Zhijun Wu e-mail: zwu@oakland.edu Sayed A. Nassar ASME Fellow e-mail: nassar@oakland.edu Xianjie Yang e-mail: yang2345@oakland.edu Fastening and Joining Research Institute, Department of Mechanical Engineering, Oakland University, Rochester, MI 48309 Pullout Performance of Self-Tapping Medical Screws This study investigates the effect of the pilot hole size, implant depth, synthetic bone density, and screw size on the pullout strength of the self-tapping screw using analytical, finite element, and experimental methodologies. Stress distribution and failure propagation mode around the implant thread zone are also investigated. Based on the finite element analysis (FEA) results, an analytical model for the pullout strength of the self-tapping screw is constructed in terms of the (synthetic) bone mechanical properties, screw size, and the implant depth. The pullout performance of self-tapping screws is discussed. Results from the analytical and finite element models are experimentally validated. [DOI: 10.1115/1.4005172] Keywords: self-tapping, medical 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 fastener 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 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 biomechanical 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 a thread shape factor as a function of thread pitch and outer and core diameters of the screw and demon- strated that increasing the thread shape factor (by decreasing the pitch) would increase the screw pullout strength in a porous mate- rial. 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 sur- face, 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] dem- onstrated 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] pre- sented 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 com- pared with standard bolt threads. Coe et al. [17] reported the linear relationship between bone mineral density (BMD, gm/cm 2 ) meas- ured by dual photon absorptiometry and the pullout strength of the screw expressed in the following empirical formula: F s ¼ 43:6 þ 499  BMD ð Þ (1) In this study, the experimental investigation explores the tensile and compressive stress-strain response of different synthetic bone materials. Based on the different deformation and failure mecha- nisms of synthetic bone materials under tension and compression testing, a damaged coupled constitutive model was used to describe the constitutive and damage behavior of each material, as well as the pullout strength and failure characterization. Finite ele- ment analysis is used to explore the screw pullout process. Stress distribution, failure/damage initiation and propagation around implant threads (along the screw axis), and the pullout strength are investigated. Based on experimental data and the finite ele- ment simulation, the effects of pilot hole diameter, screw implant depth, screw size, and synthetic bone density on the pullout strength of the self-tapping screw are explored. Both the finite ele- ment results and the analytical model are experimentally validated. Experimental Determination of Synthetic Bone Mechanical Properties It is well known that the material property of real bones varies with age, sex, location of the bone, mineral content, test condition (dry or wet), and disease such as osteoporosis [18]. As a result, extensive bone testing would be necessary on cadaver bone to iso- late the effect of various screw/structure design and pilot hole dimensions [19]; this would be time consuming and impractical. In the study, a practical alternative is to use synthetic bone (uni- cellular polyurethane foam); the material offers uniform and con- sistent physical, mechanical properties that eliminate the variability of cadaver bone. Some researchers have found that some densities of the rigid polyurethane foam exhibit closed-cell structure similar to the human cancellous bone, and the mechani- cal properties are also in the same range of those of human cancel- lous bone [20,21]. In this study, unicellular polyurethane foams with four den- sities, namely, 0.24, 0.36, 0.48, and 0.64 g/cm 3 , from Pacific Research Laboratories, Inc., Vashon, Washington, were used for the tests. Uniaxial tensile and compressive tests are conducted using a MTS 810 machine to determine the material tensile and Contributed by the Bioengineering Division of ASME for publication in the JJOURNAL OF BIOMECHANICAL ENGINEERING. Manuscript received May 12, 2011; final manuscript received September 23, 2011; published online November 18, 2011. Assoc. Editor: Mohamed Samir Hefzy. Journal of Biomechanical Engineering NOVEMBER 2011, Vol. 133 / 111002-1 Copyright V C 2011 by ASME Downloaded 20 Dec 2011 to 141.210.133.193. Redistribution subject to ASME license or copyright; see http://www.asme.org/terms/Terms_Use.cfm