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

Biomechanics

Spine 2012;37:E79–E85

Title:

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

Authors

Nitin N. Bhatia

Kenneth H. Lee

Christopher N. H. Bui

Mario Luna

George M. Wahba

Thay Q. Lee

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.

1. PLIF using bilateral interbody nonexpandable cages is one of the popular methods.

2. However, it can require extensive bony removal and nerve root retraction.

3. Expandable interbody cages may decrease the risk associated with PLIFs.

Methods.

1. Biomechanical test on 5 fresh frozen L4/L5 spinal units using a custom testing system that permits 6 df and a digital video digitizing system.

2. The specimens were tested intact, postdiscectomy,

à After interbody cage placement

à After cage placement and pedicle screw fixation

3. Extension/flexion/lateral bending/rotation (0.5~8.0 N · m), Axial compression (5~300N)

4. The angular displacement, stiffness, disc height, and sagittal alignment were determined.

Results.

1. Cage with pedicle screw fixation

àMean angular displacement for rotation and lateral bending was significantly less than all other conditions (P<0.05).

2. Statistically significant decreased ROM in lateral bending (P<0.05) for cage alone vs. postdiscectomy.

3. For the pedicle screw construct,

à Rotation showed a significantly lower percentage ROM compared with all other constructs (P<0.05)

à Lateral bending and extension-flexion showed a significantly lower percentage ROM compared with postdiscectomy (P<0.05).

4. 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).

5. Anterior disc height was restored to intact after cage alone (P<0.05).

6. No statistically significant differences in sagittal alignment

Conclusion.

PLIF using expandable lumbar interbody cage requires pedicle screw fixation.

Recommendation

Additional posterior fixation for further stabilization of construct, especially in rotation

Key Points

1. Cage alone decreases mean angular displacement and ROM (%).

2. Only after posterior pedicle fixation

   àStatistically significant decrease in mean angular displacement and ROM (%)

   àIncrease in stiffness to levels above intact

3. Restored disc height, even exceeding the height of the intact disc

4. No changes in sagittal alignment

"This comes from Spine (c) 2012 Lippincott Williams & Wilkins, Inc."