Static and Dynamic Anterior Cervical Plates: A Retrieval Analysis of Damage and Clinical Data

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INTRODUCTION: Metallic anterior cervical plates (ACPs) are frequently employed in the setting of cervical fusion to prevent graft extrusion and provide mechanical support to the anterior column. Due to the range of motion of the cervical spine, as well as the probability of interbody graft subsidence, ACPs experience a high degree of biomechanical loading, particularly at the screw-plate interface. The purpose of this retrieval study was to characterize the material-level damage of ACPs and analyze patient and surgical factors to elucidate trends in clinical implant failure.

METHODS: Fourteen two-level ACPs of two different designs, seven static and seven dynamic, were retrieved from 14 patients at our institution from 2005 to 2010, under an IRB-approved protocol. Retrieved components were examined microscopically for damage modes, including burnishing, scratching, and pitting. Additionally, each plate was inspected with scanning electron microscopy (SEM)/energy dispersive x-ray spectroscopy (EDX) in three regions: (1) potential corrosion (e.g. crevices, near screw-plate interface), (2) scratched, and (3) bulk/undamaged, as a control. Chart reviews were performed to collect patient and surgical data. Correlation coefficients of 0.20-0.40, 0.40-0.60, 0.60- 0.80, and 0.80-1.00 were considered weak, moderate, strong, and very strong, respectively.

RESULTS: The patient population included five males and nine females, with average ages at implant and revision of 51 (range 31-76) and 56 (range, 41-78), respectively, and average body mass index of 28 (range, 21-39). Terms of implantation in the static and dynamic groups were 89 (range, 34-139) and 42 (range, 23-72) months, respectively. Indications for revision included disc herniation (n=7), cervical spondylotic radiculopathy (n=6), adjacent level degenerative changes (n=3), pseudoarthrosis (n=3), and cervical stenosis (n=1). At revision, there were no reported infections, no observed wear debris, and one case of implant loosening (static group; screw back out). All dynamic plates showed evidence of pitting and scratching damage, while these damage modes were demonstrated in 86% and 57% of static plates, respectively. Burnishing damage was observed in 29% of both the static and dynamic plates. SEM/EDX showed no significant differences between the static and dynamic systems, in terms of average weight percent of the elemental constituents. Additionally, there were no significant differences when the compositions of the potential corrosion and scratched regions were compared with the bulk/undamaged regions, within each plate group; however, iron was located in the scratched regions of the plates, indicating possible iatrogenic damage and material transfer from surgical instruments.

DISCUSSION AND CONCLUSION: Although no significant compositional differences were shown between various locations on the plates via EDX, damage on dynamic plates covered a larger surface area than damage observed on the static plates, perhaps due to the variability in plate thickness near the screw holes, allowing for stress concentrations and the collection of biologic fluid. Future work is ongoing to analyze additional plate systems as well as characterize damage and corrosion within screw holes. Data will also be correlated to radiographic analysis of plate