in vitro full-field strain investigation in intact spine and spinal … · 2017-10-05 ·...
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ESB-ITA 2017, September 28-29 2017, Roma, Italy 1
Abstract - Spine is one of the most studied systems of the
human skeleton because of the several types of problems that can
injure it (accidents, pathologies, stresses). Many studies evaluated
the range of motion, stiffness and strain on hard tissues under
different loading conditions. To integrate the study with the
deformation of the soft tissues (intervertebral discs and
ligaments), Digital Image Correlation (DIC) can measure the
distribution of deformation in a contact-less way and provide a
full-field view of the examined surface under load.
This study was performed using segment of human spine
loaded in anterior bending with the use of loading machine and
DIC. All tests showed the different deformation of the specimen
in the vertebral body, intervertebral discs, ligaments and in the
posterior rods.
This work showed the feasibility and importance of
investigating the spine in a full-field way, due to the high strain
inhomogeneity in the vertebrae and intervertebral discs.
Therefore it can give useful information to design medical devices
and to surgeons to implant them.
Keywords - Biomechanics, Ligaments, Digital Image
Correlation, Intervertebral disc.
I. INTRODUCTION
INVESTIGATING the biomechanics of the spine is a
fundamental task because it could help engineers and
clinicians to design implants with a higher success ratio [1].
Up to date, the spine was characterized as a whole, without an
experimental description of the strain distribution on the
surface of the spine segment; or describing the details of a
single organ (the vertebrae, the discs and the ligaments)
without taking in account the complex boundary conditions of
the spine [2],[3]. The aim of this work was to merge these two
different methodologies: measuring the full-field strain
distribution on a intact spine segment on the vertebral bodies
and the intervertebral discs (IVDs), and on the supraspinous
spine ligaments and spine fixator after spinal stabilization.
II. MATERIAL AND METHODS
A. Specimen and DIC
A fresh-frozen human spine segment (L2 to S1) was
obtained through an ethically approved international donation
program. The anterior soft tissues (including the anterior
ligament) were removed without damaging the vertebrae, the
intervertebral discs and the other ligaments.
An high-contrast white-on-black speckle pattern [4]-[5] was
required to evaluate the strain using Digital Image Correlation
(DIC) on the spine surface. The background was prepared
staining the entire spine segment with methylene blue with a
brush. The white speckle pattern, instead, was created using
water-based paint sprayed with an optimized airgun [6] (figure
1).
A commercial 3D-DIC (Q400, Dantec
Dinamics,Skovlunde, DK) with two 5MPixels cameras and
35mm lens was employed. A field of view of 60x80 mm^2
was set for the two cameras to frame the functional spinal unit
of L3-L4 and its IVD, obtaining a pixel size of 0.03mm.
Consequently, the speckles had an average size on the camera
sensor of 6 pixels and a standard deviation of 6 pixels, as
required [6].
An optimization in zero strain condition allowed selecting
the best compromise between the measurement spatial
resolution and the measurement uncertainties. A facet size of
33 pixels, a grid spacing of 19 pixels, and a filter with a kernel
of 5x5 were set obtaining a strain uncertainties of 140
microstrain.
After testing the intact spine (see next paragraph), the
specimen was instrumented with pedicle screw fixation system
by a surgeon. In order to evaluate the strain on the rods, a
white-on-black speckle pattern was prepared, using black paint
for the background and white paint for the dots.
B. Mechanical testing
The load was applied with a uni-axial testing machine
(8032, Instron, High Wycombe, UK) in displacement control.
A compression-anterior bending was reproduced with an
eccentric compression: the point where the force was applied
had an anterior offset equal to the 20% of the antero-posterior
depth of the central intervertebral disc. The setup avoided the
transmission of any other load component (figure 1). Ten
In vitro full-field strain investigation in intact
spine and spinal fixator by means of DIC
Maria Luisa Ruspi1, Marco Palanca1, Luigi La Barbera2,3, Tomaso Villa2,3, Luca Cristofolini1
1 Alma Mater Studiorum – Università di Bologna: [email protected], [email protected],
2 Laboratory of Biological Structure Mechanics, Department of Chemistry, Materials and Chemical Engineering
“G. Natta”, Politecnico di Milano, Italy,
3 IRCCS Galeazzi Orthopaedic Institute, Milan, Italy
[email protected], [email protected], [email protected]
I
Figure 1 – Spine segment prepared with the speckle pattern: it was
illuminated by led lights. The two digital cameras acquired images
during the test.
