Can we develop self-expandable metallic stents without consideration of mechanical properties?
25 July 2014 (online)
We read with great interest the paper by Hirdes et al., in which the authors evaluated, for the first time, the mechanical properties (radial and axial force) of esophageal stents . We would like to make two comments on this paper.
Radial force is well known as the expansion force required to dilate a stricture and maintain luminal patency. Conversely, the axial force, which is a new concept, is the recovery force required after bending a self-expandable metallic stent (SEMS) and is different from flexibility . We originally proposed the axial force as a force related to the conformability of SEMS in the bile duct.
Our first comment on the Hirdes paper is that the method of measuring axial force differed from our original report . The direction of axial force is perpendicular to the axis of the SEMS. Therefore, an angle correction would be required in order to compare the value of the axial force with that in our report. Furthermore, standard methods of measuring axial and radial force need to be established.
Our second comment is that the mechanical properties of SEMSs are clinically relevant only when they are associated with clinical outcomes. The axial force matters when an SEMS is placed in a curved lumen. When fixed at the stricture, the SEMS exerts pressure on the luminal wall at both ends of the stent. A strong axial force can cause kinking or perforation at the SEMS ends, and the longest SEMS possible should be selected    . In the bile duct, an SEMS with a high axial force can compress the orifice of the pancreatic or cystic duct and is associated with cholecystitis and pancreatitis after biliary SEMS placement  . We have also reported that covered SEMSs with a high radial force showed a lower incidence of migration .
The association between the mechanical properties of esophageal SEMS and clinical outcomes has yet to be clarified. Although conformability of the SEMS and prevention of migration are important issues, as the authors acknowledged, the esophagus is a relatively straight structure without branching ducts. The axial force might not be as relevant as in the biliary or duodenal SEMS, and a high radial force may be more important to prevent migration. Therefore, we should clarify associations of mechanical properties of SEMS and clinical outcomes in each anatomical area, and development of esophageal, biliary, duodenal or colonic SEMSs should be tailored according to these results.
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