Biomechanical Comparison of Transforaminal Interbody Fusion Implants: Static vs Expandable Cages and Unilateral...

To assess differences in biomechanical stability provided by posterior or transforaminal lumbar interbody instrumentation with expandable relative to static implants and unilateral or bilateral implants.

Nine human cadaveric lumbosacral spine specimens were subjected to motion testing before instrumentation and after single-level posterior decompression and interbody instrumentation at L3-4 with randomized order of single and bilateral static and expandable interbody implants. Measurements of disk height were also performed on the intact spine and with different implant paradigms. Changes in motion at each lumbar segment, comparison of implant paradigms, and changes in disk height at the instrumented level were statistically compared using linear mixed-effects models.

Instrumentation resulted in a significant reduction in flexion, extension, axial rotation, and lateral bending at L3-4 for all interbody implant paradigms. Bilateral expandable cages resulted in a significantly greater reduction in lateral bending relative to single static implants and neared significance compared with bilateral static and unilateral expandable implants. Disk height increased with all implants, and bilateral expandable cages significantly increased height relative to unilateral and bilateral static cages.

Bilateral expandable implants confer greater stability with regard to lateral bending forces compared with unilateral expandable, unilateral static, and bilateral static cages with posterior interbody instrumentation. In addition, bilateral expandable implants confer greater ability to increase disk space height compared with static cages. Therefore, the added cost of bilateral expandable cages may be warranted in the setting of significant coronal imbalance or very collapsed disk spaces.