INTRODUCTION: Although there are several biomechanical studies of strength and stability of anterior cervical plating, there has been no investigation into the effects of misalignment of cervical plates as a risk factor for failure of the plate‐screw‐bone interface. This study assessed the torsional strength and mode of failure of cervical plates misaligned relative to the mid‐sagittal vertical axis.
METHODS: Nine 27.5 mm and nine 80 mm Atlantis Vision™ Anterior Cervical Plates (ACP) and 72 Atlantis™ Cancellous Fixed Angle screws (Medtronic Sofamor Danek, Memphis, TN) were subjected to axial torsional testing. A construct consisted of one ACP, four screws, one UHMWPE inferior block, and one Grade 30 polyurethane foam superior block (General Plastics, Tacoma, WA). (Figure 1) Polyurethane was selected for the superior block to represent vertebral cancellous bone. Three groups (n=3 specimens/group) for each plate size were evaluated. (Figure 2) Group 1 (control) included ACPs correctly aligned in the mid‐sagittal vertical axis. Group 2 specimens were positioned 20° offset from the midline. Group 3 was constructed with the ACP shifted 5 mm away from the midline and 20° offset from the midline. Testing was performed on a servo‐hydraulic test frame (MTS, Minneapolis, MN) at a rate of 0.5°/s. Torque versus angle data were recorded. The failure criterion was the first sign of pullout as determined both visually and by interpretation of graphical data.
RESULTS: The failure mechanism observed in the control groups was different from those of the misaligned implants. In the control groups, a more direct screw pullout from the foam block occurred. The failure mode for the misaligned plates (Groups 2 and 3) was a combination of the screws elongating the holes (ploughing through the block) and shear forces acting between the plate and block. The failure torque was 50% reduced for the 80 mm versus the 27.5 mm plate in the neutral position. In some cases, visual initial failure was different than the graphical interpretation. Initial screw slippage inside the block as shown graphically may have preceded the visual identification of slippage
CONCLUSIONS: We observed different failure mechanisms for neutral versus misaligned plates. Neutral aligned plates failed from direct screw pullout while misaligned plates failed from a combination of screws elongating the holes and shear forces. There was an approximately 50% reduction in torsional strength of 80 mm versus 27.5 mm ACPs in neutral position. Misalignment was shown to be a risk factor for failure of the screw bone interface especially in longer constructs. These findings may explain clinically observed failures of long anterior cervical plate constructs and those in patients with low bone mineral density.
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