title: Increase of pullout strength of spinal pedicle screws with conical core: biomechanical tests and finite element analyses.
abstract: Screw loosening can threaten pedicle screw fixation of the spine. Conical screws can improve the bending strength, but studies of their pullout strength as compared with that of cylindrical screws have shown wide variation. In the present study, polyurethane foam with two different densities (0.32 and 0.16 gm/cm3) was used to compare the pullout strength and stripping torque among three kinds of pedicle screws with different degrees of core tapering. Three-dimensional finite element models were also developed to compare the structural performance of these screws and to predict their pullout strength. In the mechanical tests, pullout strength was consistently higher in the higher density foam and was closely related to screw insertion torque (r=0.87 and 0.81 for the high and low density foam, respectively) and stripping torque (r=0.92 and 0.78, respectively). Conical core screws with effective foam compaction had significantly higher pullout strength and insertion torque than cylindrical core screws (p<0.05). The results of finite element analyses were closely related to those of the mechanical tests in both situations with or without foam compaction. This study led to three conclusions: polyurethane foam bone yielded consistent experimental results; screws with a conical core could significantly increase pullout strength and insertion torque over cylindrical; and finite element models could reliably reflect the results of mechanical tests.
authors: Department of Mechanical Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan.
authors: Ching-Chi
authors: CC
authors: Hsu
authors: Ching-Kong
authors: CK
authors: Chao
authors: Jaw-Lin
authors: JL
authors: Wang
authors: Sheng-Mou
authors: SM
authors: Hou
authors: Ying-Tsung
authors: YT
authors: Tsai
authors: Jinn
authors: J
authors: Lin
mesh_terms: D001863:Bone Screws; D020342:Finite Element Analysis; D006801:Humans; D008422:Materials Testing; D011140:Polyurethanes; D011475:Prosthesis Failure; D013123:Spinal Fusion / Q000295:instrumentation*; D013314:Stress, Mechanical; D019415:Torque; D016474:Weight-Bearing
chemical_list: D011140:Polyurethanes