Subject-Specific Finite Element Analysis of In-vivo Healthy and Tendinopathic Achilles Tendons

Abstract

The human Achilles tendon is the strongest tendon in the body, but it is also the tendon most prone to developing tendinopathy. Tendinopathy is a clinical term used to describe tendon pathology that presents clinically as a painful, thickened tendon with altered mechanical behaviour. The main conservative treatment for tendinopathy is exercise, typically in the form of calf muscle contractions. However calf exercises are likely to impose very different stresses and strains on the tendons of different individuals as a result of inter-subject variability in tendon geometry, tendon material properties, muscle mechanical properties and muscle activation patterns. Although there are reports of global stress and strain experienced by the Achilles tendon during voluntary contractions in the literature, little is known about the local stress and strain within the tendon. It is important to understand the local mechanical behaviour in tendon tissue because mechanical loading is a critical stimulus to tendon adaptation via localised mechano-biological pathways. Musculoskeletal tissues including tendons, are reported to have an optimal range in which mechanical loading produces positive tissue adaptation (an anabolic effect), with either too much or too little mechanical loading having a detrimental (catabolic) effect. Clinical efficacy of exercise-based training and rehabilitation for the Achilles tendon could be enhanced if an “optimal” loading stimulus is provided to the tendon. The general aim of this thesis was to investigate the influence of subject-specific tendon geometry and material properties on mechanical stress and damage in living free Achilles tendons with and without tendinopathy during submaximal, isometric loading. A methodological pipeline was developed to generate individualised finite element models of the living free Achilles tendon (n = 8 healthy, n = 8 tendinopathic). Subject-specific polynomial finite element meshes were rendered based on freehand three-dimensional ultrasound scans of the free tendon. Subject-specific material properties were obtained from numerical optimisation by minimising the difference between experimental measures of longitudinal strain under load and modelled strains. The tendon was defined as an incompressible, homogeneous and hyper-elastic material and implemented in CMISS. CMISS is a finite element software based on polynomial basis functions which can accommodate large deformation based of coarse polynomial meshes. Finite element analyses were subsequently conducted to determine the strain, stress and damage criteria within the tendon based on force boundary conditions obtained from the subject-specific experimentally measured ankle plantarflexor torque. The first study used three-dimensional ultrasound based measures of in-vivo free Achilles tendon geometry in conjunction with finite element analysis to determine the effects of subject-specific versus generic geometries and material properties on the stress distribution within the living tendon during a submaximal isometric contraction. The mean (SD) lengths, volumes and cross-sectional areas of the tendons at rest were 62 ± 13 mm, 3617 ± 984 mm³ and 58 ± 11 mm², respectively. The measured tendon strain at 70% maximum voluntary contraction was 5.9 ± 1.3%. Generic geometry was represented by the average mesh and generic material properties were taken from the literature. Local stresses were subsequently computed for all combinations of subject-specific and generic geometry and material properties. For a given geometry, changing from generic to subject-specific material properties had little effect on the stress distribution in the tendon. In contrast, changing from generic to subject-specific geometry had a 26-fold greater effect on tendon stress distribution. Overall, these findings highlight a strong variability between individual tendons and indicate that the tendon geometry has a greater influence on the stress distribution than the tendon material properties. Achilles tendon mechanical properties and geometry are altered in Achilles tendinopathy. The purpose of the second study was to determine the relative contributions of altered mechanical properties and geometry to free Achilles tendon stress distribution during a sub-maximal contraction in tendinopathic relative to healthy tendons. The average resting CSA of the free tendon was on average 35% greater for the tendinopathic tendons. At the same tensile force, the tendinopathic tendons experienced a strain of 7.1 ± 2.9% compared to 5.9 ± 1.3% for controls. The mean Young’s modulus for tendinopathic tendons was 40% of the corresponding control value. Finite element analyses revealed that tendinopathic tendons experience 24% less stress at submaximal loading compared to healthy tendons. The lower tendon stress in tendinopathy was due to a greater influence of tendon CSA, which alone reduced tendon stress by 30%, compared to Young’s modulus, which alone increased tendon stress by 8%. These findings suggest that the greater tendon CSA observed in tendinopathy compensates for the substantially lower Young’s modulus, and thereby protects pathological tendon against excessive stress. The purpose of the final study was to determine how tendinopathic alterations in Achilles tendon geometry and material properties affect damage load and location. Tendon damage load was assessed at a theoretical damage strain of 12%. The subject-specific damage load was found significantly higher for the healthy tendon (12.5 ± 5.0 kN) compared to the tendinopathic tendon (5.7 ± 1.5 kN). A 59% decrease in the damage load was observed when the average material properties of the healthy tendon were replaced with average tendinopathic material properties while retaining the average healthy tendon geometry. Damage load increased by 23% when the average healthy geometry was replaced by average tendinopathic geometry while retaining average healthy material properties. A substantial variation in damage location was observed across all tendons. Overall findings of this study suggest that tendinopathic alterations in material properties are more influential than corresponding alterations in tendon geometry in determining the load required to cause tendon damage. This thesis has demonstrated the feasibility of using a finite element modelling approach to investigate stress distributions in the Achilles tendons based on in-vivo, subject-specific measures of three-dimensional tendon geometry and tendon mechanical properties. Stress patterns in the Achilles tendon were found to differ substantially between individuals. Generic training and rehabilitation programs for the Achilles tendon are therefore likely to result in very different tendon stresses and strains between individuals. The general findings of this thesis point to the need for personalised training and rehabilitation for the Achilles tendon that takes account of the substantial variation in tendon geometry and material properties between individuals to ensure an optimal loading stimulus is provided that maximises positive tissue adaptation for healthy and tendinopathic tendons.