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Objective: Strain has become a viable index for evaluating abdominal aortic aneurysm stability after endovascular aneurysm repair (EVAR). In addition, literature has shown that healthy aortic tissue requires a degree of strain to maintain homeostasis. This has led to the hypothesis that too much strain reduction conferred by a high degree of graft oversizing is detrimental to the aneurysm neck in the seal zone of abdominal aortic aneurysms after EVAR. We investigated this in a laboratory experiment by examining the effects that graft oversizing has on the pressure-normalized strain (ερ+¯/pulse pressure [PP]) reduction using four different infrarenal EVAR endografts and our ultrasound elastography technique. Approximate graft oversizing percentages were 20% (30 mm phantom-graft combinations), 30% (28 mm phantom-graft combinations), and 50% (24 mm phantom-graft combinations). Methods: Axisymmetric, 10% by mass polyvinyl alcohol phantoms were connected to a flow simulator. Ultrasound elastography was performed before and after implantation with the four different endografts: (1) 36 mm polyester/stainless steel, (2) 36 mm polyester/electropolished nitinol, (3) 35 mm polytetrafluoroethylene (PTFE)/nitinol, and (4) 36 mm nitinol/polyester/platinum-iridium. Five ultrasound cine loops were taken of each phantom-graft combination. They were analyzed over two different cardiac cycles (end-diastole to end-diastole), yielding a total of 10 maximum mean principal strain (ερ+¯) values. ερ+¯ was divided by pulse pressure to yield pressure-normalized strain (ερ+¯/PP). An analysis of variance was performed for graft comparisons. We calculated the average percent ερ+¯/PP reduction by manufacturer and percent oversizing. These values were used for linear regression analysis. Results: Results from one-way analysis of variance showed a significant difference in ερ+¯/PP between the empty phantom condition and all oversizing conditions for all graft manufacturers (F(3, 56) = 106.7 [graft A], 132.7 [graft B], 106.5 [graft C], 105.7 [graft D], P < .0001 for grafts A-D). There was a significant difference when comparing the 50% condition with the 30% and 20% conditions across all manufacturers by post hoc analysis (P < .0001). No significant difference was found when comparing the 20% and 30% oversizing conditions for any of the manufacturers or when comparing ερ+¯/PP values across the manufacturers according to percent oversize. Linear regression demonstrated a significant positive correlation between the percent graft oversize and the all-graft average percent ερ+¯/PP reduction (R2 = 0.84, P < .0001). Conclusions: This brief report suggests that a 10% increase in graft oversizing leads to an approximate 5.9% reduction in ερ+¯/PP on average. Applied clinically, this increase may result in increased stiffness in axisymmetric vessels after EVAR. Further research is needed to determine if this is clinically significant. : Clinical Relevance: This research suggests that a higher degree of graft oversizing results in a higher average percent pressure-normalized strain reduction and thus increased mechanical stiffness. There is evidence suggesting that arteries become less pulsatile when they become stiffened. Therefore, decreasing aortic pulsatile flow by implanting a highly oversized graft may have deleterious long-term effects after endovascular aneurysm repair. This may have implications for future graft construction and for the choice of endograft by the surgeon, especially when considering the benefits of strain reduction to the diseased tissue that comprises the aneurysmal sac. In addition, our technique allows for the assessment of dynamic changes to stiffness after graft implantation in vivo.

Objective: Predicting success after endovascular aneurysm repair (EVAR) of abdominal aortic aneurysms (AAAs) relies on measurements of aneurysm sac regression. However, in the absence of regression, morphometric analysis alone is insufficient to reliably predict the successful remodeling of AAAs after EVAR. Biomechanical parameters, such as pressure-normalized principal strain, might provide useful information in the post-EVAR AAA assessment. Our objective was to assess the feasibility of our novel ultrasound elastography (USE) technique to detect changes in the aortic wall principal strain in patients who had undergone EVAR and determine the temporal nature of the biomechanical changes in the aorta. Methods: USE images were obtained from patients undergoing elective EVAR intraoperatively, immediately before and after endograft implantation, and at their 30-day follow-up. The maximal mean principal strain (ερ+¯) for each scan was assessed using our novel technique, which uses a finite element mesh to track the frame-to-frame displacements of the aortic wall over one cardiac cycle. The ερ+¯ in the user-defined aortic wall was then divided by the pulse pressure at the time of the scan to produce a pressure-normalized strain measurement (ερ+¯/PP), a surrogate for tissue stiffness. Paired t tests were used to compare the pre- and postoperative ερ+¯/PP and the postoperative and 30-day ερ+¯/PP. Patient 30-day sac regression and endoleak data were collected by a review of 30-day follow-up computed tomography scans. Results: USE analysis of the data from 12 patients demonstrated a significant reduction in aortic wall ερ+¯/PP (average, 0.191% ± 0.09%/kPa vs 0.087% ± 0.04%/kPa; P = .002) immediately after graft implantation, with a nonsignificant change in the ερ+¯/PP (0.091% ± 0.04%/kPa vs 0.102% ± 0.05%/kPa; P = .47) from postoperatively to 30-day follow-up. This represents an average 46.5% reduction after stent placement, with a nonsignificant 18.1% increase at 30-day follow-up. All the patients showed sac stability, except for two patients who had demonstrated 7.3-mm and 6.8-mm sac regressions. Conclusions: Our analysis has demonstrated that the presented USE technique is a feasible method for detecting significant reductions in aortic ερ+¯/PP intraoperatively after EVAR. We found that patients undergoing EVAR will experience large reductions in the ερ+¯/PP intraoperatively after graft implantation, with stabilization found at their 30-day follow-up. These preliminary data have shown that an intraoperative ερ+¯/PP reduction could be a promising correlate of post-EVAR aneurysm remodeling. Our results have also suggested that endograft design likely plays a large role in determining the aneurysm biomechanical changes immediately after implantation.