Achilles Tendon Resting Angle Relates to Tendon Length and Function.

Background: Following Achilles tendon rupture, tendon elongation leads to long term deficits in calf function. A surrogate measure of Achilles tendon length, Achilles tendon resting angle (ATRA), has been described but has not been validated against length measured using ultrasound. Therefore, the purpose of this study was to validate the ATRA against ultrasound. Secondarily, this study aimed to identify the relationship of other factors (tendon mechanical properties, heel-rise test performance) to the ATRA.

Methods: Individuals following unilateral Achilles tendon rupture were included. ATRA was measured in knee flexed and extended positions. Tendon elongation was measured using extended field of view ultrasound imaging. Continuous shear wave elastography quantified tendon mechanical properties. The relationship between variables was tested using Spearman's ρ. Subgroup analysis was used to compare subjects with less then or greater than 1 year following rupture. A total of 42 participants (with a mean of 18.2 months following rupture [SD = 35.9]) were included.

Results: Tendon elongation related with relative ATRA with knee flexed (ρ = .491, P = .001) and knee extended (ρ = 0.501, P = .001) positions. In individuals greater than 1 year following rupture, relative ATRA with the knee flexed related to shear modulus (ρ = .800, P = .01) and total work on the heel-rise test (ρ = -.782, P = .008) relative to the uninjured side.

Conclusion: Relative ATRA in both knee flexed and knee extended positions has a moderate relationship to tendon elongation within the first year following rupture. After 1 year, the relative ATRA with knee flexed may be a better indicator of tendon elongation and also related to tendon mechanical properties and heel-rise test performance.

Level Of Evidence: Level III, case-control study.