309 - Shear Loading Response of XLIF Supplemental Fixation in a Cadaveric Sp...

#309 Shear Loading Response of XLIF Supplemental Fixation in a Cadaveric Spondylolisthesis Model

Oral Posters: Lumbar

Presented by: G. Fogel

Author(s):

G.R. Fogel (1)
A.W.L. Turner (2)
Z.A. Dooley (2)
G.B. Cornwall (2)

(1) Spinepainbegone, San Antonio, TX, USA
(2) NuVasive, Inc., Biomechanics, San Diego, CA, USA

Abstract

Introduction: Spondylolisthesis is a common clinical indication for lumbar anterior interbody and posterolateral fusion. After fusion, treatment complications such as increased anterolisthesis, cage displacement, screw loosening, spinous process fracture, and decreased fusion rates have been reported. Several authors have created a model of spondylolisthesis by resection of appropriate structures in cadaveric spines. There are no in vitro studies of the stability provided by an interbody cage and supplemental fixation in a spondylolisthesis model. We have employed a spondylolisthesis model to test the stability of the XLIF cage and various supplemental fixation devices.

Methods: Seven human cadaver lumbar L4-L5 motion segments were evaluated using multidirectional non-destructive moments (±7.5 N·m) and anterior-posterior shear loading, with a custom 6 degrees-of-freedom (DOF) spine simulator. During shear testing, a static cranial-caudal compressive load (300 N) was applied via an actuator, and a posterior shear load was applied to the inferior vertebra using a motorized cable system. Intervertebral motions (ROM and A-P shear displacement) were measured opto-electronically. Each spine was evaluated under the following conditions:

(1) intact,

(2) destabilized with inferior facetectomy created with a 4 mm burr, lateral annulotomy and discectomy at L4-L5,

(3) standalone 18 mm A-P width XLIF cage,

(4) with lateral plate supplemental fixation,

(5) with lateral plate + contralateral pedicle screws (PS),

(6) with contralateral PS,

(7) with bilateral PS,

(8) with spinous process plate,

(9) with lateral plate + spinous process plate.

ROM data was normalized to intact and compared with results from a previous study without facet destabilization.

Results: ROM after destabilization increased significantly in all directions: by 1.9 times intact ROM in flexion-extension, 2.6 times in lateral bending, and 7.8 times in axial rotation. With the interbody cage alone, ROM decreased to 83% of intact in flexion-extension, 81% in lateral bending and 451% in axial rotation. The most rigid constructs in flexion-extension were those incorporating spinous process plate fixation. In lateral bending, the most rigid constructs were those with lateral plates or bilateral PS. Under axial rotation motion, bilateral PS fixation was the only condition to reduce motion to approximately intact. All conditions were less stable than corresponding conditions with intact posterior elements. Under shear loading, A-P displacement with the XLIF cage alone increased by approximately 5 times compared with the intact spine. Supplemental fixation reduced the amount of shear displacement, with bilateral PS fixation being the only fixation method to reduce displacement below the intact spine. A correlation between the A-P shear displacement and the ROM in each direction indicated the strongest relationship between shear displacement and axial rotation (R2 = 0.707) as the facet joints restrict both shear and axial rotation.

Conclusions: This is the first in vitro testing of lateral-approach interbody fusion cages with supplemental fixation in a human cadaveric spondylolisthesis model. The persistent instability found with all types of supplemental fixation including bilateral PS was significant and may explain clinical treatment complications in spondylolisthesis.