.308 | 0.530 |
| 50–54 | 1.208 | 0.833 | 1.751 | 0.318 |
| Large AAA (55–69 mm) (N=6752) | ||||
| Female sex | 1.593 | 1.285 | 1.974 | <0.001 |
| Aneurysm morphology | ||||
| Fusiform (reference) | ||||
| Saccular | 1.281 | 0.813 | 2.018 | 0.286 |
| Aneurysm diameter, mm | ||||
| 55–59 (Reference) | ||||
| 60–64 | 2.396 | 1.910 | 3.007 | <0.001 |
| 65–69 | 4.388 | 3.452 | 5.577 | <0.001 |
AAA indicates abdominal aortic aneurysm; and OR, odds ratio.
ROC Curve Analysis to Predict Rupture of Fusiform and Saccular AAAs
Diameters that predict rupture in fusiform and saccular AAAs were analyzed using ROC curve analysis and are shown in Figure 4 and Table 4. The areas under the curve of the diameter that predict rupture of fusiform and saccular AAAs were 0.830 and 0.752, respectively. A cutoff diameter with the highest predictive power for rupture was 59.5 mm in fusiform AAAs (sensitivity, 73.4%; specificity, 82.5%) and 50.5 mm in saccular AAAs (sensitivity, 63.0%; specificity, 77.7%). If the cutoff diameter was set at 55 mm in fusiform AAAs, the sensitivity and specificity for predicting rupture were 79.8% and 73.7%, respectively. As for saccular AAAs, if the cutoff diameter was set at 45 mm, the sensitivity and specificity were 71.7% and 58.1%, respectively. Furthermore, the sensitivity of the cutoff diameter of 55 mm in fusiform AAAs (79.8%) was comparable with that of 43 mm in saccular AAAs (78.3%).
Figure 4. Receiver‐operating characteristic curve analysis of rupture for fusiform and saccular abdominal aortic aneurysms (AAAs).
The areas under the curve (AUCs) of the diameter that predicts rupture of fusiform (A) and saccular (B) AAAs were 0.830 and 0.752, respectively. A cutoff diameter with the highest predictive power for rupture was 59.5 mm in fusiform AAAs (A) and 50.5 mm in saccular AAAs (B).
Table 4.
Cutoff Values, Sensitivity, and Specificity of Aneurysm Diameter for Predicting Rupture in Fusiform and Saccular AAAs
| Type of AAA | Cutoff value, mm | Sensitivity | Specificity |
|---|---|---|---|
| Fusiform AAA | >50 | 0.889 | 0.486 |
| >55 | 0.798 | 0.737 | |
| >60 | 0.673 | 0.863 | |
| Saccular AAA | >40 | 0.859 | 0.400 |
| >45 | 0.717 | 0.581 | |
| >50 | 0.630 | 0.775 |
AAA indicates abdominal aortic aneurysm.
DISCUSSION
In this retrospective cohort study, among patients who underwent primary EVAR for degenerative AAA between 2016 and 2019, 7.8% had a saccular AAA. In patients who underwent EVAR for nonruptured AAAs, saccular AAAs were operated on at smaller diameters than fusiform AAAs, which is consistent with the current treatment guidelines for AAAs. 8 , 9 , 10 Furthermore, in patients who underwent EVAR for ruptured AAAs, saccular AAAs presented at smaller diameters than fusiform AAAs, which supports these guidelines. A higher proportion of saccular AAAs that underwent EVAR were ruptured compared with fusiform AAAs that underwent EVAR in the medium‐size category, in which saccular morphology turned out to be an independent risk factor for rupture by adjusting for sex and aneurysm diameter. In addition, the cutoff diameter for predicting rupture was 9.0 mm smaller in saccular AAAs than in fusiform AAAs in the ROC curve analysis.
Most AAAs are fusiform AAAs and often occur as a result of degeneration of the aortic wall. On the other hand, saccular AAAs are rare and seem to be mainly caused by degeneration, followed by a variety of causes, such as dissection, trauma, infection, and vasculitis. 11 , 18 Although aneurysm diameter and growth rate have been widely accepted as major indications for repair of an AAA, a saccular morphology has also been considered as an indication for repair. Despite the common perception that saccular aneurysms are at high risk of rupture, not much has been known about the natural history and the risk of rupture in saccular AAAs. Furthermore, there are limited data on what diameter saccular AAAs are treated with surgery in clinical practice.
