232 - Would Resting a Lateral Interbody Cage Across the Ring Apophysis in th...

#232 Would Resting a Lateral Interbody Cage Across the Ring Apophysis in the Lumbar Spine Mitigate Endplate Violation?

Oral Posters: Lumbar

Presented by: J. Zavatsky

Author(s):

J.M. Zavatsky (1)
H. Serhan (2)
A.K. Agarwal (3)
M.K. Kodigudla (3)
A. Agarwal (3)
V.P. Goel (3)

(1) Ochsner Medical Center, Department of Orthopaedic Surgery, New Orleans, LA, USA
(2) DePuy Synthes Spine, Raynham, MA, USA
(3) University of Toledo, Departments of Bioengineering and Orthopaedic Surgery, Toledo, OH, USA

Abstract

Introduction: Supplemental fixation including lateral plating, facet or pedicle screw fixation can increase the segmental stability of interbody fusion. Cage subsidence can affect stability. Poor bone quality or endplate violation during discectomy can result in cage subsidence and affect stability. The ring apophysis is the strongest portion of the vertebral body endplate located at its periphery. When performing the lateral technique, larger cages that span the ring apophysis maximizes contact with the periphery of the endplate and could mitigate endplate violation. In this study, we evaluated the value of resting the lumbar cage across the ring apophysis to resist compressive forces with and without decortication of the endplates.

Materials and Methods: Forty specimens were obtained from 8 fresh-frozen human lumbosacral (L1-S1) spines. Bone mineral density (DEXA) scans and x-rays were obtained on all specimens. Randomization was used to assign each specimen to one of the following 4 treatment groups:

Group 1 - Intact endplate with short cage not extending to the ring apophysis (Figure 1)

Group 2 - Intact endplate with long cage spanning the ring apophysis

Group 3 - Endplate decortication with short cage

Group 4 - Endplate decortication with long cage

The minimum amount of endplate was removed to expose the trabeculae using a burr. Vertebrae were secured and tested by applying a compressive load in an MTS load cell. Load displacement data was collected at 5Hz until failure. Failure was defined as subsidence of the cage into the vertebral body > 5mm or fracture of the endplate that resulted in axial displacement of the actuator > 5mm. Load displacement curves were plotted to calculate failure loads and displacement. Failure loads were normalized with respect to bone mineral density of the specimens.

Results: Longer cages, spanning the ring apophysis, resting on intact endplates (Group 2) had a significant increase in strength and less subsidence when compared to the smaller cage group resting on intact endplates [(Group 1), (P=0.003)]. Longer cages spanning the ring apophysis of intact endplates showed a significant increase in strength in compression and resistance to subsidence when compared to similar length cages resting on decorticated endplates (P=0.028).

Conclusion: Spanning a lateral lumbar interbody cage across the ring apophysis increases the load to failure by 40% with intact endplates and by approximately 30% with decorticated endplates. Although the amount of stability required for spinal fusion is unknown, supplemental instrumentation has been shown to enhance the stiffness of the fused segment. Larger cages that span the vertebral body ring apophysis could improve the compressive strength and decrease subsidence at the operative level. Spanning lumbar cages across the ring apophysis may mitigate potential causes of subsidence including endplate violation during discectomy or poor bone quality. Utilizing this technique could provide further stability by decreasing subsidence and ultimately improve fusion rates.

Figure 1