1. Retrieval System for Intracranial Artery Stenting Yvonne Banarez 1, Kristin Lindberg 1, Natalie Pous 1, Brian Sweet 1 Advisor: Ted Larson III M.D. 2 1 Department of Biomedical Engineering, Vanderbilt University, Nashville, TN 2 Department of Neuroradiology, Vanderbilt University, Nashville, TN Overview Project Deliverables Design a retrieval system for intracranial artery stenting Develop a computer model or prototype of retrieval system Provide theoretical data to evaluate feasibility of retrieval system Retrieval System Design Device Description Specifications Vessel lumen (fully-expanded stent range): 2.0 – 5.0 mm diameter Compatible with 2.3 F delivery catheter (approximately 0.75 mm interior diameter) Compatible with 0.014 inch guidewire Control mechanism located up to 1.0 m away The retrieval system consists of three rotationally detachable arms, fused to the guidewire and connected to the proximal end of the microstent. The retrieval system is detached from the stent when it is correctly placed, by clockwise rotation of the guidewire by the physician. If incorrectly deployed, the stent can be collapsed and pulled back into the catheter by the physician and redeployed in the appropriate location. The retrieval system is fabricated in stainless steel, and the stent is made of nitinol. Conclusions and Recommendations Acknowledgements Several problems have been documented using self-expanding microstents for procedures to correct intercranial atherosclerotic stenosis. Intercranial arteries are tortuous, making longitudinal flexibility an important issue, as stents can become kinked during delivery, rendering them useless. This is particularly problematic when navigating vessels in the anterior circulation [1]. When the catheter and stent can be delivered to the proper site without any shape distortion in the stent, it can often be deployed in an incorrect place or improperly expand in an abnormality of the vessel. Often vessel sacrifice is the only technique that can be used when stent deployment is unsuccessful [2]. There is thus a very specific need, in intracranial atherosclerotic stenosis treatment and other stenting procedures throughout the body, for a retrievable microstent which can be deployed, retrieved if necessary, and redeployed in another location. U.S. Patent activity confirms this need, as several retrievable stents have been patented by some of the largest biotechnology firms in the United States [3, 4, 5, 6]. Much of the patent activity for retrievable stents focuses on applications elsewhere in the body because of the physiological constraints of intracranial arteries. Minimizing physician error and time under general anesthesia in intracranial artery stenting surgeries is paramount to successful stent placement, creating a need for a fully and easily retrievable microstent system. Stent misplacement in an aneurysm [2]. Material Selection Nitinol: 55% Nickel, 45% Titanium – intermetallic bond maintains stability Shape-memory alloy – can be formed at body temperature according to physiological specifications and returns to this shape during stent deployment biocompatibility – no thrombogenic or toxic effects, no cytotoxicity, highly resistive to corrosion FDA approved for biological use Stainless Steel: Fe-23Mn-21Cr-1Mo-1N (nickel-free medical grade) with improved tensile strength Results Theoretical Data Theoretical inward radial force data for arm length of 3.0 mm and catheter lumen radius of 375 µm, for physician-exerted forces of (a) 10 N; (b) 20 N; and (c) 30 N.  = angle between arm and longitudinal axis of catheter and vessel R1 = radius of expanded stent R2 = radius of catheter lumen L1 = length of attachment arm L2 = distance between the point of arm attachment to guidewire and point of catheter force F S = resultant inward radial force exerted by arm on stent F C = force of catheter on the arms Prototype 300 hours of experienced machinist time would be required to prototype a scaled model of our design using biomedical-grade stainless steel or a similar metal [9]. There are no materials available currently for rapid prototyping techniques that simulate the behavior of nitinol and stainless steel. Simplifying our design or changing materials was not an option, so we instead present only a computer model of our retrieval system. Rotational detachment may be very useful for intracranial artery stenting procedures due to simplicity and minimization of physician error With physician-exerted force of 30 N, the maximum inward radial force for collapsing stent is approximately 6.78 N. Our design provides higher force when stent is fully expanded. Future work: Test design using fluid profile software Prototype the design Evaluate actual inward radial force required to collapse stent FDA approval procedures: in vivo testing in rabbit vasculature Social and Market Impact Stroke affects 700,000 – 1,000,000 Americans per year Studies estimate between 10% and 29% of ischemic events are directly caused by intracranial atherosclerotic disease and related thrombosis [1, 7] Currently stent costs range from $900-$3195 Estimated cost of an additional stainless steel retrieval system: $1000 Addition of a retrieval system can save patient costs by minimizing stent waste— some studies report an average number of stents used in a single procedure of 1.7, with procedures using as many as 5 stents [8]. 1.Lutsep HL, Clark WM (2000). Association of intracranial stenosis with cortical symptoms or signs. Neurology 55: 716-718. 2.Broadbent LP, Moran CJ, Cross DT 3rd, Derdeyn CP (2003). Management of Neuroform stent dislodgement and misplacement. Am J Neuroradiol 24: 1819-1822. 3.Bolea, et al. Retrievable stent and method of use thereof. United States Patent No. 6,821,291. Nov 23 2004. 4.Amplatz, et al. Repositionable and recapturable vascular stent/graft. United States Patent No. 6,468,301. Oct 22 2002. 5.Letendre, et al. Stent which is easily recaptured and repositioned within the body. United States Patent No. 6,267,783. Jul 31 2001. 6.Rabkin et al. Methods for delivering, repositioning and/or retrieving self-expanding stents. United States Patent No. 6,837,901. Jan 4 2005. 7.Chimowitz MI, Kolkinos J, Strong J, Brown MB, Levine SR, Sillman S, Pessin MS, Weichel E, Sila CA, Furlan AJ (1995). The Warfarin-Aspirin symptomatic intracranial disease study. Neurology 45: 1488-1493. 8.Hirschfeld JW, Wilensky RL (2004). Drug-eluting stents are here-now what? Implications for clinical practice and health care costs. Cleveland Clinic J Med 71: 825-828. 9.Phil Davis, Research Development Engineer, Vanderbilt University School of Engineering Design Goals FCFC  FCFC FSFS FSFS Our Design Mesh design (adapted from U.S. Patent 6,776,693. Attachment arm assembly and guidewire. Arm assembly attachment site on stent. Proximal view of stent assembly including catheter. Arm assembly attached to stent.Stent assembly including catheter.