Comparison between short segment pedicle and lateral mass screw construction strength for resisting flexion and extension motion in sub axial cervical spine

Background: There are several kinds of posterior cervical instrumentation in vertebrae such as wiring, lateral mass, and pedicle screw. Lateral mass still becomes the main choice for instrumentation, although the pedicle screw has better pull-out strength compared to lateral mass.

Objective: The aim of this study was to compare lateral mass and pedicle screw strength in short segment construction for flexion/extension motion in the sub axial cervical spine.

Method: Six adult sub axial cervical spines were harvested from human cadavers. Each cadaver divided into Cervical 3-4 and Cervical 5-6 then 3,5 mm screw was placed with Roy-Camille technique for lateral mass and Abumi technique for pedicle. The cyclic load of flexion and extension motion was applied for each construction to evaluate the construction strength in a fatigue test. Data was collected and being tested by using a t-test.

Result: The mean cyclic load 172 cycles in lateral mas screw construction and 367 cycles in pedicle screw construction were applied until construction became failed. Pedicel screw has higher strength because pedicle structure has higher bone mineral density and longer bone-screw interface than lateral mass. Pedicle screw has longer screw than lateral mass, the mean length of screw-in pedicle was 28 mm and 14 mm for lateral mass. The longer screw used in the pedicle has the advantage for construction strength because the pedicle screw has anterior to posterior column fixation.

Conclusion: Short segment pedicle screw construction has higher fatigue strength than lateral mass screw construction in flexion/extension motion.