Histological evaluation of the aortic wall response following endovascular aneurysm repair and endovascular aneurysm sealing

Abstract

Objective: The Nellix endovascular aneurysm sealing (EVAS) system was developed as an alternative to conventional endovascular aneurysm repair (EVAR) to minimize endoleaks. A significantly higher failure rate of EVAS may be related to an interaction between the filled endobags and the AAA wall. In general, biological information on aortic remodeling after traditional EVAR is scarce. In this light, we provide here the first histologic evaluation of aneurysm wall morphology after EVAR and EVAS.

Methods: Fourteen histological human wall samples of EVAS and EVAR explantation were systematically analysed. Primary open aorta repair samples were included as reference.

Results: Compared with primary open aortic repair samples, endovascular repair aortic samples were characterized by more pronounced fibrosis, a greater number of ganglionic structures, decreased cellular inflammation, less calcification, and a lower atherosclerotic load. EVAS was specifically associated with the presence of unstructured elastin deposits.

Conclusions: The biological response of the aortic wall after endovascular repair resembles the maturation process of a scar rather than a bona fide healing response. Moreover, the inflammatory response in the aortic wall after placement of endovascular protheses is less prominent than after primary open repair. A specific post-EVAS aortic wall characteristic was unstructured elastin fragments.

Keywords: Abdominal aortic aneurysm; Endoleak; Endovascular aneurysm repair; Histology; Vascular remodeling.

Fig 1

Characteristics of the aortic wall after endovascular aneurysm sealing (EVAS) and endovascular aneurysm repair (EVAR) compared with open repair, visualized in a heatmap.

Fig 2

Histological characteristics of the aortic wall after endovascular aneurysm sealing (EVAS) and endovascular aneurysm repair (EVAR) compared with open repair. Movat Pentachrome staining of post-EVAR (A1) and post-EVAS (B2) aortic wall samples, compared to open repair (C1/C2) samples. Note that open repair samples are characterized by a substantial variability in cellular composition; C1 represents a mesenchymal cell-rich wall sample, while C2 illustrates a sample that is mesenchymal cell-depleted, but that contains extensive leukocyte infiltrations. Aneurysm walls are primarily composed of mesenchymal cells (predominantly smooth muscle cells; elongated cells in red in Movat. Nuclei stain purple). Extracellular matrix aspects highlighted in the Movat stain are elastin (black), collagen (yellow), proteoglycans (blue); green represents colocalization of proteoglycans and collagen. Both all AAA samples were characterized by extensive elastin loss and fibrosis: the extracellular matrix is rich in collagen (ochre in the Movat staining A1/B1), while poor in mesenchymal cells (A2/B3:corresponding H&E staining; arrow points to a nucleus of a mesenchymal cell. Neovessel formation is observed up into the intima (A3). Appearance of ganglion-like structures in the adventitial layer is a notable features of the post EVAR and EVAS samples (A4, arrows highlight two examples). Remnants of intraluminal thrombi were observed in EVAS wall samples (B4) and open repair samples, but were absent in the post-EVAR samples. Differences in biological responses are observed for the EVAR and EVAS repair: areas of unstructured elastin deposition were found in EVAS (B2, arrow); while in conventional EVAR and the reference open repair samples are devoid of elastin.

Fig 3

Mesenchymal cell density in post endovascular repair samples. Smooth muscle cells, myofibroblasts and fibroblasts in a post EVAS samples. Mesenchymal cell (elongated cells) are identified by Vimentin staining (red). The mesenchymal cell population is dominated by smooth muscle cells and myofibroblastic cells (identified by αSMA and Tropomyosin staining [blue]). Few single Vimentin+ elongated cells fibroblasts are present (arrow).

Fig 4

Inflammatory cells present in endovascular repair samples. EVAR, endovascular aneurysm repair; EVAS, endovascular aneurysm sealing.