The BAK spinal fusion system has been applied to laparoscopic anterior lumbar interbody fusion. The system, consisting of a pair of cylindrical implants with threads, placed symmetrically about the sagittal plane, functions by tensioning the annulus fibrosis. Cylindrical plugs of increasing size are inserted prior to the implant placement. As the procedure may affect spinal posture and disc height, we measured changes due to incremental plug insertion using human cadaveric spine specimens (L5–S1, n = 4). Multi-directional flexibility of the construct was also measured as a function of plug size. The disc height change was found to increase initially and then to level off at 13-mm diameter plugs. In the sagittal plane, the intervertebral posture first shifted towards kyphotic then came back to the initial lordotic posture with plugs of bigger size. However, changes in disc height and spine posture were not statistically significant. Comparing the neutral zone (NZ) flexibility after inserting the plugs to the intact values, neither the flexion/extension nor the axial rotation NZ showed any singificant change. In lateral bending, the NZ decreased after the insertion of 13-mm plugs (p < 0.05). Insertion of plugs of increasing size from 9 mm to 12 mm decreased the range of motion (ROM) in all directions (p < 0.05). Insertion of 13-mm and 14-mm plugs decreased the flexion/ extension and lateral bending ROM, but not the axial rotation ROM, probably indicating some injury to the annulus fibers.