According to a previous large cohort study of saccular AAAs conducted in the Netherlands, 12 saccular AAAs were operated on at smaller diameters than fusiform AAAs in the elective setting (mean, 53.0 versus 61.0 mm; P<0.001) and became symptomatic or ruptured at smaller diameters than fusiform AAAs (mean, 70.7 versus 76.5 mm; P=0.033). The authors also added that a diameter of 45 mm seems to be an acceptable threshold for surgery, based on the finding that the proportion of symptomatic/ruptured patients was similar between saccular AAAs with diameters <45 mm and fusiform AAAs with diameters <55 mm. However, the number of symptomatic/ruptured cases of saccular AAAs (n=83), especially those with a diameter <45 mm (n=7), was insufficient for a powerful statistical analysis. The present study focused on the rupture of AAAs, because patients could be described as symptomatic if the aneurysm caused a pulsing sensation or local compression symptoms, and such symptomatic patients should be differentiated from patients presenting with abdominal or back pain. Furthermore, the primary goal of the physician taking care of patients with AAA is to predict the risk of rupture, not the development of symptoms.
In the present study, the percentage of saccular AAAs out of all treated AAAs was 7.8%, which is similar to previous reports (≈5%). The median aneurysm diameter at rupture was smaller in saccular AAAs than fusiform AAAs (fusiform AAAs: 68.0 mm; saccular AAAs: 55.6 mm), suggesting that saccular AAAs are more prone to rupture. The median diameters at rupture in this study were smaller than those at symptom/rupture in the previous study from the Netherlands (fusiform AAAs: 75.0 mm; saccular AAAs: 68.0 mm). However, this may be attributable to the smaller aortic diameter in the Asian populations, 19 and attributable to the predisposition to symptomatic/ruptured presentation at smaller diameters in the Asian population. 20
The present study suggests that if AAAs are classified by size, each size range has its own unique characteristics: small AAAs might rupture regardless of sac morphology or sex, although this is rare, whereas the rupture risk of medium AAAs can be greatly affected by saccular morphology rather than sac diameter, and the rupture risk of large AAAs can be affected by sac diameter rather than sac morphology. For small AAAs, surveillance at intervals of several years is clinically acceptable for men with AAAs in the range of 30 to 40 mm. 21 Thus, conservative management is generally recommended for patients with small AAAs. 22 Consistent with this approach, the present study showed that there was a small number of nonruptured fusiform AAAs with diameters <40 mm; hence, the numbers of fusiform and saccular nonruptured AAAs in these categories were similar. Therefore, the likelihood of repair for rupture in small fusiform AAAs must have been much lower. On the other hand, large AAAs are uncontroversially indicated for repair. Saccular morphology was not an independent risk factor in large AAAs, presumably because the effect of saccular morphology on rupture was offset by that of the large aneurysm diameter or because a relatively small number of large AAAs led to type II statistical error. Perhaps the most controversial category is medium AAAs, particularly when taking into consideration the contribution of sac morphology to the risk of rupture. At least, because the percentage of saccular AAAs in the 45‐ to 49‐mm and 50‐ to 54‐mm categories in this study (6.3% and 3.8%, respectively) was similar to the percentage that was previously reported in AAAs of all sizes, the statistical analysis for medium AAAs is considered reasonable.
ROC curve analysis indicates that aneurysm diameter has an acceptable predictive power for assessing the risk of rupture in both fusiform and saccular AAAs, but it was better in fusiform AAAs. This finding suggests that aneurysm diameter may contribute less to the risk of rupture in saccular AAAs. When considering the size threshold for intervention in AAAs, sensitivity is more important than predictive power itself, because false negatives (ie, unexpected rupture) must be avoided. Using the approach to find the rupture risk in saccular AAAs that equals the one of fusiform AAA with a diameter of 55 mm, which is widely accepted as an indication for repair and is reasonable to adopt as a historical control, the sensitivity of a cutoff diameter of 55 mm in fusiform AAAs was comparable with that of a cutoff diameter of 43 mm in saccular AAAs (ie, 12 mm smaller in saccular AAAs than in fusiform AAAs). Furthermore, the cutoff diameter to predict rupture was 9.0 mm smaller in saccular AAAs than in fusiform AAAs. Therefore, it was suggested that the threshold diameter for intervention of saccular AAAs can be set 1 cm smaller than that of fusiform AAAs, although it goes without saying that a size threshold cannot be definitively determined based solely on the findings of the present study.
