Biomechanical evaluation of anterior thoracolumbar spinal instrumentation.

STUDY
DESIGN: A biomechanical study was designed to assess relative construct stabilities of modern anterior thoracolumbar instrumentations in a calf spine model with an anterior and middle column defect.
OBJECTIVES: The purpose is to compare the biomechanical stability of various anterior fixation devices in an unstable calf spine model. SUMMARY OF BACKGROUND DATA: Modern types of anterior thoracolumbar instrumentations evolved to either rods or plates. Biomechanical properties and comparative studies of these instrumentations are lacking.
METHODS: Twenty fresh calf spines (L2-L5) were used for the biomechanical tests. L2 and L5 vertebrae were used to attach the loading and base frames, respectively. Specimens underwent nondestructive biomechanical tests performed using a three-dimensional motion measuring system. In each specimen, three different cases were tested: intact spine, anterior fixation with an interbody graft after total discectomy and endplate excision of L3-L4 disc, anterior fixation only without the graft. Four anterior fixators, University Anterior Plating System, the Kaneda device, the Z-plate, and Texas Scottish Rite Hospital system were used. Each device was tested on five specimens. A polymethylmethacrylate block was inserted into the disc space to simulate the interbody grafting, and a fixation device was implanted with axial compression. Rotational angles of the L3-L4 segment stabilized by a fixation device and graft were normalized by the corresponding angles of the intact specimen to study the overall stabilizing effects.
RESULTS: With the interbody graft and fixation devices, all showed significant stabilizing effects in flexion, extension, and lateral bending. All devices restored axial rotation stability to intact specimen, but only the Kaneda device restored the torsional stability beyond the intact specimen. No statistical differences in stabilizing effects in axial rotation were found between any of the tested devices. When the graft was removed, the Kaneda device significantly decreased the motions in all directions compared with the intact motion, whereas the University plate decreased the motions in flexion, extension, and lateral bending. The Texas Scottish Rite Hospital system was found to reduce the flexion and lateral bending motions significantly, and Z-plate decreased lateral bending motions only. Stabilizing effects of the interbody graft were significant in lateral bendings for all devices. Additionally, the significant stabilizing role of the graft was noted in flexion and extension in Z-plate only. The graft did not significantly reduce the axial rotation motion in any instrumentations.
CONCLUSIONS: Modern anterior instrumentations for the thoracolumbar spine, such as the Kaneda device, Texas Scottish Rite Hospital system, Z-plate, and University plate, restored the stability in all motions when an interbody graft was inserted. The stability of fixation devices revealed that the Kaneda device is the best, particularly in restoring the torsional stability. The information on the relative stability provided by different instrumentations should help the spine surgeon in choosing the appropriate instrumentation for the particular circumstance.