Effect of a Posterior Dynamic Implant Adjacent to a Rigid Spinal Fixator.
From: Biomechanics Laboratory, Department of Orthopedics, Charité Campus Benjamin Franklin, Hindenburgdamm 30, 12203 Berlin, Germany.
Clinical biomechanics (Bristol, Avon)
- Publish Date: Oct 2006
- ISSN: 0268-0033
- Volume: 21
- Issue: 8
- Pages: 767-74
- Medium: Print
- Language: English
- Citation (JAMA): Zander Thomas, Rohlmann Antonius, Burra Nagananda K, et al. Effect of a Posterior Dynamic Implant Adjacent to a Rigid Spinal Fixator.. Oct 2006;21:767-74
Abstract
BACKGROUND: A slightly degenerated disc adjacent to a segment that has to be fused is sometimes instrumented with a dynamic fixator. The dynamic implant is assumed to reduce disc loads at that level and to preserve disc function, thus inhibiting the progression of degeneration. METHODS: A three-dimensional finite element model of the lumbar spine was used to study the effect of a dynamic implant on the mechanical behavior at the corresponding level. After studying a healthy lumbar spine for comparison, a rigid fixator and a bone graft were inserted at L2/L3. Healthy and degenerated discs were assumed at the adjacent level, i.e. L3/L4. An additional paired dynamic posterior fixator was then implemented at level L3/L4. Finally, the segment with the dynamic fixator was distracted to the height of a healthy disc. The loading cases of walking, extension, flexion and axial rotation were simulated. FINDINGS: A dynamic implant reduces intersegmental rotation for walking, extension and flexion as well as facet joint forces for axial rotation at its insertion level. Intradiscal pressure is not markedly reduced by a dynamic implant. Moreover, there are no substantial differences between the mechanical behavior of rigid and dynamic fixators. INTERPRETATION: Our model does not predict major differences in the mechanical effects between rigid and dynamic fixators despite the extreme assumption that a dynamic implant does not transfer moments. The results do not support the assumption that disc loads are significantly reduced by a dynamic implant. For axial rotation, however, dynamic fixation devices do reduce the force in the facet joint.
Mesh Headings (Keywords): Back, Biocompatible Materials, Biomechanics, Compressive Strength, Humans, Intervertebral Disk Displacement, Lumbar Vertebrae, Materials Testing, Prostheses and Implants, Spine, Weight-Bearing
Check for Full Text / PubMed Unique Identifier (PMID): 16750875
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