Biomechanical properties of expander compared with conventional screws

A pullout strength biomechanical study was performed in 20 fresh swine vertebral bodies in which titanium expander (Group 1) and conventional screws (Group 2) were placed.

The screws were inserted into the anterosuperior portion of the anterior spine, and assessment was performed after application of loads. The expander screw is composed of two parts: 1) a cover with an external portion comprising tight thin threads; and 2) a compact internal screw inserted through the cover that allows expansion. In the comparative study between the screws in Groups 1 and 2 maximum load was assessed, and the intergroup difference was significant (p = 0.00001 [t-test]); regarding load at the elasticity threshold, a significant difference was also observed (p = 0.0063). With regard to rigidity (stiffness), there was a tendency in both groups toward significance (p = 0.069). With regard to absorbed energy in the elastic phase, statistical analysis showed a significant intergroup difference (p = 0.00439). The expander screw showed a greater load-bearing capacity than the conventional screw. Adhesion to bone in relation to the applied load and displacement was greater (significant tendency) in the expander screw group than in the conventional screw group.

The expander screws exhibited a greater capacity to absorb energy in the elastic phase. They adhered better to bone, were easy to insert, and, if necessary, were simple to remove.