From a biomechanical perspective, the role of aneurysm geometry in rupture potential has been investigated in the past 2 decades. The results of previous reports on the effect of aneurysm geometry on mechanical wall stress using finite element analysis were controversial. 6 , 23 , 24 Subsequently, using computational fluid dynamics analysis, Boyd et al reported that aneurysms tended to rupture at the site of low wall shear stress, 25 and Natsume et al proposed that saccular aneurysms with sac depth/neck width >0.8 had low wall shear stress regardless of diameter, whereas in fusiform aneurysms, wall shear stress was lower as diameter increased. 26 This may be reflected in the finding of the current study that aneurysm diameter may contribute less to the risk of rupture than aneurysm morphology in medium AAAs, and the finding that the proportion of women was similar between ruptured and nonruptured cases in saccular AAAs. Akai et al attempted to identify the subgroup of saccular aneurysms that were truly at high risk of rupture and then defined horizontally long aortic aneurysms with an aspect ratio (neck width/horizontal diameter) <1.0 as true “saccular” aneurysms. 27 This study was conducted on thoracic aortic aneurysms, followed by a study on AAAs, which revealed that ruptured AAAs had a horizontally longer shape with a smaller fillet radius than nonruptured AAAs. 28 In addition, aneurysm shape in the NCD was confirmed by each vascular surgeon, and their judgments were subjective and lacked a detailed definition, except for focal or asymmetric enlargement of the aorta. Hanada et al reported that a discrepancy existed between a vascular surgeon's subjective diagnosis and an objective diagnosis using a mechanical structural analysis for AAAs. 29 As mentioned above, the present study revealed that the diameter of fusiform AAAs has more predictive power for rupture than the diameter of saccular AAAs. Further research using biomechanical approaches is expected to provide a more detailed understanding of the rupture potential of the saccular morphology.
The present study had several limitations. This was not a prospective study that followed preoperative patients with AAA from the time when their AAA diameters were small. Therefore, all patients with AAA being managed by surveillance or who died before arriving at the operating room have been excluded. In addition, this study only focused on EVAR cases because the NCD did not capture AAA morphology in patients who underwent open surgical repair. Thus, selection bias and confounding by indication exist, and the natural history of saccular AAAs is still unclear. Saccular morphology turned out to be an independent risk factor only in medium AAAs, but there is a risk of type II error in small or large AAAs because of the relatively small number in these groups. The likelihood of repair for rupture described in the current study was the proportion of rupture cases to all EVAR cases performed for AAAs, and this cannot be extrapolated to the rupture rate of AAAs, especially those with smaller diameters. Growth rate is commonly considered to be a risk for rupture, but was not captured in the NCD, and therefore was not included in the multivariable analysis. There is the idea that fusiform AAAs were treated during the follow‐up, whereas saccular AAAs were treated when found regardless of the diameter, and therefore, this differential approach could affect the outcomes. There were no specific criteria for the diagnosis of saccular configuration of AAA, and the determination of AAA shape and diameter was made by each surgeon, so saccular AAAs in this study could be morphologically heterogeneous, as mentioned above. This study focused on degenerative AAA, but the cause of saccular AAAs is sometimes difficult to discern. Interhospital variability should be considered in this study, but the available data do not include information on which hospital the individual data came from. External validity is limited because this study was conducted on an Asian population and cannot directly be translated to other populations. Finally, although the overall sample size was large, rupture cases were rare in small diameter categories and the statistical analysis might not have had sufficient power.
CONCLUSIONS
Saccular AAAs presented at smaller diameters than fusiform AAAs in patients with ruptured AAAs treated with EVAR, which supports the current idea that saccular AAAs should be treated at smaller diameters.
Sources of Funding
None.
Disclosures
Dr Hoshina received consigned research fund from Japan Lifeline Co, Ltd. Dr Takahashi is affiliated with the Department of Healthcare Quality Assessment at the University of Tokyo, which is a social collaboration department supported by grants from the National Clinical Database, Johnson & Johnson K.K., Nipro Co, and Intuitive Surgical Sàrl. The remaining authors have no disclosures to report.
Supporting information
Tables S1–S4
Acknowledgments
The study was supported by the Japanese Society of Vascular Surgery (JSVS). The authors would like to express their gratitude to Dr Kimihiro Komori (Chief Director of the JSVS), Dr Yoshikatsu Saiki (Chief of the Clinical Research Promotion Committee of the JSVS), Dr Hideaki Obara (Chief of the Database Management Committee of the JSVS), and the staff of the National Clinical Database (NCD). We would also like to thank all the hospitals participating in this NCD project for their continued efforts concerning data entry.
Author contributions: Conception and design: Drs Ozawa and Shukuzawa. Analysis and interpretation: Drs Ozawa and Shukuzawa. Data collection: Dr Takahashi. Writing the article: Dr Ozawa. Critical revision of the article: Drs Bessho, Hoshina, and Ohki. Final approval of the article: Drs Ozawa, Takahashi, Bessho, Hoshina, Shukuzawa, and Ohki. Statistical analysis: Dr Takahashi. Overall responsibility: Ohki.
This article was sent to John S. Ikonomidis, MD, PhD, Guest Editor, for review by expert referees, editorial decision, and final disposition.
Preprint posted on MedRxiv, June 10, 2023. doi: https://doi.org/10.1101/2023.06.06.23291061.
Supplemental Material is available at https://www.ahajournals.org/doi/suppl/10.1161/JAHA.123.032715
For Sources of Funding and Disclosures, see page 9.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Supplementary Materials
Tables S1–S4