Relationship Between Structure and Age in Healthy Achilles Tendons

Age is an important factor to consider with Achilles tendon injury, as variability in tendon structure during developmental growth and aging influence lower limb function and mobility. However, the overlap in structural alterations with aging and Achilles tendon injury makes it unclear which structural changes are related to age separate from tendon pathology. The objective of this study was to determine the relationship between structure and age in healthy Achilles tendons. Healthy Achilles tendons from 389 children and adults (8-79 years) were included in this retrospective analysis. Achilles tendon morphology was assessed via B-mode ultrasound of Achilles tendon length, cross-sectional area (CSA), and thickness. Mechanical properties of Achilles tendon shear modulus and viscosity were assessed via continuous shear wave elastography. The relationship between Achilles tendon structure and age was determined using General Linear Models and White's test of heteroscedasticity (to assess for unequal variance across the age span), controlling for sex, weight, and physical activity level. Healthy free Achilles tendon length (p = 0.002), thickness (p < 0.001), CSA (p < 0.001), and viscosity (p = 0.009) increased with age, supporting age-related changes in tendon structure that may limit its capacity to store and transfer energy in older adults. Full Achilles tendon length and CSA varied across the age span (p < 0.05), suggesting the Achilles tendon undergoes natural aging processes seen with most musculoskeletal tissue. Normative data on Achilles tendon structure with age will contribute to our understanding and interpretation of Achilles tendon injury pathogenesis; aiding in the design of injury prevention and treatment strategies.