Dynamic Cervical Plates: Do They Load Share at the Expense of Stability?
Andrew T Dailey, MD
Sohrab Gollogly, MD
Alexander Mohr, BS
Kent N. Bachus, PbD
Darrel Brodke, MD (Salt Lake City, UT)
Corpectomy with anterior strut grafting is a common surgical technique for degenerative and traumatic conditions of the cervical spine. Anterior cervical plates have been designed to augment initial stability and improve the fusion rate of corpectomy reconstructions. More recently, dynamic cervical plates have been introduced that allow for axial settling in order to increase load sharing with the graft and thereby improve fusion rates. The load sharing properties and stiffness in flexion/extension, lateral bending, and axial torsion were compared for the Depuy–Acromed DOC, Aesculap ABC, Synthes CSLP, and Sofamor–Danek Orion plates.
A simulated corpectomy reconstuction model using UHMWPE bodies was inserted into a custom 4–axis spine simulator designed to apply compressive axial loading and pure flexion/extension, lateral bending, and axial torsion moments. Six specimens with a 30 mm graft were tested with each device. Load sharing was calculated by measuring load born by each graft as a percentage of the applied axial load of 15 to 12ON using the Tekscan system. The stiffness of each construct was then calculated in flexion/extension, lateral bending and axial torsion by applying +–/– 2.5Nm moments in each plane and measuring displacement with the OptoTrak system. These measurements were then repeated using a 27 mm graft to simulate 1O+ACU– graft subsidence.
Analysis of the load sharing data showed that the stiffer plates (CSLP and Orion) shared a greater portion of the axial load than the dynamic plates (DOC and ABC). When the graft was shortened by 10+ACU– only the two dynamic plates shared a significant proportion of the applied axial load over the entire load range. In flexion/extension and lateral bending, the CSLP, Orion, and ABC plates were not significantly different in stiffness while the DOC plate was significantly less stiff. In axial torsion all plates were significantly different with Orion being the stiffest, followed by ABC, CSLP, and DOC plates. These findings provide more data for the debate regarding the use of instrumentation in anterior cervical reconstruction and fusion.
