Biomechanical Evaluation of an Expandable Cage in Single-Segment Posterior Lumbar Interbody Fusion

Study Design. Controlled laboratory study. Objective. To evaluate the biomechanical characteristics of a new expandable interbody cage in single-segment posterior lumbar interbody fusion (PLIF) using cadaveric lumbar spines. Summary of Background Data. One of the popular methods of treating lumbar spine pathologies involves a posterior lumbar interbody fusion using bilateral interbody nonexpandable cages. However, this method can require extensive bony removal and nerve root retraction. Expandable interbody cages may decrease the risk associated with PLIFs. Methods. Biomechanical testing was performed on 5 fresh frozen L4/L5 mobile functional spinal units using a custom testing system that permits 6 df and a digital video digitizing system. The specimens were tested intact, postdiscectomy, after interbody cage placement, and after cage placement and pedicle screw fixation. Each specimen was tested from 0.5 to 8.0 N.m for extension, flexion, lateral bending, and rotation, and from 5 to 300 N for axial compression. The angular displacement, stiffness, disc height, and sagittal alignment were determined. Results. When the cage was supplemented with pedicle screw fixation, the mean angular displacement for rotation and lateral bending was significantly less than all other conditions (P < 0.05). The percentage range of motion (% ROM) showed a statistically significant decrease in lateral bending (P < 0.05) for cage alone vs. postdiscectomy. For the pedicle screw construct, rotation showed a significantly lower percentage ROM compared with all other constructs (P < 0.05), and lateral bending and extension-flexion showed a significantly lower percentage ROM compared with postdiscectomy (P < 0.05). For all motions, stiffness of the cage and pedicle screw construct was greater than intact, with only rotation showing a statistically significant increase (P < 0.05). Anterior disc height was restored to intact after cage alone (P < 0.05). Sagittal alignment did not show statistically significant differences. Conclusion. PLIF using expandable lumbar interbody cage requires pedicle screw fixation.