Experimental comparative study of arterial implants made of silicone reinforced with polyester fabric and expanded polytetrafluoroethylene (PTFE) in rabbits aorta

ABSTRACT Objectives: the aim of this study was to compare the outcomes of a new silicone vascular prostheses with PTFE vascular prostheses, on a rabbit experimental model. Methods: forty rabbits underwent infra-renal aorta replacement with 4 mm diameter prostheses, twenty animals with PDMS and twenty animals with PTFE (control group). Retrograde aortic angiography was performed to assess patency. Histological graft samples were examined by electron microscopy to evaluate prostheses endothelialization. Results: patency rates were 100% for both grafts after 30 days; after 60 days, patency rate for PDMS was 92.3% (±7.4), and 73,8% (±13.1) at 90 days. PTFE grafts had patency rates of 87.5% (±11.7) at 60 and 90 days. No statistically significant difference was found in between groups for patency rates (p=0.62). Postoperative complications (death, paraplegia) rates (p=0.526) and aortic clamping times (p=0.299) were comparable in both groups. No statistically significant difference for stenosis was found on angiographical analysis between groups (p=0.650). Electron microscopy revealed limited anastomotic endothelial ingrowth in both prostheses. Conclusion: in this experimental model, PDMS and PTFE vascular prostheses had comparable outcomes and PDMS prosthesis could be used as a vascular graft.


INTRODUCTION
T he use of synthetic grafts for large caliber arteries has been done for years and with broad support from medical literature. In arteries such as the aorta, iliac, and femoral arteries, high flow and low resistance guarantee long-lasting patency of vascular prostheses of dacron or polytetrafluoroethylene (PTFE) [1][2][3][4][5] . However, in small-caliber vessels, none of these materials was found to be superior or equal to the saphenous vein 6-9, considered the arterial substitute of choice for peripheral revascularization 10, 11 .
In cases where there is no autologous vascular segment with sufficient diameter and extension to be used in bypass surgery, especially in distal revascularizations of the lower limb, it is necessary to resort to synthetic grafts, despite their known inferiority 11 .
Several studies have been carried out in this field in an attempt to find a substitute comparable to the autologous vein, such as cryopreserved veins 12,13 and biosynthetic materials 14,15 . However, an effective alternative to replace small vessels is still being sought 16,17 .
Polydimethylsiloxane (PDMS), or silicone, has been used in medicine since the 1960s 18,19 . Due to its characteristics, silicone has become one of the most used materials for prosthetic replacement in various contexts, such as breast and penile prostheses. The use of PDMS in the manufacture of various types of catheters for intravenous administration of substances is also widely and universally accepted. It is an inert material, which is one of the advantages for use in implants. Silicone also has excellent long-term bio-stability, low toxicity, and Fernanda appolonio rocha 1,2 ; laila Massad ribas 1 ; paulo isao sassaki-neto 1 ; nelson de-luccia 1 .

Original article A B S T R A C T A B S T R A C T
Objectives: the aim of this study was to compare the outcomes of a new silicone vascular prostheses with PTFE vascular prostheses, on a rabbit experimental model. Methods: forty rabbits underwent infra-renal aorta replacement with 4 mm diameter prostheses, twenty animals with PDMS and twenty animals with PTFE (control group). Retrograde aortic angiography was performed to assess patency.
Histological graft samples were examined by electron microscopy to evaluate prostheses endothelialization. Results: patency rates were 100% for both grafts after 30 days; after 60 days, patency rate for PDMS was 92.3% (±7.4), and 73,8% (±13.1) at 90 days. PTFE grafts had patency rates of 87.5% (±11.7) at 60 and 90 days. No statistically significant difference was found in between groups for patency rates (p=0.62). Postoperative complications (death, paraplegia) rates (p=0.526) and aortic clamping times (p=0.299) were comparable in both groups. No statistically significant difference for stenosis was found on angiographical analysis between groups (p=0.650). Electron microscopy revealed limited anastomotic endothelial ingrowth in both prostheses. Conclusion: in this experimental model, PDMS and PTFE vascular prostheses had comparable outcomes and PDMS prosthesis could be used as a vascular graft.

