Study Design. A biomechanical study was designed to assess relative constru
ct 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 variou
s anterior fixation devices in an unstable calf spine model,
Summary of Background Data, Modern types of anterior thoracolumbar instrume
ntations evolved to either rods or plates. Biomechanical properties and com
parative studies of these instrumentations are lacking.
Methods. Twenty fresh calf spines (L2-L5) were used for the biomechanical t
ests. L2 and L5 vertebrae were used to attach the loading and base frames,
respectively. Specimens underwent nondestructive biomechanical tests perfor
med using a three-dimensional motion measuring system. In each specimen, th
ree different cases were tested: intact spine, anterior fixation with an in
terbody graft after total discectomy and endplate excision of L3-L4 disc, a
nterior fixation only without the graft. Four anterior fixators, University
Anterior Plating System, the Kaneda device, the Z-plate, and Texas Scottis
h Rite Hospital system were used. Each device was tested on five specimens.
A polymethylmethacrylate block was inserted into the disc space to simulat
e the interbody grafting, and a fixation device was implanted with axial co
mpression. 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 signific
ant stabilizing effects in flexion, extension, and lateral bending. All dev
ices restored axial rotation stability to intact specimen, but only the Kan
eda 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 fl
exion, extension, and lateral bending. The Texas Scottish Rite Hospital sys
tem was found to reduce the flexion and lateral bending motions significant
ly, and Z-plate decreased lateral bending motions only. Stabilizing effects
of the interbody graft were significant in lateral bendings for all device
s. Additionally, the significant stabilizing role of the graft was noted in
flexion and extension in Z-plate only. The graft did not significantly red
uce 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, an
d University plate, restored the stability in all motions when an interbody
graft was inserted. The stability of fixation devices revealed that the Ka
neda device is the best, particularly in restoring the torsional stability.
The information on the relative stability provided by different instrument
ations should help the spine surgeon in choosing the appropriate instrument
ation for the particular circumstance.