Design of a Testing Mechanism for Stent Failure Rates and Displacement Under Extreme Bending Conditions

Abstract

Stents are commonly associated with the mechanical support of the coronary arteries to improve blood flow and retain residual plaque following various angioplasty procedures. However, they are becoming more frequently used in other vessels in the human body, such as the carotid and femoral arteries. The femoral arteries transverse through the hip region, and are the sites of potential plaque build-up. Thus the design of a stent for the specific biomechanical stresses and conditions of this location is of growing interest. The effectiveness of stent designs are quantified by their ability to survive in the human body without failing mechanically, dislocating, or invoking a major inflammatory response. Common methods of failure are mechanical, including fractures and dislocations. Several different instruments are commercially available for the testing of stents under various stresses and application frequency. However, these machines generally test with small bending angles or they apply nonphysiological axial, radial and torsional loads; thus they are not idealized for motions to mimic accurate biomechanical motion. Specifically, for the design of a stent localized in the hip region, a test for significant bending cases is necessary. The placement of stress on a mock artery should be applied solely to the ends of a mock artery to remove any radial or axial stresses not caused directly from the bending motion. Furthermore, visualization of the stent inside the mock artery is desired for tracking displacement of the stent and cycle count until failure. The ability to quantify the mechanical failure and dislocation of stent designs under extreme bending conditions is a prerequisite to the optimization of physical stent designs and of stent spacing, orientation and placement. We compare a proprietary stent-like design (Innovasc Inc., Honolulu, HI) placed at fixed intervals versus a commercially available SMART stent (Cordis Corporation, Warren, NJ). Both designs are intended to retain the arterial plaque while minimizing the stress applied to the artery wall to prevent restenosis. The new stent prototype designs are uniquely configured to enable points of stress reduction along the length of interest. Early preliminary experiments with the Innovasc design show promising results in the reduction of restenosis in porcine models. Stent designs are tested in mock arteries of latex and silicone. The mock arteries are selected for internal diameters and thicknesses to match artery properties. Two symmetric cylinders holsters are allowed to rotate freely and are affixed to the ends of the mock arteries. A simple linkage system drives the two cylinders together and apart, allowing for the mock arteries to bend at a fixed angle of 120 degrees at frequencies (∼10–20 Hz) without external stresses for one million cycles. Images are captured using a X-Stream Vision High-Speed CMOS camera (Integrated Design Tools, Tallahassee, FL) via a trigger system. Failure points and dislocations are noted and measured using the NIH ImageJ imaging software.