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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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
marcolms@drexel.edu
INTRODUCTION
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 [1]. 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
discs.

METHODS
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.
[2]. 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.

RESULTS
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
denucleation time.
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.
1.25
0
1
2
3
4
5
Norm DHNorm cROM Norm tROMNorm cNZ
Normalized to Intact
5min 10min15min20min
*
*
***
*
*
*
*
*
*
*
*
**
*
0
0.25
0.5
0.75
1
50N200N400N 800N 1400N
Normalized Stiffness
5min10min15min20min
*
*
*
*
*
*
*
*
*
*

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 HeightCompressionAxial RotationLateral BendingFlexion/Extension
0
1
2
3
4
5
6
DH
cROM
tROM
cNZ
ROM
NZ
ROM
NZ
ROM
NZ
Normalized to intact
*
*
*
*
*
*
*
*
*
*
0.00
0.25
0.50
0.75
1.00
1.25
1.50
-7.0Nm-4.0Nm-2.0Nm -0.5Nm0.5Nm2.0Nm4.0Nm7.0Nm
Normalized Stiffness
Axial Rotation Lateral BendingFlexion/Extension
*
*
**
*
*
*


DISCUSSION
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.

REFERENCES
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

ACKNOWLEDGEMENTS
Synthes Spine for Funding.
53rd Annual Meeting of the Orthopaedic Research Society
Poster No: 1062