(K-426) Comparing Laser Fenestration vs Mechanical Puncture for In-Situ Abdominal Aortic Aneurysm Repair

An abdominal aortic aneurysm (AAA) is a condition that results in a balloon-like dilation that, without treatment, poses a risk of rupture (ranked the 13^th leading cause of death in the United States)¹. The current surgical repair of AAA techniques includes 1) highly invasive open surgical repair (OSR), involving removing the dilated portion of the aorta and replacing it with a cylindrical graft material, 2) minimally invasive endovascular aortic repair (EVAR), involving sealing the inside of the aneurysm using an internally placed stent graft rather than excising the aneurysm wall², and similarly 3) fenestrated endovascular aortic repair (FEVAR), a minimally invasive procedure involving the placement of a customized stent graft with fenestrations that allow for blood to flow to branching arteries³. However, there are limitations to the customized devices. They often have long wait periods and there is only one FDA approved stent graft for this repair technique⁴. The presented study compares two potential techniques to create in situ fenestrations using mechanical puncture or a laser. In this study, we compare the maximum mechanical puncture force and factor of safety observed in each experimental test of the mechanical puncture experiment to the imaged burn depth observed in the laser fenestration using an energized beam. These characteristics will help us determine which repair technique is safer and should be chosen for in situ fenestration.

The differences between mechanical puncture and laser fenestration will be determined through compression testing and microscopic imaging, followed by analysis and quantification of these metrics. A model was designed for mechanical puncture testing and 3D printed to hold the specimen taut and disallow needle movement during testing. To test the spring scale component of the testing equipment, a 100g precision weight was used to verify that the calibration of the spring scale was accurate for testing. Each experimental test (stent graft, porcine aorta (Figure 1), and stent graft with porcine aorta) was punctured at least three times using a 20-gauge needle. The ASTM guideline for testing bursting strength of textiles, section 8.1, recommended a 125mm (5 in) square specimen⁵, but we used approximately a 32mm (1.25 in) specimen due to restrictions in our testing protocol and test equipment, Instron BioPlus model 5543A (Instron Inc., Norwood, MA, USA). The rate of compression was 3mm/min for each test specimen. Once the data was collected, an ANOVA was conducted, and the data was analyzed from the statistical test to help determine the differences between mechanical puncture treatments. The laser fenestration, CVX-300 Excimer Laser System by Philips, was tested at intervals of 1, 2, 3 and 4 seconds, then the depth of the burn was measured to help characterize the effectiveness. The laser fenestrations were burned through stent graft placed on top of porcine aorta. The imaged sections were created with cryostat and viewed under a microscope to measure any quantifiable burn depth.

Results and Discussion: The maximum force was observed for mechanical puncture before the needle punctured the specimen. The factors of safety for the stent graft and aorta were calculated and were 1.37 and 1.20, respectively. The ANOVA showed no significant differences between the three experimental test groups (Figure 2). Since there is no significant difference in the force required to puncture through the stent graft and the aorta, the risk of incorrectly puncturing the aorta is high, which is also supported by the low factors of safety. The thicknesses of the stent graft, aorta, and stent graft with aorta were measured before testing and are reported as 0.618 mm, 2.10 mm, and 2.63 mm, respectively. The laser fenestration results show burn depths of 43.0 µm, 34.0 µm , 15.0 µm , and 1560 µm for one through four-second intervals, respectively. This information indicates that the one, two, and three-second intervals show insignificant burn depths. While the four second interval was significant (Figure 3), it did not burn through the aorta since its thickness was greater than the burn depth.

Conclusions: The findings indicate that mechanical puncture is not an efficient fenestration method because the calculated factor of safety was low, the clinician may not be aware if both the stent graft and aorta are punctured. Laser fenestration has many benefits, including the ability for customization to each patient, the procedure is minimally invasive, but most importantly is the precision in burn depth and the ability to burn through the stent graft without burning the aorta. For these reasons, laser fenestration should be used for the in-situ fabrication of stent-grafts.

We would like to thank the Michael G. Wells pitch competition hosted by the Innovation Institute at the University of Pittsburgh and the NIH (Grant HL157646) to provide the resources to perform this research.