Pullout strength comparison of two methods of orienting screw insertion in the lateral masses of the bovine cervical spine.

We undertook a biomechanical study to compare the pullout strength of 3.5-mm AO screws placed in two different orientations within the bovine cervical spine. The first set of screws were oriented obliquely and passed through the lateral mass, as recommended by the AO group. The orientation of the second set was anterior to posterior through the lateral mass, as recommended by Roy-Camille. All screw holes were drilled and tapped by a spinal surgeon experienced with both techniques. Pullout force was measured on an Instron materials testing machine using a self-centering screw-holding chuck and loading rate of 0.833 mm/sec. Although the bone strength in the Roy-Camille orientation was greater (46.7 N/mm versus 36.1 N/mm, p < 0.05), the overall mean pullout force for the AO orientation was greater (607 N versus 471 N, p < 0.025) due to the longer length of bone available for screw purchase (17.0 mm versus 10.3 mm).