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: marialuisa.ruspi2@unibo.it, marco.palanca2@unibo.it,

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

luigi.labarbera@polimi.it, tomaso.villa@polimi.it, luca.cristofolini@unibo.it

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.

2. Brandolini, N., L. Cristofolini, and M. Viceconti, Experimental

Methods for the Biomechanical Investigation of the Human Spine: A Review. Journal of Mechanics in Medicine and Biology, 2014.

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