Strength of fixation of anterior vertebral body screws

Study Design. The technique of hole preparation and the placement or orientation of anterior transvertebral screws in relation to the endplates were the subjects of two experiments. Objectives. Experiment 1-to determine if anterior transvertebral body screws have higher pull-out strengths after unicortical predrilling compared with a bicortical predrilling; Experiment 2-to determine the effect of the screw's orientation in the vertebral body on its resistance to loosening. Summary of Background Data. There are a variety of surgical techniques for using anterior transvertebral screws in the vertebral body without any clear guide-lines as to which techniques optimize the strength of fixation. Methods. In Experiment 1, 31 cadaveric vertebral bodies were tested for pull-out strength of transvertebral body screws placed after unicortical predrilling compared with bicortical predrilling. In Experiment 2, 48 cadaveric vertebra bodies were tested with transvertebral screws inserted using four different screw orientations. Results. There is no statistically significant difference in pull-out strength of anterior transvertebral body screws inserted after unicortical compared with bicortical predrilling (Experiment 1). The resistance to loosening is greatest when transvertebral screws are oriented in the ''superior oblique'' position (Experiment 2). Conclusions. Experiment 1-preparation of the far cortex by predrilling is not necessary. Experiment 2-transvertebral screws should be obliquely oriented so that the forces applied to the screws are resisted by both of the endplates.