How Controlled is the Expansion of VIATORR CX?

Abstract

Purpose: To investigate and compare the physical properties of the new generation Gore VIATORR-Controlled Expansion Endoprosthesis (VCX) to those of the predecessor VIATORR stent in an in vitro experimental setup.

Materials and methods: A total of 12 stents (8 VCX; 4 VIATORR; GORE, USA) were examined. Radial resistive force (RRF) and chronic outward force (COF) were assessed using a radial force testing machine (RX-650, Machine Solutions Inc., USA). To assess the radial forces of the VCX above 8 mm, balloon expansion was performed between cycles.

Results: All VCX stents show an abrupt decrease in COF at an external diameter of 8.3 mm; RRF decreases likewise at an external diameter of 8.5 mm. The predecessor VIATORR stent without the "controlled expansion" feature shows linear radial force reduction until full expansion at a diameter of 10 mm. The physical properties of the VCX can be altered by balloon modulation. Point of COF (RRF) reduction shifts to 8.5 mm (8.6 mm), 8.6 mm (8.8 mm) and 9.3 mm (9.6 mm) following modulation with a 8 mm, 9 mm and 10 mm balloon.

Conclusions: The VCX shows an abrupt and disproportionate decrease in COF and RRF at an external diameter of 8.3 mm, thus passive expansion to its nominal diameter of 10 mm is not to be expected. By means of balloon dilatation the physical properties of the stent can be altered, enabling customized TIPS creation. The previous VIATORR stent shows continuous COF and RRF until total expansion.

Keywords: Gore VIATORR-Controlled Expansion Endoprosthesis (VCX); Hepatic encephalopathy; In vitro mechanical test; Physical properties; Portosystemic over-shunting; Transjugular intrahepatic portosystemic shunt (TIPS).

Fig. 1

Image of the tested stents: VIATORR-controlled expansion stent (VCX) inside its catheter (A) and fully released (B). Predecessor VIATORR stent (W.L. Gore & Associates Inc., Flagstaff, AZ, USA) as a reference (C)

Fig. 2

Radial force testing machine: A RX-650 (Machine Solutions Inc., Flagstaff, AZ, USA) with Stent, B close up of crimping jaw with stent inside

Fig. 3

Schematic drawing of circumferential stent compression during crimping (A) and stent self-expansion (B) performed on a radial force tester. Schematic drawing of deformation characteristics of a self-expanding (nitinol) stent during crimping and expansion (C). The test starts fully expanded at 10 mm (1). The stent is then crimped to a nominal outer diameter of 5 mm (2) and self-expands back to the initial 10 mm. The force exerted during crimping is the radial resistive force, the force exerted during self-expansion is the chronic outward force

Fig. 4

Results of the radial force measurements for the different stents. The predecessor VIATORR stent expanded completely to an outer diameter of 10 mm (black line). For the new generation devices (yellow line) a step in resistive radial force is visible at 8.47 ± 0.06 mm diameter at out-of-the-box condition. This step was moved toward 8.56 ± 0.01 mm, 8.81 ± 0.04 mm and 9.62 ± 0.06 mm diameter after inflation with an 8 mm (red line), 9 mm (green line) and 10 mm balloon (blue line), respectively

Fig. 5

At 37 °C out of the box the VCX did not expand uniformly to the same size and took on a dogbone shape: it expanded to 9.1 mm in the center and to 9.7 mm at the edges. The uncovered edge of the stent even expanded to almost 10.5 mm

Fig. 6

Design of VIATORR-Controlled Expansion Endoprosthesis (VCX) and predecessor VIATORR stent (Gen. 1, W.L. Gore & Associates Inc., Flagstaff, AZ, USA). The new feature of controlled expansion sleeve guarantees the size of the diameter during implantation