Axial and Angled Pullout Strength of Bone Screws in Normal and Osteoporotic Bone Material

Abstract

Orthopedic surgery often involves the use of bone screws to stabilize fractures. Screw fixation can be extremely difficult in osteoporotic (OP) bone because of its compromised strength. Pullout strength is commonly used to measure screw fixation strength. In this study, axial and angled screw pullouts (ranging from 0° to 40°) were performed on 0.09 g.cm-3, 0.16 g.cm-3 and 0.32 g.cm-3 polyurethane (PU) foam. The PU foam was used to model different levels of low density human cancellous bone, including OP bone. Screw pull-out tests were conducted under displacement control at a rate of 0.1 mm.s-1. Load and displacement values were recorded; the maximum load during screw pullout was defined as the pullout strength of the screw. Two different titanium-alloy bone screws were used to test for any effect of thread type (i.e. cancellous or cortical) on the screw pull-out strength. Failure of screw fixation was observed from the stripping of the internal screw threads within the PU foam. Compared to the cortical screw, the cancellous bone screw had the highest pullout strength under almost all conditions of pullout angle and PU foam density. For both screws, the pullout strength increased with increasing PU foam density. Results indicate that the PU foam density, rather than the screw angle or thread type, is the most influential factor affecting screw pullout strength. These findings are important in understanding the behavior of screw fixation and screw failure within an OP bone model.