1 * Fellow ASME Copyright © 2011 by ASME Proceeding of the ASME 2011 Pressure Vessels and Piping Division Conference July 17-21, 2011, Baltimore, Maryland, USA DRAFT PVP2011-57594 STUDY OF PULLOUT PERFORMANCE OF SELF-TAPPING SCREWS FOR HUMAN BONE Zhijun Wu, Sayed A. Nassar*, Xianjie Yang Fastening and Joining Research Institute Department of Mechanical Engineering Oakland University Rochester, MI, 48309, USA ABSTRACT The study investigates the pullout strength of self-tapping pedicle screws using analytical, finite element, and experimental methodologies with focus on medical device applications. The stress distribution and failure propagation around implant threads in the synthetic bone during the pullout process, as well as the pullout strength of pedicle screws, are explored. Based on the FEA results, an analytical model for the pullout strength of the pedicle screw is constructed in terms of the synthetic bone material properties, screw size, and implant depth. The characteristics of pullout behavior of self-tapping pedicle screws are discussed. Both the analytical model and finite element results are validated using experimental techniques. Keywords: self-tapping, pedicle screw, pullout strength INTRODUCTION Since the concept of placing a screw in the lateral masses of the cervical spine for stabilization purposes, was presented by Roy-Camille [1] in 1963, self-tapping screws have been widely used in orthopedic joints after surgery and during the healing process [2, 3]. This type of threaded fasteners is primarily screwed into the bone without any tension in the screw or clamping force in the bone. Applications include neck and spine injuries, as well as hip and knee and replacements. The strength of the screw connection is one of the main concerns in the post-surgery recovery and the long term mobility of the patient. Hence, it is important that a high reliability level is ensured for those self-tapping screws used in medical devices [4-7]. Although pure pullout may not be a common failure mode in clinical applications, pullout testing is thought to be a good predictor of screw fixation strength [7]. Many bio-mechanical studies have demonstrated that numerous parameters affect screw pullout resistance [8-16]; this includes the screw geometries (outer and core diameter, and pitch), pilot hole dimensions, implant depth, and the bone-mineral density. Chapman et al. [3] presented thread shape factor as a function of thread pitch and outer and core diameters of the screw and demonstrated that increasing the thread shape factor (by decreasing the pitch) would increase the screw pullout strength in a porous material. Hearn et al. [4] stated that the primary factors affecting the holding strength are screw outer and core diameters and the length, which determine the area of cylindrical load bearing surface, and the ultimate shear strength of the bone. Becker et al. [10], and Hsu et al. [6] concluded that the insertion depth and the outer diameter of the screw have a significant effect on the pullout strength. Oktenoglu et al. [9] stated that the mean pullout strength in the cases of nonpilot hole preparation was greater than that in the cases with pilot hole. Hsu et al. [6], and Battula et al. [8] demonstrated that a higher density of synthetic bone consistently yields better pullout strength. Although many researchers have qualitatively explored the effect of the various factors, only a few studies have addressed the pullout failure mode of screws and the damage initiation and propagation in the bone. Some analytical models were used for estimating the pullout strength of screws. Chapman et al. [3] presented a formula to predict the pullout force that would strip the internal thread, as a function of bone material shear strength and the shear area of the threads. The model did not consider the failure propagation in the bone and the special thread shape compared with standard bolt threads. Coe et al. [17] reported the linear relationship between bone mineral density (BMD, gm/cm 2 ) measured by dual photon absorptiometry and the pullout strength of the screw expressed in the following empirical formula ( ) BMD F s × + = 499 6 . 43 (1) In this study, the experimental investigation explores the tensile and compressive stress-strain response of different synthetic bone materials. Generated stress-strain data is then Proceedings of the ASME 2011 Pressure Vessels & Piping Division Conference PVP2011 July 17-21, 2011, Baltimore, Maryland, USA PVP2011-57594