The purpose of this biomechanical study was designed to assess construct stabilities of modern anterior thoracolumbar instrumentation in a calf spine model with an anterior and middle column defect. Twenty tresh calf spines(L2-L5) were used for the biomechanical tests with an anterior and middle columm defect. L2 and L5 vertebrae were used to attach the loading and base frames. Specimens underwent nondestructive biomechanical tests performed using a 3-D motion measuring system. In each specimen, three different cases were tested: 1) intact spine 2) anterior fixation with an interbody graft following total discectomy and endplate excision of L3-L4 disc 3) anterior fixation only without the graft. Four anterior fixators, University Anterior Plating System(UNIV), the Kaneda device, the Z-plate, and Texas Scottish Rite Hospital system(TSRH) were used. A PMMA hlock was inserted into the disc space to simulate the interbody grafting, and a fixation device was implanted with axial compression. Modern anterior instrumentation for the thoracolumbar spine such as the Kaneda device, TSRH device, Z-plate, and UNIV plate restored the stability in all motions, when an interbody graft has been 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 hy different instrumentations should help the spine surgeon in choosing the appropriate instrumentation for the operation.