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Evaluation of Balloon Withdrawal Forces (Full Title Below)
Evaluation of Balloon Withdrawal Forces with Bare-Metal Stents, Compared with Taxus™ and Cypher™ Drug-Eluting Coronary Stents: Balloon, Stent and Polymer Interactions
ABSTRACT: Background. There have been reports of serious complications related to difficulty removing the deflated Taxus™ stent delivery balloon after stent deployment. The purpose of this study was to determine whether the Taxus SIBS polymer was “sticky” and associated with an increase in the force required to remove the stent delivery balloon after stent deployment, using a quantitative, ex-vivo model. Methods. Balloon-polymer-stent interactions during balloon withdrawal were measured with the Taxus Liberté™, Liberté bare-metal stent (BMS; no polymer = control), the Cordis Cypher™ drug-eluting stent (DES; PEVA/PBMA polymer) and the BX Velocity™ (no polymer). We quantitatively measured the force required to remove the deflated stent delivery balloon from each of these stents in simulated vessels at 37° C in a water bath. Balloon withdrawal forces were measured in straight (0 degree curve), mildly curved (20 degree curve) and moderately curved (40 degree curve) simulated vessel segments. Results. The average peak force required to remove the deflated balloon catheter from the Taxus Liberté DES, the Liberté BMS, the Cypher DES, and the Bx Velocity BMS were similar in straight segments, but were much greater for the Taxus Liberté in the moderately curved segments (1.4 lbs vs. 0.11 lbs, 0.11 lbs and 0.12 lbs, respectively; pJ INVASIVE CARDIOL 2010;22:113–116 The introduction of drug-eluting stents (DES) has advanced the percutaneous treatment of coronary artery disease by substantially reducing the incidence of in-stent restenosis.1,2 Up until recently, the Cypher™ (Cordis Corp., Miami Lakes, Florida) and the Taxus™ (Boston Scientific, Natick, Massachusetts) were the only DES that were available in the United States (U.S.).3 The Cypher DES contains the drug sirolimus which elutes from a PEVA/PBMA (polyethylene-co-vinyl acetate/poly N-butyl methacrylate) polymer, and the Taxus DES elutes the drug paclitaxel from a SIBS (styrene-isobutylene-styrenetriblock) polymer.3 Since their approval, DES have become the stents of choice for 60–80% of the ~600,000 hospital discharges for coronary artery stenting performed in the U.S. annually.4 Following the launch of the Taxus stent in 2004, complications due to both balloon deflation problems and to difficulties in removal of deflated Taxus stent delivery balloons were reported. A number of these complications have been fatal or have led to serious complications including emergency bypass surgery to remove the “stuck” balloon. Between 2004 and the second quarter of 2006, there were 390 “stickiness” or balloon withdrawal issues reported to the FDA’s (U.S. Food and Drug Administration) MAUDE (Manufacturer and User Facility Device Experience Database) site5 for the Taxus Express™ and Liberté™ (European reports to FDA) stents. In contrast, there are virtually no similar complications reported with the bare-metal Express, Liberté, Cypher™ or other DES.5 One typical MAUDE report taken from the FDA website reads; “It was reported that during a coronary DES treatment procedure, difficulty removing the balloon from the stent was encountered. The physician successfully deployed a Taxus Liberté 2.5 x 24 mm DES. Upon withdrawal, the physician felt the fully deflated balloon was ‘sticking to the stent.’ Multiple inflations and deflations were unsuccessful. Physician then decided to use additional force to successfully remove the balloon.”5 Numerous other similar cases are reported, including a number with severe dissections from “sucking in” of the guiding catheter and/or tearing of the distal stented segment from the force of balloon withdrawal.5 It is unclear from this and numerous other case reports whether these issues are related to the stent design, or the interaction between the polymer coating of the stent (i.e., the SIBS polymer) and the balloon. The purpose of this study was to examine the force of withdrawal of the Taxus balloon from the Taxus stent as compared to a control BMS of the same design (Liberté), and compared to another polymer coated DES (Cypher), along with its bare metal equivalent (BX Velocity) in a reproducible ex vivo model.
