Abstract
Background context
Current spine arthroplasty devices require disruption of the annulus fibrosus for
implantation. Preliminary studies of a unique annulus-sparing intervertebral prosthetic disc (IPD) found that preservation of the annulus resulted
in load sharing of the annulus with the prosthesis.
Purpose
Determine flexibility of the IPD versus fusion constructs in normal and degenerated
human spines.
Study design/setting
Biomechanical comparison of motion segments in the intact, fusion and mechanical nucleus
replacement states for normal and degenerated states.
Patient setting
Thirty lumbar motion segments.
Outcomes measures
Intervertebral height; motion segment range of motion, neutral zone, stiffness.
Methods
Motion segments had multidirectional flexibility testing to 7.5 Nm for intact discs, discs reconstructed using the IPD (n=12), or after anterior/posterior
fusions (n=18). Interbody height and axial compression stiffness changes were determined
for the reconstructed discs by applying axial compression to 1,500 N. Analysis included stratifying results to normal mobile versus rigid degenerated
intact motion segments.
Results
The mean interbody height increase was 1.5 mm for IPD reconstructed discs versus 3.0 mm for fused segments. Axial compression stiffness was 3.0±0.9 kN/mm for intact compared with 1.2±0.4 kN/mm for IPD reconstructed segments. Reconstructed disc ROM was 9.0°±3.7° in flexion
extension, 10.6°±3.4° in lateral bending, and 2.8°±1.4° in axial torsion that was
similar to intact values and significantly greater than respective fusion values (p<.001).
Mobile intact segments exhibited significantly greater rotation after fusion versus
their more rigid counterparts (p<.05); however, intact motion was not related to motion
after IPD reconstruction. The NZ and rotational stiffness followed similar trends.
Differences in NZ between mobile and rigid intact specimens tended to decrease in
the IPD reconstructed state.
Conclusion
The annulus-sparing IPD generally reproduced the intact segment biomechanics in terms
of ROM, NZ, and stiffness. Furthermore, the IPD reconstructed discs imparted stability
by maintaining a small neutral zone. The IPD reconstructed discs were significantly
less rigid than the fusion constructs and may be an attractive alternative for the
treatment of degenerative disc disease.
Keywords
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Article info
Publication history
Published online: June 22, 2009
Accepted:
April 20,
2009
Footnotes
FDA device/drug status: approved for this indication (Fusion devices [cages, spacers]).
Author disclosures: GRB (stock ownership, Dynamic Spine, LCC).
The study was funded by NIH (grant no. 1 R43 AR048732-01A1).
Identification
Copyright
© 2009 Elsevier Inc. Published by Elsevier Inc. All rights reserved.
