AN NEW APPROACH ON RADIAL EXPANSILE FORCE MEASUREMENT OF SELF-EXPANDABLE ESOPHAGEAL METAL STENTS

The expansile force is a marginally important property for the selection and the design of a stent. There have been a few studies on the measurements of esophageal stent's expansile force. The physical properties that they measured, however, were not applicable to the clinical situation in the aspect that they measured neither the true radial force nor the expansile pressure, although the intraluminal pressure and the compliance of a patient's esophagus can be measured with currently available esophageal manometry. We developed a novel method of measurement that closely simulates the real implanting environment. We could measure the true radial force as well as the pressure with this method. A pair of square blocks with a hemi-cylindrical trough was used to compress a stent uniformly in radial direction. The stent was installed between two blocks and the expansile force was quasi-statically measured with universal tensile testing machine when the blocks started to part. In order to get the relations between the expansile force and the radial deformation, a series of differently sized cylindrical block pairs were used for each stent. Six kinds of esophageal stents: covered and uncovered Y2P SR, covered Y3E SR (Stentech, Korea), covered and uncovered Ultraflex (Boston Scientific, USA) and uncovered NiTi (Tae Woong, Korea), were measured in this study. The expansile forces measured with this system were observed to be somewhat bigger than those with the conventional methods[1]. This is because the new method is more realistic to implanting situation than the previous simplified methods. Since the actual radii of deformed stents were given in the test, the true pressure was easily calculated with the expansile force. In general, the expansile force was gradually increased as the radial deformation was increased. However, covered Y2P SR and covered Y3E SR showed a sudden drop of force in the middle of the force-deformation curve. We think that the weaving pattern and the coverage of those particular stents caused the unusual behavior. This phenomenon is very important due to the fact that the stent could be locked at this radius before it is fully expanded. Also, the expansile force-deformation curve showed nonlinear behavior when the deformation was large. For this reason, the actual expansile force for the radii in the clinical situation rather than the calculated force from the hoop strength should be measured. We expect that the data of the new method are directly applicable to the clinical selection of stents used in esophagus, in biliary tracts, and in various hollow organs where obstructive illnesses occur. We also expect that they can be effectively used for the stent design.