Proceedings VII Meeting Italian Chapter of the European Society of Biomechanics (ESB-ITA 2017) 28-29 September 2017, Rome - Italy
ISBN: 978-88-6296-000-7
ESB-ITA 2017, September 28-29 2017, Roma, Italy 2
preconditioning cycles were applied between 0 and 1.0 mm of
compression at 0.5Hz. A compression of 3.0 mm was applied
in 0.1mm steps, while DIC images were acquired at each step.
We repeated the test: the first time to evaluate the strain on
natural spine, the second time to evaluate the strain on spinal
fixator. The test was designed to avoid specimen damage,
based on some preliminary tests [7].
III. RESULTS AND DISCUSSION
DIC system evaluated successfully the strains of the entire
functional spinal unit (vertebra and IVD from the frontal view,
and supraspinous ligaments from the posterior view) and of the
fixation system.
With the intact spine, the IVD reached larger deformations
than the vertebrae, as expected (figure 2). On the frontal part
of the IVD, the maximum principal strain was in the order of
+50000 microstrain (tension) and it was aligned
circumferentially, while the minimum principal strain was in
the order of -150000 microstrain (compression) and was
aligned axially (figure 2). These strain maps showed no-
homogeneous strain distribution on the IVD, revealing strain
peaks in the central side of the disc. In the frontal part of the
vertebral bone, the strains were lower than in the IVD: the
maximum principal strain was in the order of +10000
microstrain (tension), while the minimum principal strain was
in the order of -5000 microstrain (compression) (figure 2).
On the posterior view of the specimen, the strain on the
supraspinous ligament and the two rods were evaluated during
the anterior bending (figure 3). The ligaments, after the
fixation, had strain in the order of ten thousand microstrain,
like the strain evaluated on the bars. The strain on the rods can
be easily achieved with traditional strain gauges but, using
full-field measurement technique, stress/strain concentration
region can be highlighted. As in this configuration, the left rod
is more strained than the right rod, and larger strain were in the
medial side of the rods. Moreover, the measure was not
affected by the rods orientation.
IV. CONCLUSION
This study aimed to examine multi-vertebrae segment of
spine providing a full-field strain distribution on hard tissues
(vertebrae), soft tissues (intervertebral discs, supraspinous
ligaments) and spinal fixators. The spine was accounted as a
whole, considering the complex boundary condition and
described in details with particular emphasis to the soft tissues
(intervertebral disc and ligament), which cannot be analysed
using traditional techniques. This study showed a non-
homogeneous distribution of the strain on the IVD and its
interfaces with vertebrae, that shows the importance of
implementing a full-field analysis to explore the spine
biomechanics. The spinal fixators, can be explored using this
technique as well, in order to focus the weak points of this
system with a high failure rate. Finally, the use of DIC to
analyse the spine can increase the knowledge of the
biomechanics of the spine, opening the way to new basic and
transactional researches (understanding the role of ligaments,
studying fixator devices, analysing failures that occur after
surgery).
REFERENCES
1. Lau, D., et al., Proximal junctional kyphosis and failure after spinal deformity surgery. SPINE, 2014. 39.
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14(01): p. 1430002.
3. Danesi, V., et al., Effect of the In Vitro Boundary Conditions on the Surface Strain Experienced by the Vertebral Body in the Eastic
Regime.pdf. Journal of Biomechanical Engeneering, 2016. 138.
4. Palanca, M., et al., Exploring the strain distribution of thoracolumbar spine segments: An application of Digital Image
Correlation. Medical Engineering and Physics, 2017.
5. Palanca, M., et al., The evaluation of strain on spine segments in a contactless way and full-field view. ESB, 2016.
6. Palanca, M., G. Tozzi, and L. Cristofolini, The use of digital image
correlation in the biomechanical area: a review. International Biomechanics, 2015. 3: p. 1-21.
7. Cristofolini, L., In vitro evidence of the structural optimization of
the human skeletal bones. J Biomech, 2015. 48(5): p. 787-96.
Figure 3 - Posterior view of spine segment during anterior bending:
strain distribution on the two rods and on the supraspinous ligament
Figure 2 – Anterior view of spine segment during anterior bending:
strain distribution on the vertebrae and intervertebral discs
Proceedings VII Meeting Italian Chapter of the European Society of Biomechanics (ESB-ITA 2017) 28-29 September 2017, Rome - Italy
ISBN: 978-88-6296-000-7