Achilles Tendon Elasticity and Deformation Patterns in Young and Middle-Aged Adults Evaluated Using Quantitative Ultrasound Approaches

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Abstract

Tendon injuries can be debilitating, and injury incidence is linked with aging. The Achilles tendon complex is particularly susceptible to injury during middle-age (e.g. tendinopathy), though the effects of aging on tendon mechanics are poorly understood. In fact, recent in vivo studies aiming to measure the effects of aging on Achilles tendon stiffness have shown contradictory results. These inconsistencies may arise due to limitations in methodological approaches, which often focus on the evaluation of gross tissue motion and may mask complex deformation patterns. Thus, the purpose of this dissertation was to utilize innovative ultrasound approaches to quantify high resolution in vivo tendon deformations and muscle-tendon mechanical properties in young and middle-aged adults. Using ultrasound elastography, we observed complex deformation patterns within the Achilles tendon, with deep portions of the tendon observed to undergo significantly more displacement than superficial portions. These non-uniform deformation patterns persisted with altered loading conditions and knee flexion angles, and may reflect variable motion between the tendon fascicles arising from the soleus and gastrocnemius muscles. We also observed small, but significant, posture- and load-dependent changes in displacement magnitudes. Supersonic Shear Imaging was used separately to characterize stretch-dependent variations in shear wave speed along the Achilles tendon and aponeurosis. Tendon shear wave speed increased with passive stretch, likely reflecting its strain-stiffening behavior, and varied significantly along the tendon length, with the proximal tendon (i.e. gastrocnemius aponeurosis) exhibiting lower shear wave speeds than the distal tendon (i.e. free tendon). Performing the same tasks, middle-aged adults exhibited evidence of significantly more uniform tendon deformation patterns and an increase in tendon compliance. The results of this dissertation demonstrate that Achilles tendon mechanics are highly non-uniform and change with age, highlighting the importance of using high resolution quantitative ultrasound techniques to assess spatial variations in tendon properties. Age-related changes in tendon mechanics were also observed to occur near regions prone to injury (e.g. the gastrocnemius aponeurosis). These observations suggest a link between the effects of aging on tendon mechanics and injury development, though future work will be critical to interpret these results in the context of injury and rehabilitation.

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