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 0.16gm/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 reliubly reflect the results of mechunicul tests.

A parametric study of cylindrical pedicle screw design implications on the pullout performance using an experimentally validated finite-element model

“…It is interesting to emphasize at this point that the number of numerical studies dealing with the investigation of the mechanical behaviour of a bone screw under pure pull-out loads is rather small [19][20][21][22][23][24] compared to the respective number of experimental studies. From these studies the only cases where a FE model was used for the prediction of the pullout force of different bone screws were those by Zhang et.…”

“…Especially for the screw design and its influence on the pullout force many experimental [6][7][8][9][10][11][12][13][14][15][16][17][18] and numerical [19][20][21][22][23][24] studies have been carried out. Additionally some researchers have attempted to quantify the factors that influence the pullout force of a bone screw using equations developed for "machine" screws (i.e.…”

“…The parameters studied were the outer radius, inclination, pitch and depth of the thread, as well as the screw purchase length. Another parameter that can influence a screw's pullout force is the conical angle of its core [22,23]. This parameter was not included to the present study which is focused on cylindrical screws.…”

Estudo da influência do torque de inserção na resistência ao arrancamento dos parafusos pediculares

“…As the pilot hole's diameter increases, a larger amount of bone is removed, and a smaller amount of bone is available to be compacted around the implant, thus reducing the insertion torque. The correlation between insertion torque and implants' pullout resistance has been presented as a controversial subject in literature reports addressing the matter, where reports exist on the correlation between insertion torque and pullout resistance (1,5,(9)(10)(11)(12) next to other reports not finding such correlation (13,14) . Screws' resistance to pullout is a complex phenomenon and dependant on a number of factors (2,15) .…”

“…All these variables influence a fixation system's stability and may affect the end outcome of the treatment when a vertebral fixation system fails (1)(2)(3)(4)(5) . The objective of this study was to assess the influence of insertion torque on pedicular screws' pullout resistance.…”