Analytical models for predicting mechanical properties of self-expandable metal stents with cover membrane

Abstract

Abstract-Various mechanical characteristics of stents were analyzed and mathematical models were developed in order to predict expansive pressure of stents. Given the geometry and material properties of a stent, one can utilize these models to predict its expansive pressure properties. Then, these models were verified with the test results derived from some prototype and commercially available stents. The models allow for the characterization of mechanical properties of stents and may be instrumental in developing clinically efficacious stents. Keywords -Self-expandable metal stent, radial expansive pressure, analytic model I. INTRODUCTIONSuccessful stenting is crucial for maintaining the patency of the organs with luminal obstruction. Three types of stents are commonly used: bare stent (wire only), coated stent (polymer coating on wire), and covered stent (polymer membrane on its peripheral surface).The clinical applicability of these stents largely depends on thorough understanding of their mechanical properties, one of which is radial expansive pressure. In the present study, we propose the mathematical models by which one can predict radial expansive pressures of coated and covered stents. A load is applied to elongate a stent in longitudinal direction. The load acting on the stent modeled with combination of n wires can be expressed as a function of pitch angle as in the following equation: and β is pitch angle, I moment of inertia, I p polar moment of inertia, E Young's modulus, G shear modulus, and n number of wires of a stent. B. Coated typeA coated stent exerts two types of forces against radial compression: the spring restoring force (F wire ) exerted by metal wires and polymer knot force (F coat ) derived from the moments by the knots of polymer coating. The helical spring model [1] is employed to calculate wire spring force in this type of stent. The polymer knots are considered as torsional springs and the spring moments are calculated. Fig. 2 shows stent in initial state and compressed state in θ-z plane (cylindrical coordination). When stent is compressed in radial direction as shown in Fig. 2(D), the moment from compressed torsional springs at each knots are exerted on wires of stent. The moment by one knot (M knot ) is ) (where k knot is a torsional spring constant. The moment exerted on one wire becomeswhere KNOTS wire is the number of knots in the wire. Thus, polymer knot force of a stent (F coat ) can be expressed as ANALYTICAL MODELS FOR PREDICTING MECHANICAL PROPERTIES OF SELF-EXPANDABLE METAL STENTS WITH COVER MEMBRANE Abstract Subject Terms Report Classification unclassified Classification of this page unclassified Classification of Abstract unclassified Limitation of Abstract UUNumber of Pages 4