title: Unzipping of small diameter stents: an in vitro study.
abstract: OBJECTIVE
  The purpose of this study was to determine if small-diameter stents can be unzipped in vitro.
  
  
  BACKGROUND
  Small-diameter stents can relieve stenosis in infant blood vessels. As the child grows, refractory stenosis may result. If an implanted stent can be intentionally fractured along its length-"unzipped," it can be redilated to the eventual adult vessel diameter.
  
  
  METHODS
  Stents of diameters ≤6 mm were dilated using angioplasty balloons until they fractured. The change in length-diameter (dL/dD ratio), and the yield-point-force (σy ) for each stent was calculated.
  
  
  RESULTS
  Thirty-four coronary (CS), 11 biliary, and 10 nitinol peripheral stents (median diameter = 4, 5, and 6 mm; range = 2.75-4.5, 4-6, 6 mm, respectively) were tested. Stainless-steel (SS) CS unzipped predictably at twice their nominal diameter with minimal shortening (n = 24, median dL/dD = 0.4). Nitinol stents fractured in a disorganized fashion. The remaining stents unzipped, had disorganized fractures, and shortened significantly (dL/dD>1). A dL/dD ratio of<1 had a strong, positive correlation with the ability to unzip (Pearson rho = 0.94). By multivariate regression analysis, SS alloy, and closed-cell design were found to be significant predictors (P < 0.05) for unzipping. Optimal cut-off points for stents to unzip included, strut-thickness = 112 µm, alloy-density = 7.7 g/cm(3) , dL/dD ratio = 0.12 and σy = 108Mpa (Youden's index = 0.8, 0.4, 0.8, and 0.5, respectively).
  
  
  CONCLUSIONS
  Stainless-steel, coronary stents of a closed-cell design unzip at twice their nominal diameter without significant shortening when serially dilated. This study may encourage the implantation of small stents in infant blood vessels and aid in selection of appropriate stent type.
authors.affiliation: Division of Pediatric Cardiology, LeBonheur Children's Hospital, University of Tennessee Health Science Center, Memphis, Tennessee.
authors.forename: Shyam K
authors.initials: SK
authors.lastname: Sathanandam
authors.forename: Lauren M
authors.initials: LM
authors.lastname: Haddad
authors.forename: Saradha
authors.initials: S
authors.lastname: Subramanian
authors.forename: Dena
authors.initials: D
authors.lastname: Wright
authors.forename: Ranjit
authors.initials: R
authors.lastname: Philip
authors.forename: Benjamin Rush
authors.initials: BR
authors.lastname: Waller
mesh_terms: D000497:Alloys; D000800:Angioplasty, Balloon / Q000295:instrumentation*; D015906:Angioplasty, Balloon, Coronary / Q000295:instrumentation; D019544:Equipment Failure Analysis; D005240:Feasibility Studies; D008422:Materials Testing; D011312:Pressure; D011474:Prosthesis Design; D011475:Prosthesis Failure; D013193:Stainless Steel; D015607:Stents; D013314:Stress, Mechanical
chemical_list: D000497:Alloys; D013193:Stainless Steel; C013616:nitinol
keywords: balloon dilation; fracture; stents; unzip