Material Selection and Force Requirements for the Use of Pre- Curved Needles in Distal Tip Manipulation Mechanisms

Pre-curved needles are used in a variety of medical applications for both passive and active control of instrument position. In one application, the deployment of a pre-curved stylet from a concentric outer cannula can be used to achieve lateral positioning of the distal tip of the stylet. This paper outlines how the material and geometry of the stylet can be chosen to ensure that it will not yield and thus repeatably return to its pre-curved shape when deployed from a cannula. Using this methodlogy we calculate the maximum strain for a range of stylet geometries and show that nitinol is required for the stylet material in order for then to not plastically deform. Then, sixteen stylets of varying diameter (0.508, 0.635, 0.838 and 0.990 mm) and radius of curvature (10, 20, 30 and 40 mm) were manufactured. Experiments were performed with four different diameter cannulas (20, 18, 16 and 14 gauge) to measure the forces required to deploy the stylets from and retract them back inside the cannulas. Retraction forces were measured between 0.3 and 13.9N, and deployment forces were measured between 0.2 and 7.0N. For a given cannula it was found that force increases as stylet diameter increases and bend radius decreases. INTRODUCTION In medicine, percutaneous needle insertion pertains to any medical procedure where the skin is punctured with a rigid needle or probe to access inner organs or tissue. As opposed to surgery, these procedures offer the advantages of reduced invasiveness and shorter recovery times for patients. Typically the procedures are performed under image-guidance such as computed tomography (CT), fluoroscopy, ultrasound or magnetic resonance imaging (MRI) that provide high resolution images of the patient anatomy. After a target is identified in the body a needle insertion point is chosen so as to avoid obstructing structures (such as ribs and blood vessels) and the needle is then manually inserted towards the target. Once the needle has been placed, the radiologist will often desire to adjust the location of its distal tip. Reasons for this could be to correct for a targeting error due to instrument misalignment and deflection or to sample/treat multiple points adjacent to each other. However, reorientation of the needle once inside the body is difficult as there are forces from the tissue that resist the pivoting motion. Radiologists do attempt to reposition the distal tip of the instrument once it is inside the body by overcompensating when realigning the needle; however, these medical instruments are usually quite thin and easily bend. Thus, the radiologist is often forced to retract the needle and attempt to re-insert it along the correct trajectory. However, this approach leads to another needle insertion for the patient that can increase the risk of complications. One approach that can be used to reposition the distal tip of a medical instrument is to deploy a pre-curved stylet from inside a concentric outer cannula as illustrated in Figure 1. Upon deployment of the stylet from the distal tip of the cannula, the stylet will then take its preformed shape and laterally deflect in the direction corresponding to the stylet curvature and the cannula angular position. Through rotation and translation of the outer cannula with respect to the patient and translation of the stylet relative to the cannula, the distal tip can be repositioned within a volume. Such a mechanism is thus