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Strength of fixation of anterior vertebral body screws
W C Horton1, S F Blackstock, J T Norman
1Department of Orthopaedics, Emory University, Decatur, Georgia, USA.
Study Design:
The technique of hole preparation and the placement or orientation of anterior transvertebral screws in relation to the endplates were the subject 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 And Background Data:
There are a variety of surgical techniques for using anterior transvertebral screws in the vertebral body without any clear guidelines 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 vertebral 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.