DIRECT MEASUREMENT OF INTERVERTEBRAL DISC MAXIMUM SHEAR STRAIN IN SIX DEGREES OF
FREEDOM: MOTIONS THAT PLACE DISC TISSUE AT RISK OF INJURY.
+*Costi, J J; *Stokes, I A; *Gardner-Morse, M G; *Laible, J P; **Scoffone, H M; *Iatridis, J C
+University of Vermont, Burlington, VT
[email protected]
%/mm) and compression (9.0±0.5 %/mm) produced the largest regional
INTRODUCTION:
MSS. Lateral shear was significantly larger than anterior and posterior
During certain motions, the disc is at risk of injury and this most
shear, and compression was significantly larger than posterior shear
commonly occurs in the annulus [1]. Axial compression coupled with
(P<0.015). No significant differences existed between compression,
various combinations of excessive flexion, lateral bending or axial
lateral shear and anterior shear (P>0.24). For the rotation motions, lateral
rotation have been shown to lead to disc injury. However, similar
bending had significantly larger regional MSS than all other tests
injuries have also been caused by repetitive motion at lower, more
physiological ranges of motion. The primary objectives of this study
(5.8±1.6 %/°, P<0.001), with no significant differences between the
were to determine the regions of largest shear strain experienced by disc
remaining rotation motions (P=1).
tissues in six degrees of freedom (DOF), since shear is considered a
The physiological MSS produced at the maximum reported lumbar
likely tissue failure criterion [2], and to identify the physiological
segmental range of motion for each DOF was greatest for lateral
rotations and displacements that may place the disc at greatest risk for
bending, which produced physiological MSS that were significantly
large tissue strains and injury.
larger than all other motions (57.8±16.2%, P<0.001). In addition,
METHODS:
physiological MSS for flexion was also significantly larger than for all
Nine lumbar human disc segments from three spines were tested.
remaining motions (38.3±3.3%, P<0.001). No significant differences
Five 1.5 mm diameter lead beads were placed at the center, mid-sagittal
were present between the remaining motions (P>0.25), with
and lateral margins of each vertebral endplate. A grid of tantalum wires
physiological MSS values ranging from 9.4±1.3% for axial rotation to
of 0.25 mm diameter was inserted into the mid-transverse plane of the
14.4±1.1% for lateral shear.
disc using a needle, needle-guide and a positioning translation stage. The
disc periphery was marked by stretching an elastic band tagged with
fifteen to twenty 5 mm length tantalum wire segments. The disc was
Figure 2.
then equilibrated under a 100 N compressive preload in a 0.15M PBS
Contour plots
bath at 4C for three hours. After equilibration, left/right anterolateral
showing the
stereo radiographs of the specimen were taken and the position was
MSS for
recorded as the initial unloaded (datum) position (Figure 1).
translation tests
(%/mm, a-d)
and rotation
tests (%/°, e-h).
Note: Lateral
shear, axial
rotation and
lateral bending
values include
pooled values
based on
presumed
symmetry.
Figure 1. Stereo
radiograph pair
showing the grid
of wires in the
disc, endplate
and calibration
beads, and
peripheral disc
wire segments.
Axial compression was then applied using a 6 DOF hexapod robot to
a nominal value of 2 mm after which stereo radiographs were taken and
the disc returned to the datum position, followed by axial rotation (±4°),
AP and lateral shear (±2 mm), flexion/extension (±6°), and lateral
bending (±5°). The positions of the wires and beads in the radiographs
were manually digitized. Stereo-photogrammetry was used to
reconstruct lines corresponding to the wires, bead centers and midpoints
of the circumferential disc markers. A regular grid comprised of fournode quadrilateral elements was then created within the disc
circumference, and average displacements at each grid intersection for
all specimens were interpolated to the regular grid coordinates. Relative
loaded-unloaded maximum shear strains (MSS) at each grid node were
calculated and expressed as %/mm for translation tests and %/° for
rotation tests.
Mean regional MSS values at each of nine anatomical regions were
defined by partitioning the grid. These regions were: anterior, left/right
anterolateral, left/right lateral, nucleus, left/right posterolateral, and
posterior. For each input displacement, the regions with the largest MSS
were identified, and the largest regional MSS values were pooled
wherever several regions had values that were not significantly different
from each other (ANOVA with Bonferroni-adjusted post-hoc
comparisons using p<0.05 as significant).
To identify the physiological rotations and displacements that may
place the disc at greatest risk for large tissue strains and injury, the
mean±95% confidence interval of the pooled regional MSS were
multiplied by the maximum reported physiological lumbar segmental
motion for each DOF [3, 4]. The MSS at the extremes of physiological
motion are referred to as physiological MSS.
RESULTS:
The regions of largest regional MSS for each displacement and
rotation were found in the posterior, posterolateral and lateral regions of
the disc (Figure 2). For the translation motions, lateral shear (9.6±0.7
DISCUSSION:
This study has identified the lumbar segmental motions that produce
physiological MSS comparable with the known failure strain of disc
tissue and that may place the disc at greatest risk of injury. Lateral
bending and flexion place the disc at greatest risk. The exact failure
criterion for intervertebral disc tissue is not known, and MSS was used
because it is related to maximum and minimum principal strains, and it
was shown that disc tears may be initiated by large interlamellar shear
strains that dominate over radial and circumferential annular fiber strains
[5]. These results provide improved understanding of disc behaviors
under loading and may also be of value validating finite element models.
REFERENCES:
[1] Vernon-Roberts et al. 1997, Spine, 22(22): 2641. [2] Goel et al.
1995, Spine, 20(6): 689. [3] Pearcy et al. 1984, Spine, 9(3):294 [4]
White & Panjabi 1990, Clinical Biomechanics of the Spine, J.B.
Lippincott Co. [5] Iatridis & Gwynn 2004, J. Biomech., 37(8): 1165.
AFFILIATED INSTITUTIONS FOR CO-AUTHORS:
**Rensselaer Polytechnic Institute, Troy, NY
Supported by NIH R01 AR 49370.
53rd Annual Meeting of the Orthopaedic Research Society
Poster No: 1138