Would Resting a Lateral Interbody Cage across the Ring Apophysis in the Lumbar Spine Mitigate Endplate Violation?
Authors: Briski David C. , Waddell Bradford S. , Cook Brandon W. , Zavatsky Joseph M. 
Published: May 2015

Introduction Several operative approaches are commonly utilized to access the anterior column for lumbar interbody fusion. These include anterior (ALIF), posterior (PLIF and TLIF), and lateral (transpsoas) techniques. Lateral lumbar interbody fusion has an increased popularity because of its biomechanical and complication-related advantages over the anterior and posterior interbody surgical approaches. Historically, stand-alone cages were associated with instability and supplemental fixation is advocated. Cage subsidence can also affect segmental stability. When performing the lateral technique, larger cages that span the ring apophysis maximize contact of the cage with the periphery of the endplate and could mitigate potential endplate issues secondary to either violation during discectomy or poor bone quality. Materials and Methods Eight fresh-frozen human lumbosacral spines (L1–S1) were used for this study. The specimens were placed in the MTS machine, such that the superior endplate was horizontal. Each lumbar spine had five candidate endplates (L1–5) for a total of 40 specimens. Specimens were randomized into four groups, with 10 specimens per group. Treatment 1: Intact endplate with short cage not extending to the ring apophysis. Treatment 2: Intact endplate with long cage spanning the ring apophysis. Treatment 3: Endplate decortication with short cage. Treatment 4: Endplate decortication with long cage spanning the ring apophysis. Load displacement data were collected at 5 Hz until failure. Failure was defined as subsidence of the cage into the vertebral body > 5 mm or fracture of the vertebral endplate that resulted in axial displacement of the actuator > 5 mm. Load displacement curves were plotted to calculate failure loads and displacement (subsidence). Failure loads were normalized with respect to bone mineral density of the specimens. Results Longer cages spanning the ring apophysis provided more strength in compression and less subsidence relative to shorter cages, regardless of endplate integrity. Longer cages, spanning the ring apophysis, resting on intact endplates (Group 2) had a significant ( p < 0.05) increase in strength and less subsidence when compared with the smaller cage group resting on intact endplates (Group 1) ( p = 0.003). Thus, with intact endplates, the longer cage configuration (Group 2) demonstrated significantly less subsidence than the shorter cage group (Group 1). With decorticated endplates, the longer cage group that spanned the ring apophysis demonstrated an increase in strength and less subsidence than the shorter cage group, although it was not statistically significant ( p = 0.052). Longer cages spanning the ring apophysis of intact endplates showed a significant ( p < 0.05) increase in strength in compression and resistance to subsidence when compared with similar length cages resting on decorticated endplates ( p = 0.028). Conclusion Spanning the ring apophysis increases the load to failure by 40% with intact endplates and by approximately 30% with decorticated endplates. Larger cages that span the vertebral body ring apophysis could improve the compressive strength and decrease subsidence at the operative level secondary to endplate violation during discectomy or poor bone quality.