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Biomechanical evaluation of a posterior non-fusion instrumentation of the lumbar spine.

Eur Spine J. 2011 Dec 20;

Authors: Schmoelz W, Erhart S, Unger S, Disch AC

Abstract
PURPOSE: Numerous posterior non-fusion systems have been developed within the past decade to resolve the disadvantages of rigid instrumentations and preserve spinal motion. The aim of this study was to investigate the effect of a new dynamic stabilization device, to measure the screw anchorage after flexibility testing and compare it with data reported in the literature. METHODS: Six human lumbar spine motion segments (L2-5) were loaded in a spine tester with pure moments of 7.5 Nm in lateral bending, flexion/extension and axial rotation. Specimens were tested intact, after instrumentation of the intact segment, after destabilization by a nucleotomy and after instrumentation of the destabilised segment with the new non-fusion device (Elaspine). After flexibility testing all screws were subjected to a pull-out test. RESULTS: Instrumentation of the intact segment significantly reduced the RoM (p < 0.002) in flexion, extension and lateral bending to 49.7, 44.6 and 53% of the intact state, respectively. In axial rotation, the instrumentation resulted in a non-significant RoM reduction to 95% of the intact state. Compared to the intact segment, instrumentation of the destabilized segment significantly (p < 0.05) reduced the RoM to 69.8, 62.3 and 79.1% in flexion, extension and lateral bending, respectively. In axial rotation, the instrumented segment showed a significantly higher RoM than the intact segment (137.6% of the intact state (p < 0.01)). The pull-out test showed a maximum pull-out force of 855.1 N (±334) with a displacement of 6.1 mm (±2.8) at maximum pull-out force. CONCLUSIONS: The effect of the investigated motion preservation device on the RoM of treated segments is in the range of other devices reported in the literature. Compared to the most implanted and investigated device, the Dynesys, the Elaspine has a less pronounced motion restricting effect in lateral bending and flexion/extension, while being less effective in limiting axial rotation. The pull-out force of the pedicle screws demonstrated anchorage comparable to other screw designs reported in the literature.

PMID: 22205112 [PubMed – as supplied by publisher]

Effect of Graded Facetectomy on Biomechanics of Dynesys Dynamic Stabilization System.

Spine (Phila Pa 1976). 2011 Dec 23;

Authors: Kiapour A, Ambati D, Hoy RW, Goel VK

Abstract
ABSTRACT: Study Design. Finite element (FE) method was used to compare the biomechanics of L3-S1 lumbar spine with graded facetectomy before and after placement of Dynesys.Objective. To evaluate the biomechanics of Dynesys as a function of graded bilateral facetectomies.Summary of Background Data. Spinal fusion or posterior dynamic stabilization systems are used to restore stability following facetectomies.Methods. The intact FE spine was modified to simulate decompression at L4-L5 with 50%, 75% and total facetectomy with/without dynamic stabilization with Dynesys. Biomechanics of the implanted level was investigated under different physiological loadings.Results. Total facetectomy increased the motion in extension (8.7° v/s. 2.7° for intact) and axial rotation (8.4° v/s. 2.4° for intact). However the decrease in motion in the Dynesys model ranged from 65% in axial rotation to 80% in flexion for all facetectomies, except in the total facetectomy axial rotation case (motion higher than the intact case). The Center of Rotation (COR) of dynamic stabilized segment moved inferior/posterior in partial facetectomy and superior/posterior in total facetectomy with respect to the intact and destabilized cases. The Dynesys screws observed peak stresses up to 28% higher than those of a rigid fixation system in certain loadings such as lateral bending and extension. The critical loosening torque applied to the screws in total facetectomy case was 6 times the partial facetectomy case in axial rotation.Conclusions. Partial facetectomy had a minimal effect on range of motion on the DYNESYS implanted segment. However in the case of total facetectomy the motion increased by almost 40% in flexion and 200% in axial rotation. The higher stresses applied to the screws in Dynesys in specific loadings may lead to higher risk of screw failure in Dynesys compared to in a generic rigid fixation construct.

PMID: 22198353 [PubMed – as supplied by publisher]

Preliminary evaluation of posterior dynamic lumbar stabilization in lumbar degenerative disease in Chinese patients.

Chin Med J (Engl). 2012 Jan;125(2):253-6

Authors: Jia YH, Sun PF

Abstract
BACKGROUND: There has been some controversy related to the use of the Wallis system, rather than disc fusion in the treatment of patients with degenerative spine disease. Furthermore, there are no reports concerning the application of this dynamic stabilization system in Chinese patients, who have a slightly different lifestyle with Western patients. The aim of this study was to assess the safety and efficacy of the dynamic stabilization system in the treatment of degenerative spinal diseases in Chinese patients.
METHODS: The clinical outcomes of 20 patients with lumbar degenerative disease treated by posterior decompression with the Wallis posterior dynamic lumbar stabilization implant were studied. All of the patients completed the visual analogue scale and the Chinese version of the Oswestry Disability Index. The following radiologic parameters were measured in all patients: global lordotic angles and segmental lordotic angles (stabilized segments, above and below adjacent segments). The range of motion was then calculated.
RESULTS: Nineteen patients (95%) were available for follow-up. The mean follow-up period was (27.25 ± 5.16) months (range 16 – 35 months). The visual analogue scale decreased from 8.55 ± 1.21 to 2.20 ± 1.70 (P < 0.001), and the mean score on the Chinese version of the Oswestry Disability Index was improved from 79.58% ± 15.93% to 22.17% ± 17.24% (P < 0.001). No significant changes were seen in the range of motion at the stabilized segments (P = 0.502) and adjacent segments (above, P = 0.453; below, P = 0.062). The good to excellent result was 94.4% at the latest follow-up. No complications related to the use of the Wallis posterior dynamic lumbar stabilization occurred.
CONCLUSIONS: It was found to be both easy and safe to use the Wallis posterior dynamic lumbar stabilization implant in the treatment of degenerative lumbar disease, and the early therapeutic effectiveness is good. The Wallis system provides an alternative method for the treatment of lumbar degenerative disease.

PMID: 22340555 [PubMed – in process]

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