ABSTRACT

To determine the maximum length of the pedicular screw via measuring the interpedicular distance and the distance between the interfacet joint line and the anterior cortex of the first lumbar (L1) vertebra.

This is a retrospective case series study (n=75, 25 women, 50 men). We evaluated the anteroposterior (AP) and lateral X-rays in order to measure the interpedicular distance in AP views and facet joint-anterior cortex distance in the lateral views of the L1 vertebra. By measuring these distances, we observed a triangular equation and found a coefficient to determine the maximum pedicular screw length.

The sum of the square of the sides of a right angled triangle equals the square of the hypothenus. Half of the interpedicular distance and the distance between the interfacet joint line and anterior cortex make the sides of a right angled triangle. The maximum pedicular trajectory is the hypothenus of this triangle. The relation with the pedicle screw trajectory and the half of the interpedicular distance brings out a coefficient which is 3.448.

When half of the interpedicular distance is measured in an AP view, this distance times 3.448 equals the maximum pedicular screw length. This is an easy way of determining the screw length preoperatively.