title: A fibre-reinforced poroviscoelastic model accurately describes the biomechanical behaviour of the rat Achilles tendon.
abstract: BACKGROUND
  Computational models of Achilles tendons can help understanding how healthy tendons are affected by repetitive loading and how the different tissue constituents contribute to the tendon's biomechanical response. However, available models of Achilles tendon are limited in their description of the hierarchical multi-structural composition of the tissue. This study hypothesised that a poroviscoelastic fibre-reinforced model, previously successful in capturing cartilage biomechanical behaviour, can depict the biomechanical behaviour of the rat Achilles tendon found experimentally.
  
  
  METHODS
  We developed a new material model of the Achilles tendon, which considers the tendon's main constituents namely: water, proteoglycan matrix and collagen fibres. A hyperelastic formulation of the proteoglycan matrix enabled computations of large deformations of the tendon, and collagen fibres were modelled as viscoelastic. Specimen-specific finite element models were created of 9 rat Achilles tendons from an animal experiment and simulations were carried out following a repetitive tensile loading protocol. The material model parameters were calibrated against data from the rats by minimising the root mean squared error (RMS) between experimental force data and model output.
  
  
  CONCLUSIONS
  All specimen models were successfully fitted to experimental data with high accuracy (RMS 0.42-1.02). Additional simulations predicted more compliant and soft tendon behaviour at reduced strain-rates compared to higher strain-rates that produce a stiff and brittle tendon response. Stress-relaxation simulations exhibited strain-dependent stress-relaxation behaviour where larger strains produced slower relaxation rates compared to smaller strain levels. Our simulations showed that the collagen fibres in the Achilles tendon are the main load-bearing component during tensile loading, where the orientation of the collagen fibres plays an important role for the tendon's viscoelastic response. In conclusion, this model can capture the repetitive loading and unloading behaviour of intact and healthy Achilles tendons, which is a critical first step towards understanding tendon homeostasis and function as this biomechanical response changes in diseased tendons.
authors.affiliation: Department of Biomedical Engineering, Lund University, Lund, Sweden.
authors.forename: Hanifeh
authors.initials: H
authors.lastname: Khayyeri
authors.affiliation: Department of Biomedical Engineering, Lund University, Lund, Sweden.
authors.forename: Anna
authors.initials: A
authors.lastname: Gustafsson
authors.affiliation: Department of Biomedical Engineering, Eindhoven University of Technology, Eindhoven, Netherlands.
authors.forename: Ashley
authors.initials: A
authors.lastname: Heuijerjans
authors.affiliation: Department of Mechanical Engineering, Lappeenranta University of Technology, Lappeenranta, Finland.
authors.forename: Marko K
authors.initials: MK
authors.lastname: Matikainen
authors.affiliation: Department of Clinical Neurophysiology, Kuopio University Hospital, Kuopio, Finland; Department of Applied Physics, University of Eastern Finland, Kuopio, Finland.
authors.forename: Petro
authors.initials: P
authors.lastname: Julkunen
authors.affiliation: Department of Clinical and Experimental Medicine, Linköping University, Linköping, Sweden.
authors.forename: Pernilla
authors.initials: P
authors.lastname: Eliasson
authors.affiliation: Department of Clinical and Experimental Medicine, Linköping University, Linköping, Sweden.
authors.forename: Per
authors.initials: P
authors.lastname: Aspenberg
authors.affiliation: Department of Biomedical Engineering, Lund University, Lund, Sweden.
authors.forename: Hanna
authors.initials: H
authors.lastname: Isaksson
mesh_terms: D000125:Achilles Tendon / Q000502:physiology*; D000818:Animals; D001696:Biomechanical Phenomena; D002138:Calibration; D003094:Collagen / Q000378:metabolism; D004548:Elasticity; D020342:Finite Element Analysis; D008954:Models, Biological; D016062:Porosity; D051381:Rats; D013314:Stress, Mechanical; D014783:Viscosity
chemical_list: D003094:Collagen