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Finite element biomechanics of cervical spine interbody fusion

The biomechanical responses of cervical discectomy coupled with fusion using five types of interbody fusion materials were determined. Titanium core, titanium cage, tricortical iliac crest, tantalum core, and tantalum cage fusion materials were used. Smith-Robinson and Bailey-Badgley procedures were analyzed. A validated three-dimensional anatomically accurate three-segment C4-C5-C6 finite element model of the spine was developed using close-up computed tomography images and cryomicrotome anatomic sections. It was exercised in compression, flexion, extension, lateral bending, and axial torsion modes for the intact case and for the two surgeries with five implant materials. External response in terms of the stiffness and angular rotation, and the internal response in terms of the disc and the vertebral stresses were determined. The Smith-Robinson technique resulted in the highest increase in external response under all modes of loading for all implant materials. The Bailey-Badgley technique produced a higher increase in the disc and the vertebral body stresses than the Smith-Robinson technique. Our study assists in a better understanding of the biomechanics by delineating the changes in the extrinsic and intrinsic characteristics of the cervical spine components. Because of the inclusion of three-levels (C4-C5-C6) in the finite element model, it is possible to determine the internal mechanics of the various components at the altered and the unaltered adjacent intervertebral components secondary to these surgical procedures.