The role of the nucleus pulposus in neutral zone human lumbar intervertebral disc mechanics.
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.Clinical biomechanics (Bristol, Avon) 05/2013; · 1.76 Impact Factor
THE ROLE OF THE NUCLEUS PULPOSUS IN NEUTRAL ZONE HUMAN
LUMBAR INTERVERTEBRAL DISC MECHANICS
*Cannella, M; **Arthur, A; *Allen, S; *Joshi, A; ***Vresilovic, E; +*Marcolongo, M
+*Department of Materials Science and Engineering, Drexel University, Philadelphia, PA
Removal of the nucleus pulposus is common in discectomy
procedures. While the clinical outcome of these procedures is generally
good, a comprehensive understanding of the role of the nucleus in
intervertebral disc mechanics is still warranted. Numerous groups have
examined the role of the nucleus in mechanical behavior of the disc,
indicating that the removal of the nucleus leads to increased
displacement and that the more nucleus removed, the more displacement
increases. Koebbe et al. state that high clinical success rates are possible
with full disc decompression . We hypothesize that the contribution
of the nucleus pulposus to the mechanical behavior of the disc is more
dominant through the neutral zone than at the farther limits of applied
loads and moments. In this work, we investigated the effect of partial
denucleation through a 2 mm incision on the compressive, tensile,
bending and torsional behavior of the intervertebral disc as examined
through anterior column units (ACUs) of human lumbar intervertebral
Two separate groups of six ACUs were harvested from six human
lumbar spines (3M/3F). The average specimen age was 47±19years,
ranging from 30 to 64years, for the first group and 49±13years, ranging
from 26 to 65years, for the second group. Each ACU was prepared,
potted, and kept moist throughout testing. Initial intervertebral disc
height under zero load was measured by a calibrated X-ray image
acquired with an FIS-Fluoroscan III (Fluoroscan Imaging Systems,
Northbrook, IL). The average intact disc height was 9.1±2.1mm,
ranging from 5.4mm to 10.9mm, and 7.2±1.3mm, ranging from 6.2mm
to 9.5mm, for the first and second groups, respectively.
The first group of specimens (Data Set 1) was tested in axial loading
after several incremental steps of partial denucleation. Preconditioning
was performed in displacement control at 3% of initial disc height (DH)
for 50 cycles with a sawtooth wave shape at 1Hz using a servohydraulic
dynamic test system (Model no. 8874, Instron, Corp., Norwood, MA).
The intact disc was then tested in load control with a 5 cycle sawtooth
waveform at 0.1Hz from 150N tension to 1500N compression.
Intradiscal pressure was collected inside the nucleus pulposus through
the anterior annular wall. Under a 50N compressive load (to simulate
lying prone), a portion of the nucleus tissue was removed by an
automated vacuum tissue removal system (Nucleotome, Model 22500,
Clarus Medical, LLC, Minneapolis, MN). Tissue removal was
performed using a posterolateral approach in four 5 minute intervals. At
the end of each denucleation period, the tissue removed from the disc
was collected, dried and weighed, and the disc was tested with the same
protocol described above.
The second group of specimens (Data Set 2) was tested in axial
loading, axial rotation, lateral bending and flexion/extension after full
denucleation. After pre-conditioning (as above), the ACUs were loaded
using a custom test jig modeled after that reported by Spenciner et al.
. The test specimen and a six degree-of-freedom (DOF) load cell
(Model MC3A, AMTI, Inc., Watertown, MA) were positioned above an
XY table that was used to relieve shear forces. Although off-axis
moments were in general less than 10% of the applied moment (peak at
19%), the difference in off-axis moments for any test condition on a
given disc was less than 5%. Five cycles of tension/compression (-150N
to 1000N, sinusoidal waveform, 0.1Hz), axial rotation, lateral bending
and flexion/extension (±7.5Nm, sinusoidal waveform, 0.1Hz) were
applied separately to each intact ACU. Each disc was then denucleated
as above for 20min. and dry mass was measured. The full loading
regime was re-applied to the denucleated specimens.
For each data set, range of motion (ROM), neutral zone (NZ) and
stiffness were calculated for the intact and denucleated states of each
ACU using the fifth loading cycle. Paired t-tests were used to determine
whether the denucleated state was significantly different from the intact
state using p < 0.05.
The effect of partial denucleation on compressive biomechanics is
shown in Figure 1. The disc height was reduced with denucleation,
while the compressive neutral zone increased significantly (p<0.05) for
each denucleation point except 5 min, ranging from about 1.5-2.0 times
that of the intact discs (Figure 1a). The compressive stiffness followed
an interesting trend where low loading levels (under 400 N) showed a
reduction in compressive stiffness compared to intact levels, the higher
loading regimes (greater than 400 N) generally showed stiffness values
that were not different from intact specimens. Intradiscal pressure
dropped 10-20% from the intact condition, but did not vary with applied
load. However intradiscal pressure showed some dependence on
Figure 1: Axial loading with increased denucleation times shows
increased motion (a) and reduced compressive stiffness (b) through
the neutral zone in comparison to the intact ACUs.
Norm DHNorm cROM Norm tROMNorm cNZ
Normalized to Intact
50N 200N 400N 800N 1400N
For fully denucleated ACUs that were subjected to axial, bending and
torsional loading, there was a large increase in normalized neutral zone
displacement for each mode of loading. The stiffness values for each
loading mode were below the level of the intact specimens in the lower
loading regime, but equal to or higher than the intact levels as the
loading increased (Figure 2).
Figure 2: Axial loading, bending and torsion lead to an increase in
displacement through the neutral zone in comparison to the intact
condition for ACUs (a); stiffness values in bending and torsion were
reduced below that of the intact condition in lower load levels, but
not at higher loading (b).
Disc HeightCompression Axial RotationLateral BendingFlexion/Extension
Normalized to intact
Axial RotationLateral Bending Flexion/Extension
It can be hypothesized that the initial loading of the disc, through the
neutral zone, relies on the intradiscal pressure created by the nucleus to
tense the annulus fibrosus. With this tensioning, the disc can operate in
the normal, intact condition. However, when the intradiscal pressure is
compromised (with denucleation) there is then a lag where the load-
displacement curves translate along the displacement axis. By the time
the disc is loaded to a high level, the excessive displacement is
minimized or reduced completely bringing the levels of stiffness of the
denucleated conditions approximately equivalent to the levels of the
intact condition for the ACUs examined in this study. The long-term
clinical consequences of partial or total removal of the disc are unclear to
date. While used as a method to alleviate pain, the long term quasi-static
effects of an unstable disc (as well as the dynamic effects that were not
addressed here) are not well-understood. The relationship, if any,
between mechanical instability of the disc and a clinically painful disc
are worthy of continued study as are the role that the nucleus plays in
creating pain and accelerated degeneration.
1. Koebbe, CJ, et al. Neurosurg Focus, 2002. 13(2): p. E3.
2. Spenciner, D., et al. Spine J, 2006. 6(3): p. 248-57.
AFFILIATED INSTITUTIONS FOR CO-AUTHORS
**Synthes Spine, West Chester, PA; ***Department of Biomedical
Engineering, Drexel University, Philadelphia, PA
Synthes Spine for Funding.
53rd Annual Meeting of the Orthopaedic Research Society
Poster No: 1062