Anesthetic technique
The animals were anesthetized with a

Surgical technique
We performed a median laparotomy, with transperitoneal approach, which allowed the dissection of 3 to 4 cm of the infrarenal aorta. We used a self-static retractor, developed specifically for the research, to prevent evisceration during the exposure. We carefully preserved the lumbar arteries. Before aortic clamping with microsurgical forceps, we administered sodium heparin

RESULTS
We performed the procedures on 40 animals, 20 in each group.
Twenty-five animals, 14 from the PDMS group and 11 from the PTFE group, survived until late evaluations.
All surviving animals underwent angiographic control at the end of the observation period and were analyzed using Kaplan-Meier curves.
We observed early mortality (up to two days after the procedure) in eight animals (three in the PDMS group and five in the PTFE group). Seven animals developed paraplegia in the immediate postoperative period (three from the PDMS group and four from the PTFE group) and were euthanized. These animals were not considered in the long-term follow-up.
There was no difference between the groups regarding postoperative complications (death and paraplegia -Fisher's exact test, p = 0.526).

Surgical characteristics
The walls of the PDMS prosthesis had characteristics of flexibility, complacency and ease of handling, which allowed easy passage of the needle and retention and containment of the suture lines. In addition, the prosthesis provided for easily palpable pulse. We The prostheses were removed after euthanasia, and samples were sent to scanning electron microscopy; the images were obtained using the Philips XL30 system (FEI, Hillsboro, Oregon, USA). Optical microscopy was not possible due to the characteristics of the material, which was explanted by the microtome when attempting to prepare the slides.

Macroscopic analysis
The two types of prostheses were surrounded by fibrous scar tissue externally. In all animals receiving implants of PTFE prostheses, they were more adhered, with thicker incorporation tissue, resulting in greater difficulty in the cleavage plane of identification with the tube and removing it for microscopic analysis. All PDMS prostheses showed a more discreet tissue reaction, with more delicate tissue incorporation, which allowed the removal of the specimen to be performed more easily, since the cleavage plane was easily identified and dissected.
There was no case of infection or formation of pseudoaneurysm in the suture lines in either group.

PDMS prostheses had no aneurysmatic dilation.
Within the occluded grafts, we observed whitish thrombi, which were probably caused by intimal hyperplasia, later confirmed by scanning electron microscopy.
In prostheses with viable flow, in the internal region the suture lines were covered with a shiny tissue, continuous to the endothelium of the native artery.

Scanning electron microscopy
We used electron microscopy to provide additional information. We observed endothelial growth of the native vessel towards the prosthetic grafts, covering the suture lines, characterizing the formation of a neointima layer. This formed layer covered the region of the anastomoses and grew just a few millimeters further, with no complete endothelization of the PDMS prosthesis did not cause any deformity, and after removing them, the prosthesis resumed the tubular shape.
The PTFE prosthesis has little elasticity and more rigid walls, which made the needle more difficult to penetrate and the suture was more laborious. In addition, this type of prosthesis did not integrate to the native artery as well as the PDMS prostheses due to the rigidity of the walls. Another characteristic observed was the frequent bleeding through the needle holes in the PTFE, which did not occur in PDMS prostheses. The placement of clamps in this type of prosthesis caused the walls to be marked and remain with some initial deformation, but that can be corrected manually.

Prosthesis evaluation
We followed the animals in the PDMS group for an average of 72 days, with a maximum of 108 days.
The animals in the PTFE group were followed for an average of 66 days, with a maximum of 90 days.
In the PTFE group (n = 11), the flow in 30 days was 100%, and in 60 and 90 days it was 87.5% (± 11.7). The logrank test showed no statistically significant difference between the groups' flow rates (p = 0.629) (Figure 3).
The risk of occlusion for each group can be seen in Figure   4.
We observed no hemodynamically significant stenosis in any animal in both groups.   the internal surface in any of the prostheses. In some cases, we found more exuberant endothelial growth, corresponding to intimal hyperplasia (Figure 5). Experimental models in rabbits are considered good models for evaluating small-caliber vessels, with a diameter between 1 and 4 mm [29][30][31][32] . The vascular physiology of rabbits is similar in many ways to that of humans, but there are some disadvantages, such as the Rocha Experimental comparative study of arterial implants made of silicone reinforced with polyester fabric and expanded polytetrafluoroethylene (PTFE) in rabbits aorta mentioned limitations, the model used seems to be valid for small-caliber vascular grafts [29][30][31][32] .