Effects of misalignment on static torsional strength of anterior cervical plate systems

There is little understanding of cervical plate misalignment as a risk factor for plate failure at the plate-screw-bone interface.

To assess the torsional strength and mode of failure of cervical plates misaligned relative to the midsagittal vertical axis.

Plastic and foam model spine segments were tested using static compression and torsion to assess effects of misaligned and various lengths anterior cervical plate (ACPs).

Different length ACPs and cancellous fixed angle screws underwent axial torsional testing on a servo-hydraulic test frame at a rate of 0.5°/s. A construct consisted of one ACP, four screws, one ultrahigh-molecular weight polyethylene inferior block, and one polyurethane foam superior block. Group 1 had ACPs aligned in the midsagittal vertical axis, group 2 plates were positioned 20° offset from the midline, and group 3 had the ACP shifted 5 mm away and 20° offset from midline. Torques versus angle data were recorded. The failure criterion was the first sign of pullout determined visually and graphically.

Group 1 had a more direct screw pullout during failure. For the misaligned plates, failure was a combination of the screws elongating the holes and shear forces acting between the plate and block. The misaligned plates needed more torque to failure. The failure torque was 50% reduced for the longer versus the shorter plates in the neutral position. Graphically shown initial screw slippage inside the block preceded visual identification of slippage in some cases.

We observed different failure mechanisms for neutral versus misaligned plates. Clinically, misalignment may have the benefit of needing more torque to fail. Misalignment was a risk factor for failure of the screw-bone interface, especially in longer plate constructs. These comparisons of angulations may be a solid platform for expansion toward a more applicable in vivo model.