INTRODUCTION: Lateral mass screw fixation is used in numerous pathologic conditions of the cervical spine. We tested the hypothesis that differences in thread pitch would affect fixation strength of lateral mass screws placed using bicortical fixation. Specifically we hypothesized that the failure characteristics of fine‐threaded screws (smaller pitch, reduced thread area) would be reduced compared to a cancellous threaded screw (larger pitch, increased thread area).
METHODS: Two screw designs were compared. The fine thread screw (FTS) (Oasys (Stryker)) and the cancellous thread screw (CTS) (Axon (Synthes)) were each designed for lateral mass fixation. The FTS has a thread pitch of 1.1 mm, while the CTS has a pitch of 1.75 mm. Five fresh human cervical spines were used. The subaxial cervical levels C3, C4, C5, and C6 were instrumented bilaterally using standard technique with one each of the screws (FTS or CTS). Each screw was subjected to axial pull‐out testing at a rate of 1 mm/min. Load‐displacement, peak load to failure, and stiffness were recorded/calculated and analyzed (p>0.05).
RESULTS: Forty lateral mass screw placements were tested (20 FTS and 20 CTS). The FTS screws and the CTS screws demonstrated comparable peak loads at pullout (mean: 663N vs. 634N, respectively). Similarly the FTS screws and the CTS screws had comparable pullout stiffness (1563N/mm vs. 1268N/mm, respectively). The difference between the screws in pullout stiffness and peak loads was not significant. No association was found between peak load and either cervical level or side of placement.
DISCUSSION: The difference in thread pitch typical of fine‐thread cortical bone screws and coarse‐thread cancellous screws did not affect the pull out strength and failure characteristics of lateral mass fixation when bicortical fixation technique was employed. Placement technique and patient characteristics are probably more influential when it comes to ultimate fixation strength.