Cervical stability with lateral mass plating: unicortical versus bicortical screw purchase

Abstract

Purpose of study: The purpose of this study was to determine if there is a significant difference in stability between cervical spines instrumented with lateral mass plates affixed with unicortical versus bicortical Magerl screws.Methods used: Eleven human, cadaveric, cervical spines were harvested and radiographed, and all soft tissues except for supporting ligamentous structures were removed. Segments C3 through C5 were mounted in polymethylmethacrylate and instrumented segmentally with Axis lateral mass plates (Medtronic Sofamor-Danek) and screws using the Magerl technique. Fixation in the lateral masses was either bicortical (21 mm screws) or unicortical. Bicortical constructs were tested in all 11 spines. Unicortical constructs were subdivided into short screws (10 mm) in six specimens and long screws (up to but not through anterior cortex) in eight specimens. Nondestructive testing, using an MTS Bionix 858 machine, was carried out in flexion, extension, axial rotation and lateral bending at 0.45, 0.9, 1.35 and 1.8 Nm, respectively. Loads were applied sinusoidally, and data were recorded on the third cycle. The tests were then repeated after C3–C5 laminectomy. Analysis of variance was used to determine differences in construct stability. A p value of .05 was considered significant.of findings: There were no significant differences in stability between bicortical and long unicortical constructs in flexion, extension or axial torsion either with or without laminectomy. In lateral bending, bicortical and long unicortical constructs demonstrated a significant difference in stability but only after laminectomy (p=.0004). Short unicortical screw constructs demonstrated less stability without laminectomy in lateral bending (p<.0001), and with laminectomy in all bending modes: flexion/extension (p=.038), torsion (p=.005) and lateral bending (p=.0004). Bicortical screw constructs yielded equal or greater average stiffness than the unicortical constructs in all modes of testing.Relationship between findings and existing knowledge: Virtually all authors have recommended bicortical lateral mass screw purchase since the technique was first described by Roy-Camille. Bicortical fixation is associated with a small but finite risk of injury to the cervical neurovascular structures. Unicortical screw purchase should decrease or even eliminate these complications. Our results suggest that unicortical purchase using the Magerl technique allows for a biomechanically stable lateral mass plate construct, especially when the unicortical screw length is maximized. Clinical studies are necessary before recommending screw fixation.Overall significance of findings: Cervical lateral mass fixation with long, unicortical Magerl screws (up to but not through the anterior lateral mass cortex) may allow for a biomechanically stable construct while minimizing or even eliminating risk to cervical neurovascular structures.Disclosures: Device or drug: Axis lateral mass plates (Medtronic Sofamor-Danek). Status: Not approved.Conflict of interest: Anthony Muffoletto, AAS lateral mass instrumentation supplied by Medtronic Sofamor Danek.