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.

Compressive properties of commercially available polyurethane foams as mechanical models for osteoporotic human cancellous bone.

BACKGROUND: Polyurethane (PU) foam is widely used as a model for cancellous bone. The higher density foams are used as standard biomechanical test materials, but none of the low density PU foams are universally accepted as models for osteoporotic (OP) bone. The aim of this study was to determine whether low density PU foam might be suitable for mimicking human OP cancellous bone. METHODS: Quasi-static compression tests were performed on PU foam cylinders of different lengths (3.9 and 7.7 mm) and of different densities (0.09, 0.16 and 0.32 g cm(-3)), to determine the Young's modulus, yield strength and energy absorbed to yield. RESULTS: Young's modulus values were 0.08-0.93 MPa for the 0.09 g cm(-3) foam and from 15.1-151.4 MPa for the 0.16 and 0.32 g cm(-3) foam. Yield strength values were 0.01-0.07 MPa for the 0.09 g cm(-3) foam and from 0.9-4.5 MPa for the 0.16 and 0.32 g cm(-3) foam. The energy absorbed to yield was found to be negligible for all foam cylinders. CONCLUSION: Based on these results, it is concluded that 0.16 g cm(-3) PU foam may prove to be suitable as an OP cancellous bone model when fracture stress, but not energy dissipation, is of concern.