The role of the nucleus pulposus in neutral zone human lumbar intervertebral disc mechanics.

Department of Materials Science and Engineering, Drexel University, 3141 Chestnut Street, Philadelphia, PA 19104, USA.
Journal of Biomechanics (Impact Factor: 2.72). 02/2008; 41(10):2104-11. DOI: 10.1016/j.jbiomech.2008.04.037
Source: PubMed

ABSTRACT To study the effect of denucleation on the mechanical behavior of the human lumbar intervertebral disc through a 2mm incision, two groups of six human cadaver lumbar spinal units were tested in axial compression, axial rotation, lateral bending and flexion/extension after incremental steps of "partial" denucleation. Neutral zone, range of motion, stiffness, intradiscal pressure and energy dissipation were measured; the results showed that the contribution of the nucleus pulposus to the mechanical behavior of the intervertebral disc was more dominant through the neutral zone than at the farther limits of applied loads and moments.

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    ABSTRACT: Post-operative patient motions are difficult to directly control. Very slow quasi-static motions are intuitively believed to be safer for patients, compared to fast dynamic motions, because the torque on the spine is reduced. Therefore, the outcomes of varying axial rotation angular loading rate during in vitro testing could expand the understanding of the dynamic behavior and spine response. To observe the effects of the loading rate in axial rotation mechanics of lumbar cadaveric spines via in vitro biomechanical testing. An in vitro biomechanical study in lumbar cadaveric spines. Fifteen (15) lumbar cadaveric segments (L1-S1) were tested varying loading frequencies of axial rotation. Five (5) different frequencies were normalized with the base line frequency (0.125Hz n=15), in this analysis: 0.05 Hz (n=6), 0.166 Hz (n=6), 0.2 Hz (n=10), 0.25 Hz (n=10) and 0.4 Hz (n=8). The lowest frequency (0.05 Hz) revealed significant differences (P<0.05) for all parameters measured (torque, passive angular velocity, axial velocity, axial reaction force and energy loss) with respect to all other frequencies. Significant differences (P<0.05) were observed in the following: torque (0.4 Hz with respect to 0.2 Hz and 0.25 Hz), passive sagittal angular velocity (0.4 Hz with respect to all other frequencies; 0.166 Hz with respect to 0.25 Hz), axial linear velocity (0.4 Hz with respect to all other frequencies), reaction force (0.4 Hz with respect to 0.2 Hz and 0.25 Hz). Strong correlations (R(2)>0.75, P<0.05) were observed between reaction force with intradiscal pressure and axial rotation angular displacement with intradiscal pressure. Intradiscal pressure (P<0.05) was significantly larger in 0.2 Hz in comparison to 0.125 Hz. Evidences suggest that measurements at very small frequencies (0.05 Hz) torque, sagittal angular velocity, axial velocity, reaction force and energy loss is significantly reduced, when compared with higher frequencies (0.166 Hz, 0.2 Hz, 0.25 Hz, 0.4 Hz). Higher frequencies increase torque, reaction force, passive sagittal angular velocity and axial velocity with respect to lower frequencies. Higher frequencies induce a greater intradiscal pressure in comparison to lower frequencies.
    The spine journal: official journal of the North American Spine Society 11/2013; · 2.90 Impact Factor
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    ABSTRACT: The intervertebral disc maintains a balance between externally applied loads and internal osmotic pressure. Fluid flow plays a key role in this process, causing fluctuations in disc hydration and height. The objectives of this study were to quantify and model the axial creep and recovery responses of nondegenerate and degenerate human lumbar discs. Two experiments were performed. First, a slow compressive ramp was applied to 2000 N, unloaded to allow recovery for up to 24 h, and re-applied. The linear-region stiffness and disc height were within 5% of the initial condition for recovery times greater than 8 h. In the second experiment, a 1000 N creep load was applied for four hours, unloaded recovery monitored for 24 h, and the creep load repeated. A viscoelastic model comprised of a "fast" and "slow" exponential response was used to describe the creep and recovery, where the fast response is associated with flow in the nucleus pulposus (NP) and endplate, while the slow response is associated with the annulus fibrosus (AF). The study demonstrated that recovery is 3-4X slower than loading. The fast response was correlated with degeneration, suggesting larger changes in the NP with degeneration compared to the AF. However, the fast response comprised only 10%-15% of the total equilibrium displacement, with the AF-dominated slow response comprising 40%-70%. Finally, the physiological loads and deformations and their associated long equilibrium times confirm that diurnal loading does not represent "equilibrium" in the disc, but that over time the disc is in steady-state.
    Journal of the mechanical behavior of biomedical materials. 10/2011; 4(7):933-42.
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    ABSTRACT: BACKGROUND: Titanium pedicle screw-rod instrumentation is considered a standard treatment for spinal instability; however, the advantages of cobalt-chromium over titanium is generating interest in orthopedic practice. The aim of this study was to compare titanium versus cobalt-chromium rods in posterior fusion through in vitro biomechanical testing. METHODS: Posterior and middle column injuries were simulated at L3-L5 in six cadaveric L1-S1 human spines and different pedicle screw constructs were implanted. Specimens were subjected to flexibility tests and range of motion, intradiscal pressure and axial rotation energy loss were statistically compared among five conditions: intact, titanium rods (with and without transverse connectors) and cobalt-chromium rods (with and without transverse connectors). FINDINGS: All fusion constructs significantly (P<0.01) decreased range of motion in flexion-extension and lateral bending with respect to intact, but no significant differences (P>0.05) were observed in axial rotation among all conditions. Intradiscal pressure significantly increased (P≤0.01) after fusion, except for the cobalt-chrome conditions in extension (P≥0.06), and no significant differences (P>0.99) were found among fixation constructs. In terms of energy loss, differences became significant P≤0.05 between the cobalt-chrome with transverse connector condition with respect to the cobalt-chrome and titanium conditions. INTERPRETATION: There is not enough evidence to support that the cobalt-chrome rods performed biomechanically different than the titanium rods. The inclusion of the transverse connector only increased stability for the cobalt-chromium construct in axial rotation.
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