364 - Prediction of Changes in Biomechanical Behavior of the Lumbar Spine Fo...

#364 Prediction of Changes in Biomechanical Behavior of the Lumbar Spine Following Alteration of Lordotic Angles Due to Cage Insertion

Oral Posters: Deformity

Presented by: K. Park

Author(s):

K.M. Park (1)
S.U. Kuh (2)
K.Y. Lee (3)
S.J. Lee (1)

(1) Inje University, Gimhae, Korea, Republic of
(2) Gangnam Severance Spine Hospital, Seoul, Korea, Republic of
(3) Sejong University, Seoul, Korea, Republic of

Abstract

Study Purpose: Interbody spinal fusion cages are known to be very effective in promoting spinal fusion and restoring sagittal alignment for the patients with degenerative disease and pain. It is reported that cage insertion and its positioning could significantly affect segmental lordosis. Despite the large number of clinical and biomechanical reports on its efficacies, studies on the changes in spinal curve due to insertion of the cage and resulting changes in biomechanical behavior are very limited. The purpose of this study was to investigate changes in biomechanical characteristics such as range of motion (ROM), load-sharing between the disc and the facet, and subsidence due to changes in lordotic curve after cage insertion.

Methods: At first, real-time changes in lordotic angle were assessed with x-ray of a patient while varying the cage placement from anterior to posterior disc spaces of L4-5 during the surgery (Department of Neurosurgery, Spine and Spinal Cord Institute, Gangnam Severance Spine Hospital, Seoul, Korea). Before the insertion, the lordotic curve was observed at 19.5º and was changed to 23º, 18.5º, 17º following the anterior, center, and posterior placements. Post-op FE models were constructed after taking account of each angle changes from a validated 3-D FE model of the intact lumbar spine (L2-S1). Three different cage angles were considered for this study 0°, 4° and 8°(PEEK Lumbar CageTM, Genoss, Korea). Simulation of interbody fusion with cage was done after laminectomy and discectomy. The bone-implant interface behavior was accomplished via “tie” contact condition to assume complete postoperative bony union. Pure flexion (10Nm) and extension (10Nm) moments with a compressive follower load of 400 N were applied at the superior endplate of the L2 vertebral body. The inferior endplate of S1 vertebral body was constrained in all directions. Hybrid protocol was used to assess the ROM at the index and adjacent levels. Load-sharing and facet load was calculated from summation of the nodal forces. Likelihood of subsidence of the cage was assessed by predicting the peak von mises stresses (PVMS) and stress distribution underneath the cage. ABAQUS/Standard V6.10 (Simulia Corp., Providence, RI, USA) was used for FE analysis.

Results: Changes in ROM due to cage positioning were more pronounced after taking account of actual changes in lordotic angle (up to 10%). Particularly, ROM at the index level and adjacent levels were more sensitive during flexion. On the other hand, without lordotic angle corrections the ROM at both index and adjacent levels were relatively insensitive to the cage positioning. However, as for load-sharing, facet load and likelihood of cage subsidence, the differen between before and after the changes in lordotic angle remain relatively unafeected (less than 3%). Simliar to positioning of cage results, the angle of cage with lordotic angle variation did not show significant difference in ROM, load-sharing, facet load at adjacent level and likelihood of subsidence.

Discussion and Conclusion: This study suggested that the biomechanical efficacies of the cage insertion on the analysis of ROM in particular can be better predicted after taking account of lordotic angle changes in a patient due to implantation.