Ultrasound Elastography to Quantify Average Percent Pressure Normalized Strain Reduction Associated with Different Aortic Endografts in 3D-Printed Hydrogel Phantoms

OBJECTIVE Strain has become a viable index for evaluating abdominal aortic aneurysm (AAA) stability following endovascular aneurysm repair (EVAR). Additionally, 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 AAAs following EVAR. We investigated this in a laboratory experiment by examining the effects that graft-oversizing has on pressure-normalized strain (ερ+¯/PP) reduction using four different infra-renal EVAR endografts and our ultrasound elastography (USE) technique. Approximate graft-oversizing percentages were 20% (30mm phantom-graft combinations), 30% (28mm phantom-graft combinations) and 50% (24mm phantom-graft combinations). METHODS Axisymmetric, 10% by mass polyvinyl alcohol phantoms, were connected to a flow simulator. USE was performed before and after implantation with the four different endografts: (A) 36 mm Polyester/stainless steel, (B) 36 mm Polyester/electropolished nitinol, (C) 35mm PTFE/nitinol and (D) 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 ten maximum mean principal strain (ερ+¯) values. ερ+¯ was divided by pulse pressure to yield pressure normalized strain (ερ+¯/PP). An ANOVA 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 ANOVA 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<0.0001 for grafts A-D). There was a significant difference when comparing the 50% condition to the 30% and 20% conditions across all manufacturers by post hoc analysis (p<0.0001). 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<0.0001). CONCLUSION This brief report suggests that a ten percent 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 following EVAR. Further research is needed to determine if this is clinically significant.