Abstract
Purpose
As human soft tissue is anisotropic, non-linear and inhomogeneous, its properties are difficult to characterize. Different methods have been described that are either based on contact or noncontact protocols. In this study, three-dimensional (3D) digital image correlation (DIC) was adopted to examine the mechanical behaviour of the human Achilles tendon. Despite its wide use in engineering research and its great potential for strain and displacement measurements in biological tissue, the reported biomedical applications are rather limited. To our knowledge, no validation of 3D DIC measurement on human tendon tissue exists.
The first goal of this study was to determine the feasibility to evaluate the mechanical properties of the human Achilles tendon under uniaxial loading conditions with 3D Digital Image Correlation. The second goal was to compare the accuracy and reproducibility of the 3D DIC against two linear variable differential transformer (LVDT's).
Methods
Six human Achilles tendon specimens were prepared out of fresh frozen lower limbs. Prior to preparation, all limbs underwent CT-scanning. Using Mimics software, the volume of the tendons and the cross sectional area at each level could be calculated. Subsequently, the Achilles tendons were mounted in a custom made rig for uni-axial loading. Tendons were prepared for 3D DIC measurements with a modified technique that enhanced contrast and improved the optimal resolution. Progressive static loading up to 628,3 N en subsequent unloading was performed. Two charge-coupled device camera's recorded images of each loading position for subsequent strain analysis. Two LVDT's were mounted next to the clamped tendon in order to record the displacement of the grips.
Results
3D DIC strain measurement proved to be technical feasible on human tendon tissue if an adapted preparation protocol is used. A spatial resolution of 0,1 mm was reached. Accuracy analysis shows a very low scatter, comparable to that obtained in steel applications (0,03%). When compared to the LVDT measurements, DIC showed excellent correlation (R = 0.99). Apart from the longitudinal strain component, an important transverse strain component was revealed in all specimens (fig 1). Also a significant amount of slip was observed at the clamps. Through the non-contact nature of the measurement, this could be quantified and the analysis became independent of any slip (fig 2). The strain distribution was of a strongly inhomogeneous nature, both within the same specimen (fig 1) and amongst different specimens.
Conclusion
3D DIC is a very promising technique for strain measurement of human collagenous tissue. Accuracy analyses indicate a very low scatter, comparable to that obtained in traditional steel applications. The major advantages of the DIC technique over the LVDTs is the 3D, non-contact, full-field nature of the analysis and the possibility to analyse multidirectional strain, without disturbing slip effects in the grips.
