Stiffness Matters: Part I-The Effects of Plate Stiffness on the Biomechanics of ACDF In Vitro.

Study Design: In vitro biomechanical testing of human cadaveric cervical and goat cervical motion segments.
Objective: The aim of this study was to measure the effects of plate stiffness on load-sharing, instantaneous axis of rotation (IAR), and posterior element loading after anterior cervical discectomy and fusion (ACDF).
Summary of Background Data: ACDF is intended to create an environment, which facilitates sufficient stability and biomechanical conditions to promote bone formation. The relationship between cervical plate stiffness, load-sharing, and the IAR is complex. The ideal cervical plate is sufficiently stiff to limit interbody motion but is compliant enough to facilitate load-sharing rather than stress-shielding.
Methods: Anterior cervical plates of distinct bending stiffnesses were applied to human and goat cervical motion segments following ACDF. A validated custom force-sensing interbody implant was placed in the disc space to measure load-sharing in the spine. Interbody loads, posterior element strain, and the IAR were measured during flexion/extension for each plate.
Results: Load-sharing in the interbody space, posterior element strain, and the location of the IAR were all significantly affected by plate stiffness. More compliant plates resulted in more load sharing, less posterior element strain, and a more dorsally located IAR relative to stiffer plates.
Conclusion: A more compliant plate fosters more consistent load-sharing through the entire range of flexion/extension, which may promote faster bone formation and better fusion. A more compliant plate causes less posterior element strain, which may reduce facet joint loads and in turn reduce facet joint arthrosis. An ideal plate may be one that is stiff enough to minimize interbody motion and yet compliant enough to allow consistent load-sharing and minimal increase in posterior element strain.
Level of Evidence: N/A.