Stability of ventral,dorsal and combined spondylodesis in vertebral body prosthesis implantation

The purpose of this study was to evaluate the biomechanical characteristics of short-segment anterior, posterior, and combined instrumentations in lumbar spine vertebral body replacement surgery. Eight fresh frozen human cadaveric thoracolumbar spine specimens (T12-L4) were prepared for biomechanical testing. Pure moments (2.5, 5, and 7.5 Nm) of flexion-extension, left-right axial torsion, and left-right lateral bending were applied to the top vertebra in a flexibility machine and the motions of L1 vertebra with respect to L3 were recorded with an optoelectronic motion measurement system after preconditioning. One anterior, two posterior pedicle screw systems, and two combined instrumentations were tested. Load-displacement curves were recorded and neutral zone (NZ) and range of motion (ROM) were determined. The anterior instrumentation, after vertebral body replacement, showed greater motion than the intact spine, especially in axial torsion. Posterior instrumentation provided greater rigidity than the anterior instrumentation, especially in flexion-extension. The combined instrumentation provided superior rigidity in all directions compared to all other instrumentations.