Date of Award

12-2008

Document Type

Thesis

Degree Name

Master of Science (MS)

Program

Biomedical Engineering and Imaging

Research Advisor

Denis J DiAngelo, Ph.D.

Committee

Brian Kelly, Ph.D. Gladius Lewis, Ph.D.

Keywords

Anterior plate, spinal fixation, compliant force sensing graft, cervical plate, biomechanics, spinal reconstruction, in-vitro testing

Abstract

INTRODUCTION: Pseudoarthrosis is a relatively rare complication following anterior cervical arthrodesis, and is felt to be related to stress shielding. To address this, dynamic anterior cervical plates have emerged to maintain load sharing as an arthodesis matures. Dynamic plates can translate bidirectionally (allow translation in compressive and tensile loads), or unidirectionally (allow translation in compression, but maintain its position under tensile loads). The objective of this study was to compare the graft load mechanics between three plated conditions during settling using compliant interbody load cells: A static (fixed, non-moving) plate, a unidirectional multi-level translational plate design, and a bidirectional two-level translational plate design.

METHODS: Six fresh human cadaveric cervical spines (C2-T1) were procured and mounted in a programmable testing apparatus and tested in flexion-extension, left-right lateral bending, and left-right axial rotation under displacement control to a load limit of 3.0 Nm or 30 degrees of motion. Four different spine conditions were evaluated: the harvested (H) condition and three types of instrumented conditions containing two-level discectomy and graft at C4-C5 and C5-C6: a unidirectional translational plate, a bidirectional translational plate, and a rigid static plate. Compliant force sensing grafts were placed in the discectomized regions prior to plate application that permitted up to 2mm of deformation during compressive loading. The ATLANTIS® Translational cervical plating system (Medtronic Spinal and Biologics, Memphis, TN) was used for the plated spine conditions with fixed-angle screws. Measurements included vertebral motion, applied load and moment, and graft loads. Normalized flexibility data, motion data, and graft load data were compared using a one-way Repeated Measures ANOVA (p<0.05) and SNK tests.

RESULTS: A significant reduction in flexion+extension motion occurred at the operated levels of all instrumented spines compared to the harvested spine. The normalized flexion+extension motion for the unidirectional plate was significantly less than the bidirectional plate. During flexion, the unidirectional plate shortened an average of 2 mm (1 mm per level). Once shortened, a significant increase in the baseline graft preload occurred.

CONCLUSION: In this study, a compliant interbody load cell was developed and used to determine the graft load properties of three different anterior cervical plate designs: static, unidirectional and bidirectional translational. Both translational plates demonstrated better load-sharing properties than the static plate. The results showed that the unidirectional plate had less motion in flexion/extension, while maintaining a more continuous graft loading than the bidirectional plate. Also, the ratchet mechanism of unidirectional translational plate was validated in the study, where the plate was able to translate under compressive load and maintain its position under tension when ratcheted. The ability to limit motion and maintain compression with a unidirectional dynamic plate may have improved rates of fusion; however, this will need to be evaluated further in an in vivo and/or clinical model.

DOI

10.21007/etd.cghs.2008.0261

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