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Increase of pullout strength of spinal pedicle screws with conical core: Biomechanical tests and finite element analyses
Author(s) -
Hsu ChingChi,
Chao ChingKong,
Wang JawLin,
Hou ShengMou,
Tsai YingTsung,
Lin Jinn
Publication year - 2005
Publication title -
journal of orthopaedic research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.041
H-Index - 155
eISSN - 1554-527X
pISSN - 0736-0266
DOI - 10.1016/j.orthres.2004.11.002
Subject(s) - finite element method , core (optical fiber) , structural engineering , conical surface , biomechanics , orthodontics , materials science , engineering , medicine , composite material , anatomy
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/cm 3 ) 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 . © 2004 Orthopaedic Research Society. Published by Elsevier Ltd. All rights reserved.