Abstract
Background Context
Stand-alone nonbiologic interbody fusion devices for the lumbar spine have been used
for interbody fusion since the early 1990s. However, most devices lack the stability
found in clinically successful circumferential fusion constructs. Stability results
from cage geometry and device/vertebral endplate interface integrity. To date, there
has not been a published comparative biomechanical study specifically evaluating the
effects of endplate engagement of interbody devices.
Purpose
Lumbar motion segments implanted with three different interbody devices were tested
biomechanically to compare the effects of endplate engagement on motion segment rigidity.
The degree of additional effect of supplemental posterior and anterior fixation was
also investigated.
Study Design/Setting
A cadaveric study of interbody fusion devices with varying degrees of endplate interdigitation.
Outcome Measures
Implanted motion segment range of motion (ROM), neutral zone (NZ), stiffness, and
disc height.
Methods
Eighteen human L23 and L45 motion segments were distributed into three interbody groups
(n=6 each) receiving a polymeric (polyetheretherketone) interbody spacer with small
ridges; a modular interbody device with endplate spikes (InFix, Abbott Spine, Austin,
TX, USA); or dual tapered threaded interbody cages (LT [Lordotic tapered] cage; Medtronic,
Memphis, TN, USA). Specimens were tested intact using a 7.5-Nm flexion-extension,
lateral bending, and axial torsion flexibility protocol. Testing was repeated after
implantation of the interbody device, anterior plate fixation, and posterior interpedicular
fixation. Radiographic measurements determined changes in disc height and intervertebral
lordosis. ROM and NZ were calculated and compared using analysis of variance.
Results
The interbody cages with endplate spikes or threads provided a statistically greater
increase in disc height versus the polymer spacer (p=.01). Relative to intact, all
stand-alone devices significantly reduced ROM in lateral bending by a mean 37% to
61% (p≤.001). The cages with endplate spikes or threads reduced ROM by ∼50% and NZ
by ∼60% in flexion-extension (p≤.02). Only the cage with endplate spikes provided
a statistically significant reduction in axial torsion ROM compared with the intact
state (50% decrease, p<.001). Posterior fixation provided a significant reduction
in ROM in all directions versus the interbody device alone (p<.001). Anterior plating
decreased ROM over interbody device alone in flexion-extension and torsion but did
not have additional effect on lateral bending ROM.
Conclusion
The cages with endplate spikes or threads provide substantial motion segment rigidity
compared with intact in bending modes. Only the cages with endplate spikes were more
rigid than intact in torsion. All devices experienced increased rigidity with anterior
plating and even greater rigidity with posterior fixation. It appears that the endplate
engagement with spikes may be beneficial in limiting torsion, which is generally difficult
with other “stand-alone” devices tested in the current and prior reports.
Keywords
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Article info
Publication history
Published online: May 21, 2009
Accepted:
March 30,
2009
Received in revised form:
March 6,
2009
Received:
September 12,
2008
Footnotes
FDA device/drug status: approved for this indication (Tapered threaded cage; PEEK interbody spacer; Modular spacer).
Author disclosures: ALF, BPB (research support medtronic, Abbott Spine); JES (consultant and speaker, Abbott Spine). JLC (royalties, Medtronic; consultant, Medtronic and Abbott Spine; speaker, Abbott Spine).
Identification
Copyright
© 2009 Elsevier Inc. Published by Elsevier Inc. All rights reserved.
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