Determination of the length of anteromedial screw trajectory by measuring interforaminal distance in the first sacral vertebra.

STUDY DESIGN: Measurements of the length of pediculocorporeal screw trajectory were performed on bony sacra. Outer interforaminal distances of the first sacral vertebra were measured on bony sacra and anteroposterior sacral radiographs.

OBJECTIVES: To demonstrate the correlation of the length of pediculocorporeal screw trajectory of the first sacral vertebra on bony specimens with outer interforaminal distances on their anteroposterior radiographs.

SUMMARY OF BACKGROUND DATA: Posterior fixation of the lumbosacral vertebra has been used in the treatment of unstable lumbar spine disorders. To achieve the strongest biomechanical stability and to avoid injury to anterior sacral structures for anteromedial insertion of the posterior transpedicular screw to the first sacral vertebra (S1), it is crucial to determine the optimum screw length for both unicortical and bicortical screw placement.

METHODS: Fifty-one dry bony adult sacra were measured and correlated with anteroposterior lumbosacral radiographs. The length of S1 screw trajectory, which was accepted as between inferolateral border of superior articular facet of S1 and sacral promontory, and the outer interforaminal distances of anterior S1 foramens were measured on bony specimens. The outer interforaminal distances of anterior S1 foramens were also measured on anteroposterior sacral radiographs.

RESULTS: The outer interforaminal distances were not statistically different from the length of the S1 screw trajectory on bony specimens. Radiographs of the S1 vertebra have also shown that outer interforaminal distances were not statistically different from the length of the S1 screw trajectory.

CONCLUSION: In this study, we described an easy and a reliable method to determine the length of anteromedial screw by measuring outer interforaminal distance of S1 vertebra on anteroposterior radiograph of the sacrum.