M.Sc Thesis

M.Sc StudentRaz Einat
SubjectRecruitment Viscoeleasticity of Tendons
DepartmentDepartment of Biomedical Engineering
Supervisor PROFESSOR EMERITUS Yoram Lanir
Full Thesis textFull thesis text - English Version


Background:  There is still no agreement on the nature of tissues viscoelasticity and on its reliable modeling. We speculate that disagreements between previous observations stem from difficulties of separating between viscoelastic and preconditioning effects, since both are manifested by similar response features. In the present study this and related issues were investigated in the tendon since it is a simple structured prototype of other soft tissues.

Hypothesis: A general viscoelastic formulation for the tendon which is based on the collagen fibers properties and on their gradual recruitment with stretch is a valid representation of its response under both stress relaxation and creep tests.

Methods: Sheep digital tendons were preconditioned under strain that was higher by 1% compared with the one used in subsequent testing. Each specimen was then subjected to multiple test schemes: stress relaxation and quick release or creep. A stochastic micro-structural viscoelastic theory was developed based on the collagen fibers properties and on their gradual recruitment with stretch. Model parameters were estimated from stress relaxation data and predictions were then contrasted against creep data. Following its validation, the new recruitment viscoelasticity (RVE) theory was compared, both theoretically and experimentally, with the quasi-linear viscoelastic (QLV) theory.

Results:  The applied preconditioning protocol produced pure subsequent viscoelastic response. The proposed RVE model provided excellent fit to both stress relaxation and creep data. Both analytical and numerical comparisons showed that the new RVE theory and the popular QLV one are equivalent under deformation schemes at which no fibers buckle. Otherwise, the equivalence breaks down: QLV may predict negative stress, in contrast to data of the quick release tests, while RVE predicts no such negative stress.

Conclusions: The results are consistent with the following conclusions: fully preconditioned tendon exhibits pure viscoelastic response; non-linearity of the tendon viscoelasticity is induced by gradual recruitment of its fibers; a new structure based recruitment viscoelastic (RVE) theory is a reliable representation of the tendon viscoelastic properties under both stress relaxation and creep tests;  the quasi-linear viscoelastic theory (QLV) is equivalent to the RVE one (and valid) only under deformations in which no fibers buckle. The results also suggest that the collagen fibers themselves are linear viscoelastic.