Pullout Strength Of Bone Screws Research Articles

Can barb thread design improve the pullout strength of bone screws?

AimsTo draw a comparison of the pullout strengths of buttress thread, barb thread, and reverse buttress thread bone screws.MethodsButtress thread, barb thread, and reverse buttress thread bone screws were inserted into synthetic cancellous bone blocks. Five screw-block constructs per group were tested to failure in an axial pullout test. The pullout strengths were calculated and compared. A finite element analysis (FEA) was performed to explore the underlying failure mechanisms. FEA models of the three different screw-bone constructs were developed. A pullout force of 250 N was applied to the screw head with a fixed bone model. The compressive and tensile strain contours of the midsagittal plane of the three bone models were plotted and compared.ResultsThe barb thread demonstrated the lowest pullout strength (mean 176.16 N (SD 3.10)) among the three thread types. It formed a considerably larger region with high tensile strains and a slightly smaller region with high compressive strains within the surrounding bone structure. The reverse buttress thread demonstrated the highest pullout strength (mean 254.69 N (SD 4.15)) among the three types of thread. It formed a considerably larger region with high compressive strains and a slightly smaller region with high tensile strains within the surrounding bone structure.ConclusionBone screws with a reverse buttress thread design will significantly increase the pullout strength.Cite this article: Bone Joint Res 2021;10(2):105–112.

The Effect of “Toggling” on the Pullout Strength of Bone Screws in Normal and Osteoporotic Bone Models

This study tests the hypothesis that screw toggling will reduce the pullout strength of bone screws. Pullout strength of cortical (cylindrical) and cancellous (tapered by 4°) bone screws were measured without and after toggling (movement caused by a force perpendicular to the screw axis) by ± 1 mm in polyurethane foam intended to mimic normal (density 0.32 g.cm-3) and osteoporotic (density 0.16 g.cm-3) bone. Toggling had no significant effect in decreasing the strength of fixation of cortical or cancellous screws inserted in the normal and osteoporotic bone models. Analysis of the screws that had been toggled showed that there was no significant difference (p > 0.05) between the pullout strength of cancellous and cortical screws (when pulled out from the same material). The results provide insights into some mechanical aspects of the pullout strength of bone screws and will aid understanding of the mechanism of screw toggling in vivo.

The effect of screw insertion angle and thread type on the pullout strength of bone screws in normal and osteoporotic cancellous bone models

Screw fixation can be extremely difficult to achieve in osteoporotic (OP) bone because of its low strength. This study determined how pullout strength is affected by placing different bone screws at varying angles in normal and OP bone models. Pullout tests of screws placed axially, and at angles to the pullout axis (ranging from 10° to 40°), were performed in 0.09 g cm −3, 0.16 g cm −3 and 0.32 g cm −3 polyurethane (PU) foam. Two different titanium alloy bone screws were used to test for any effect of thread type (i.e. cancellous or cortical) on the screw pullout strength. The cancellous screw required a significantly higher pullout force than the cortical screw ( p < 0.05). For both screws, pullout strength significantly increased with increasing PU foam density ( p < 0.05). For screws placed axially, and sometimes at 10°, the observed mechanism of failure was stripping of the internal screw threads generated within the PU foam by screw insertion. For screws inserted at 10°, 20°, 30° and 40°, the resistance to pullout force was observed to be by compression of the PU foam material above the angled screw; clinically, this suggests that compressed OP bone is stronger than unloaded OP bone.

Investigation of fixation screw pull-out strength on human spine

For the purpose of fixation and stabilizing the spine, the pull-out strength of the screw is one of the most important factors to be considered. The material properties of the bone, coupled with the principal dimensions of the screw such as major diameter, minor diameter, pitch and purchase length, may affect the pull-out strength of the fixation screw. In this study, the effects of various factors on the bone screw pull-out strength were studied using finite element method of analysis. A three-dimensional finite element model simulating the threaded connection of bone and surgical screw was constructed. The behavior of both the bone and the screw during screw pull-out were analyzed and discussed. The effect of the screw parameters on the screw pull-out strength was also investigated. The results showed that failure of the connection was due to bone shearing which occurred along a cylindrical surface determined by the outer perimeter of the screw and penetrated length. The distribution of the shear stress along the threaded length in the bone was nearly uniform. The effect of the major diameter on the screw pull-out strength was more significant than those of the minor diameter and the pitch. The minor diameter hardly affected the pull-out strength.

Effect of insertion torque on bone screw pullout strength.

The effect of insertion torque on the holding strength of 4.5-mm ASIF/AO cortical bone screws was studied in vitro. Screw holding strength was determined using an Instron materials testing machine (Bristol, United Kingdom) on 55 lamb femora and 30 human tibiocortical bone sections. Holding strength was defined as tensile stress at pullout with rapid loading to construct failure. Different insertion torques were tested, normalizing to the thickness of cortical bone specimen engaged. These represented low, intermediate, high, and thread-damaging insertion torque. All screws inserted with thread-damaging torque and single cortex engaging screws inserted to high torque tightening moments showed diminished holding strength. This loss of strength amounted to 40%-50% less than screws inserted with less torque.