Methods
Model. We utilized a reproducible, ex vivo model to measure the force required to remove a deflated balloon from a stent deployed in a simulated human coronary artery. The force required to remove a balloon from within a deployed stent reflects the balloon-polymer-stent interaction during the process of balloon withdrawal. For these studies a stone cutting tool was used to etch grooves (~ 5 mm in diameter) of three different curvatures in two separate 3/4 inch thick granite slabs. A straight groove (0° curve) was cut in one slab, and two curved grooves (mildly curved (20° curve), and moderately curved (40° curve) were cut in the second granite slab (Figures 1A, B and C). The curvatures of 20° and 40° represent the measured (mean) stent curvature over the deployed stent length (28 mm), and not to the angulation of the curve cut into the granite. For each experiment a silastic tube (3 mm inner diameter) was placed in these groves to simulate a coronary artery. This tubing allowed expansion of a stent into the tube wall, simulating the placement of a stent in a human coronary artery (Figures 1 B and C). During the experiments, the granite slabs and silastic tubes were submerged in distilled water in a heated muscle bath at 37 degrees centigrade. The stents were delivered into the proper position in the silastic tube within the granite slab groove over a 0.014 inch guidewire. All stents were deployed at 15 atmospheres using a standard inflation device. The balloon was then fully deflated, and placed on full negative pressure from the balloon inflation device, with the stent fully expanded in the tubing. The shaft of the balloon catheter was then connected to a calibrated Mark-10 BG10 force gauge (Mark–10 Corp., Copiague, New York) using a dual roller grip (Figures 2A and 2B). The force gauge was mounted on a motorized Mark-10 ESM test stand (Mark–10), which then moved vertically at 3.0 mm per second to remove the stent delivery balloon from the stent (Figure 2). Data collection. The RS-232 BG10 sensor was used to record 60 force measurements/second from the Mark-10 BG10 device during the withdrawal of the deflated balloon from the deployed stent. These force data points were then transferred to a PC using the Win Wedge 32 data collection software (Mark–10). Data were collected until the stent catheter and balloon were completely removed from within the deployed stent. This procedure was performed with exactly the same methodology in the straight, mildly curved and moderately curved segments for each of the four stent systems. Six experiments were performed with each of the stent systems in each of the three curves (72 total experiments using 72 stents). A new piece of silastic tubing was used for each experiment to ensure consistent testing conditions. Stent types. In these experiments, four different types of stents were used: Taxus Liberté DES (SIBS polymer), Liberté (Taxus control with no polymer), Cypher DES (PEVA/PBMA polymer) and BX Velocity (Cypher control with no polymer). All experiments (with all four stent systems) were performed using stents that were 28 mm length and 3.0 mm diameter to control for any bias that could be related to differences in stent diameter or length. The serial data obtained from these experiments using the Mark-10 BG10 device were categorized into 12 groups depending on the stent type and the vessel curvature (4 stent types x 3 vessel curvatures). Comparisons were then performed between the following groups: Taxus Liberté DES and Cypher DES, Taxus Liberté DES and Liberté, and Cypher DES and BX Velocity in each of the vessel types (straight, mild curve and moderate curve) (Table 1). Statistical analysis. The maximum force value and a force-time graph were determined for each of the 12 groups by taking the average of the 6 sets in that group. ANOVA for repeated measures (Microsoft Excel) was used to compare the peak force required during balloon withdrawal among these 4 stent systems in each of these three test conditions (straight, mild curve and moderate curve). A p-value Results The peak forces of balloon removal for each stent type from the straight, mildly curved and moderately curved vessels are shown in Figures 3 and 4. The maximum forces (mean) required to remove the balloon catheter from a Taxus Liberté DES, a Liberté BMS, a Cypher DES and a BX Velocity BMS were 1.4 lbs, 0.105 lbs, 0.105 lbs, and 0.12 lbs, respectively, in the moderately curved vessel. These results demonstrated a highly significant increase in force required to remove the Taxus Liberté compared to the forces required with the Liberté BMS, the Cypher stent or the BX Velocity (p Discussion The SIBS polymer used on the Taxus stent is a form of rubber and adhesive compound with elastomeric properties.6 The SIBS polymer has some favorable characteristics as a drug carrier. SIBS is a biologically inactive and biostable compound with abilities to retain the drug (paclitaxel) on the stent, provide relatively uniform drug delivery, prevent mechanical disruption during processing and deployment, and has a relatively long shelf life.7–9 Although the SIBS polymer has properties that make it a suitable drug delivery polymer, this compound also has unfavorable physical properties related to its adhesive behavior. The SIBS polymer has been widely used in the commodity industry as an adhesive because of its outstanding cohesive properties and excellent bond strength.9,10 It is this property of the polymer that helps it stick to the stent. The polymer is “tacky” to the touch and may encourage balloon sticking, particularly after prolonged contact with the balloon after the stent is crimped onto the balloon. At least one case from the MAUDE data site describes missing fragments of the balloon after a difficult balloon withdrawal, with subsequent confirmation of balloon material adherent to the implanted Taxus stent, visualized by intravascular ultrasound.5 The controlled, quantitative experiments of the current study clearly demonstrate a major behavioral difference in balloon withdrawal forces required when using the Taxus Liberté DES compared to other stent delivery systems. The increased force required for balloon withdrawal was particularly evident in curved vessels. In this ex vivo model there was a highly significant difference (p Conclusions The SIBS polymer of the Taxus Liberté DES appears to be “sticky” and is associated with high forces required for the deflated balloon withdrawal when the stent is deployed in curved segments. This finding may help to explain the clinical complications that have been reported concerning balloon withdrawal difficulties with this device.________________________________________________
From Borgess Research Institute, Borgess Medical Center, Kalamazoo, Michigan. Disclosures: The authors disclose that Johnson & Johnson provided the stents to Borgess Hospital to conduct this study. Tim Fischell has potential conflict(s) of interest information with licensing of patents to Abbott Vascular, Cordis Corp./Johnson & Johnson and Boston Scientific. He is a member of the Scientific Advisory Board for Abbott Vascular. Manuscript submitted October 20, 2009, provisional acceptance given November 16, 2009, final version accepted November 19, 2009. Address for correspondence: Tim A. Fischell, MD, FACC, Director, Cardiovascular Research, Borgess Research Institute, Borgess Medical Center, 1521 Gull Road, Kalamazoo, MI 49048. E-mail: tafisc@gmail.com________________________________________________
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