The effect of divergent screw placement on the initial strength of plate-to-bone fixation

Numerous implants exist that allow screws to be placed at varying angles for lag fixation or to fix additional fragments. This study determined how placing screws at different angles affects fixation strength.

Using a bone model, we first investigated the pullout strength of screws inserted at varying angles; then, we studied the strength of plate-bone constructs with end screws placed at divergent angles.

Varying the screw angle from 0 to 10 to 20 degrees progressively weakened the screw pullout resistance. No additional decrease was found in varying the angle further. In contrast, the strength of fixation of plate to bone was higher for constructs with screws placed at 20 or 30 degrees off of perpendicular when tested in gap-open bending and axial compression (all p < 0.05). No such differences were found in torsion.

The pullout strength with angled screws is reduced, but this does not translate into reduced strength of the bone-to-plate interface.