Abstract: BACKGROUND: The thoracic aortic stent system instruments generally consist of covered stent, bare metal stent and corresponding delivery system. They may trigger more concerns about the safety of thrombosis in clinical practice by reason of the complexity in structure of such instruments. 
OBJECTIVE: To provide a more rational idea for the design and analysis of premarket safety evaluation of such implantable vascular devices combined with covered stent and bare stent.
METHODS: Three experimental pigs were selected and implanted with the thoracic aortic stent system. The system includes the thoracic aortic stent system transporter, covered stent and bare stent. The coated stent was inserted into the descending aorta of the experimental pig, and the bare stent was subsequently led in to concatenate the distal end of the covered stent. During the operation, the surface thrombosis of the extracorporeal part of the transporter was observed. Seven days after the surgery, the covered stent, transporter and bare stent were taken out to perform scanning electron microscopy. Meanwhile, the vessels contacted the bare stent and coated stent were separated to process histological observation.
RESULTS AND CONCLUSION: (1) Gross observation displayed that no thrombosis or thrombosis-related lesions were observed in the important organs of the experimental pigs, and no thrombotic occlusion was observed in the lumen of the blood vessels at the implantation site. (2) Scanning electron microscopy showed that thrombosis of several microns to hundreds of microns could be seen on the surface of the transporter of the coated stent system and the bare stent system, and thrombosis of several microns to hundreds of microns could be seen on the surface of the coated stent, bare stent and the vascular surface contacted with the stent. (3) Histological observation showed intimal hyperplasia at the contact site of the coated stent. Most of the new tissue was composed of fibrin network and a large number of red blood cells. Simultaneously, inflammatory cells were infiltrated inside the tissue, and inflammatory cell aggregation was observed at the edge of a few new tissues, and new small vessels were formed locally in the new tissues. Furthermore, the intima of the vessels at the contact site of the bare stent was proliferated, and the new tissue was mostly composed of necrotic cells, with local aggregation of inflammatory cells. (4) The results revealed that the influence of the implantable instruments used in combination with coated stents and bare stents on the blood vessels after implantation varied with different components. Consequently, it was required to consider the particularity of the composition of the stent itself and evaluate and analyze each component in combination with the way of its clinical usage.

Key words: thoracic aortic stent system, implantable vascular device, thrombosis in vivo, stent graft, bare stent, delivery system