Application of Stereo-Digital Image Correlation to Full-Field 3-D Deformation Measurement of Intervertebral Disc

This paper presents the application of a Stereo-Digital Image Correlation (Stereo-DIC) based approach for full-field deformation measurements on a porcine intervertebral disc (IVD) under in-vitro loading. Full-field capabilities of Stereo-DIC provide useful information on the IVD structure-function relationship needed for designing novel disc replacement devices on the basis of biomimetic concepts. In this work, the use of a multi-view Stereo-DIC system allows full-field measurement over more than 180 degrees of the IVD surface. This is achieved by sequentially moving a single camera through seven fixed positions in order to cover the desired angle of vision. Ad hoc data processing and merging procedures are calibrated for a cylindrical sample. Strain maps are determined for a rubber cylinder subjected to rigid-body motions and then to uniform compression. These preliminary procedures serve to evaluate strain-mapping errors as well as to assess repeatability and accuracy of camera re-positioning. Once the entire procedure is verified and calibrated, a fresh porcine functional spine unit is loaded under anterior, posterior and lateral compression. From displacement maps obtained with the multi-view set-up, it is possible to gather information on both IVD bulging and surface strains. Results are consistent with the IVD global behaviour under standard testing protocols reported in the recent literature. Furthermore, detailed information on local variations in structural response and stiffness properties occurring when load is applied in different regions of the IVD are obtained. The present approach allows to correlate structural response with the entire 3-D deformation and strain field and not just with a single displacement/strain component and thus it could be effectively used for explaining in-homogeneity observed in human discs degenerative patterns.