Clin Endosc.  2023 Sep;56(5):633-649. 10.5946/ce.2022.201.

Evaluation of the mechanical properties of current biliary self-expandable metallic stents: axial and radial force, and axial force zero border

• Yamagata W
• Fujisawa T
• Sasaki T
• Ishibashi R
• Saito T
• Yoshida S
• No S
• Inoue K
• Nakai Y
• Sasahira N
• Isayama H

• KMID: 2546142
• DOI: http://doi.org/10.5946/ce.2022.201

Abstract

Background/Aims
Mechanical properties (MPs) and axial and radial force (AF and RF) may influence the efficacy and complications of self-expandable metallic stent (SEMS) placement. We measured the MPs of various SEMSs and examined their influence on the SEMS clinical ability.
Methods
We evaluated the MPs of 29 types of 10-mm SEMSs. RF was measured using a conventional measurement device. AF was measured using the conventional and new methods, and the correlation between the methods was evaluated.
Results
A high correlation in AFs was observed, as measured by the new and conventional manual methods. AF and RF scatterplots divided the SEMSs into three subgroups according to structure: hook-and-cross-type (low AF and RF), cross-type (high AF and low RF), and laser-cut-type (intermediate AF and high RF). The hook-and-cross-type had the largest axial force zero border (>20°), followed by the laser-cut and cross types.
Conclusions
MPs were related to stent structure. Hook-and-cross-type SEMSs had a low AF and high axial force zero border and were considered safest because they caused minimal stress on the biliary wall. However, the increase in RF must be overcome.

Keyword

Axial force; Biliary obstruction; Mechanical property; Radial force; Self-expandable metallic stent

Figure

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  Fig. 1. Types of biliary self-expandable metallic stents. (A) Cross-type stents are knitted to form an X-shape, and the wires do not separate (WallFlex). (B) Hook-and-cross-type stents are formed by hook and cross knitting, in varying proportions. In hook knitting, the wires form a V-shape, and are separated from each other by bends (Hanaro). (C) Zigzag-type stents consist of a wavy wire shaped into vertically connected rings forming a cylinder. The laser-cut stent is based on the zigzag stent (EPIC).

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  Fig. 2. Overview of the self-expandable metallic stents evaluated in the present study.

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  Fig. 3. Radial force measurement device. The stent samples were inserted into the cylinder and then expanded. Expansion and resistance forces were measured.

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  Fig. 4. Relationship between hoop force (HF) and radial force. The HF was modified to accord with the original concept of radial force. Radial force was calculated by multiplying HF by 2π (6.28). RF, radial force.

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  Fig. 5. (A) Conventional manual method of measuring axial force (AF). This method measures the linearizing force of self-expandable metallic stent bending at 60°. (B) New AF measurement method. This method automatically measures torque in a straight stent position using an AF measurement device.

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  Fig. 6. Axial force (AF) measurement method using the new device. (A) AF measurement at onset. (B) AF measurement during the bending phase. The yellow arrow in the photo indicates the direction in which the arm moves. (C) Transition from the bending to straightening phase at 90°. (D) AF measurement during the straightening phase. The axial force zero border was measured as the angle at which the torque force was <0.05 mNm during the straightening phase.

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  Fig. 7. Radial force versus stent diameter curves. The same self-expandable metallic stents type, differing only in diameter, had similar curves. Expansion force was first measured using the cylinder until the stent was fully expanded. The diameter of the cylinder decreased, and the resistance force was measured (solid line: expansion force; dotted line: resistance force).

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  Fig. 8. Radial force versus stent diameter curve for the ComVi, WallFlex, and Niti-S stents (solid line: expansion force; dotted line: resistance force).

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  Fig. 9. Axial force measured by the new and conventional methods; a strong correlation was found (y=9.2359x, R=0.9366, p<0.001).

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  Fig. 10. Axial force versus angle curves. The axial force of stents of various diameters was measured using the new measurement device. The stent was bent to 90° and then straightened (solid line: bending phase; dotted line: straightening phase).

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  Fig. 11. Axial force versus stent angle curves for ComVi, WallFlex, and Niti-S series stents (solid line: bending phase; dotted line: straightening phase).

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  Fig. 12. Axial force zero border (red, hook-and-cross type; yellow, cross-type; blue, laser-cut-type).

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  Fig. 13. Radial (RF) and axial force (AF) scatterplots of 10 mm self-expandable metallic stent (SEMS). Outward RF at 4 mm and AF in the straightening phase with the stent at 60°, as measured by the new method, are plotted (red, hook-and-cross type; yellow, cross-type; blue, laser-cut-type).

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