Fracture of self-expanding nitinol stents stressed in vitro under simulated intravascular conditions

Abstract

The use of intravascular stents in the superficial femoral artery (SFA) continues to be controversial due to the potential for compression and fracture in the tortuous physical environment of the adductor canal. The purpose of this study was to (1) characterize the types and ranges of stent distortion theoretically produced by extremity movement and (2) use these ranges as parameters for in vitro long-term fatigue testing of commercially available self-expanding nitinol stents. Nitinol self-expanding stents were placed in the SFAs of cadavers and lateral view radiographs were obtained with the limb in various degrees of hip and knee flexion. The measured degrees of axial shortening and bending of the stent were estimated by planimetry and used for in vitro fatigue testing, which was undertaken using specially designed equipment. Six different commercially available nitinol self-expanding stents-Protégé EverFlex (EV3, Minneapolis, Minn), S.M.A.R.T. Control (Cordis/Johnson & Johnson, Miami Lakes, Fla), Luminexx (C.R. Bard, Murray Hill, NJ), LifeStent FlexStar (Edwards Lifesciences, Irvine, Calif), and Xceed and Absolute (Abbott Vascular, Santa Clara, Calif)-were mounted in elastic silicone tubing, bathed in phosphate buffered saline at 37 degrees +/- 2 degrees C, and examined for fracture after 10 million cycles of chronic deformation. For unstented arteries, the distal SFA/proximal popliteal artery exhibited the greatest axial compression (23%) vs the middle SFA (9%) or popliteal artery (14%) at 90 degrees /90 degrees knee/hip flexion. For stented arteries, the popliteal artery exhibited the most axial compression (11%) vs the middle SFA (3%) or distal SFA/proximal popliteal artery (6%) at 90 degrees /90 degrees knee/hip flexion. Axial compression of the stented popliteal artery at 70 degrees /20 degrees knee/hip flexion was 6% with a deflection angle of 33 degrees . These parameters were used for chronic in vitro fatigue testing, which produced a range of responses in commercially available stents. Chronic 5% axial compression resulted in high rates of fracture of Luminexx (80%) and LifeStent FlexStar (50%), with lower fracture rates for Absolute (3%), Protégé EverFlex (0%), and S.M.A.R.T. Control stents (0%). Chronic 48 degrees bending deformation resulted in high rates of fracture in Protégé EverFlex (100%), S.M.A.R.T. Control (100%), and Luminexx stents (100%), with lower rates in Absolute (3%) and LifeStent FlexStar (0%). Nitinol self-expanding stents undergo both axial and bending deformation when implanted into the superficial femoral and popliteal arteries. Commercially available stents exhibit a variable ability to withstand chronic deformation in vitro, and their response is highly dependent on the type of deformation applied.