Global Iliac Branch Study

Background and significance Abdominal aortic aneurysms (AAAs) affect 0.5% to 1.3% of women and 4.0% to 7.6% of men age 65 and older1-4. Infrarenal AAAs frequently extend distally through the aortic bifurcation and into the common, external, and/or internal iliac arteries,5,6 presenting a challenge for aneurysm repair. Current guidelines prioritize repair strategies which preserve blood flow to the internal iliac artery (IIA) with the goal of reducing the complications of buttock claudication and pelvic ischemia.7-9 As endovascular intervention has become standard practice for those with eligible anatomy, 7 branched endovascular devices have been developed to preserve IIA blood flow when repairing AAAs with iliac involvement.

With the ongoing development of devices and techniques, the range of iliac anatomy eligible for endovascular repair has increased. Most notably, multiple iliac branch devices (IBDs) are being used to treat AAAs involving highly tortuous iliac arteries. A variety of IBDs are available globally, and as such, there is an opportunity to determine if certain anatomic factors- specifically tortuous and non-conformable iliac arteries- may predict device complications.

To begin exploring this question, the Global Iliac Branch Study (GIBS)- a partnership between 5 centers across the United States, Europe, and Asia- was developed to evaluate performance of branched endografts based on anatomical assessments, with analysis of CT scans using centralized and standardized analysis protocols at the AortaCore Imaging Lab at the University of Wisconsin-Madison.

Previous work In addition to elevated risk of conversion to open repair,10 iliac tortuosity has previously been associated with increased risk of "conformability" events, including seal zone failure, type III endoleak, occlusion, stenosis, compression, and device migration.11-13

Previously, the applicant and her faculty mentor have explored the interaction between iliac tortuosity and device conformability in patients treated with the W.L.GORE EXCLUDER iliac branch endoprosthesis (IBE).14 Non-conformation was defined as a decrease in the total iliac index of tortuosity beyond 15%. This study found that non-conformation, as well as internal iliac artery diameter of less than 10 mm, were associated with adverse iliac events after iliac branch endoprosthesis (odds ratio, 8.2 [P = .02] and odds ratio, 12.3 [P = .02], respectively).

Similar to the aforementioned studies demonstrating increasing ranges of anatomic eligibility with the development of more complex and accommodating devices, the applicant and her mentor explored device eligibility for endovascular repair (EVAR) of small AAA over two years in a paper published in November 2021 in the Journal of Vascular Surgery which found high levels of anatomic eligibility for EVAR at baseline, and preservation of this eligibility over two years.15 These findings, like those previously mentioned, suggested that advancements in EVAR device design have improved anatomic eligibility for the procedure. Compared to open repair, EVAR is associated with decreased periprocedural morbidity and hospital length of stay,16 and as such is the preferred approach, as well as a major target for innovation in the field of vascular surgery.

Methods and analysis CT images of 400 subjects with IBDs from 5 centers around the globe will be transferred and analyzed at the University of Wisconsin-Madison AortaCore Imaging Lab under an Institutional Review Board (IRB) approved protocol. Post-processing workstations using Aquarius Intuition (version 4.4.12, TeraRecon) will be used to measure morphology data. Each CT will be fit with a manually corrected centerline that is equidistant from the outer walls of the aneurysmal vessels. Measurements will be completed at both the pre-treatment and post-treatment intervals; these include common, external, and internal iliac artery diameters and lengths, common iliac artery index of tortuosity (Common Iliac Index; Centerline length of the common iliac artery (CIA) divided by the shortest straight-line distance between the CIA origin and iliac bifurcation), total iliac artery index of tortuosity (Total Iliac Index; centerline length of the CIA plus centerline length of the external iliac artery, divided by the shortest straight-line distance between the CIA origin and terminal external iliac artery), and presence of the double iliac sign (when any portion of the iliac vessel is tortuous enough to be duplicated in a single cut of an axial computed tomography slice.)17 Figures 1 and 2 demonstrate these measurement techniques for capturing and quantifying tortuosity.

Measurement of these values before and after device deployment allows the determination of differences in iliac artery length pre- and post-treatment, which is used to quantify device "conformability."18 Notably, devices which produce minimal changes to native anatomy are considered more "conformable" and devices that make greater changes to iliac lengths and angle anatomy are considered "non-conformable" (Figure 3.)

In addition to pre-treatment demographics and comorbidities, including age, sex, and geographic origin, associations between iliac tortuosity indices and the aforementioned CT measurements will be determined in relation to clinical events collected by each center including conversion to open repair, re-intervention, endoleak, occlusion, compression, patient-reported symptoms and adverse events, and death.

Continuous variables will be compared using the unpaired Student t test if normally distributed and the Mann-Whitney U test or Wilcoxon matched pairs test if they are not normally distributed. Categorical variables will be compared using the Fisher exact test or Pearson's x2 test.

Expected outcomes As previously described, in preliminary data14 we have identified that a postoperative change in iliac tortuosity ("non-conformability") predicts adverse events in a sample size of 98 patients. We hypothesize that a larger sample size of 400 patients will confirm this relationship between non-conformability and adverse events, and enable identification of pre-operative predictors of non-conformability. Furthermore, we anticipate that the inclusion of a more diverse selection of endovascular devices will more broadly capture the relationship between device conformability and iliac tortuosity beyond what can be determined based on commercially available devices in the United States.