Achilles tendon resting angle relates to tendon length and function

Abstract

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 rho. Sub-group analysis was used to compare subjects with less then or greater than one year following rupture.

Results

42 participants (mean(SD) of 18.2(35.9) months following rupture) were included. Tendon elongation related with relative ATRA with knee flexed (rho=0.491, p=0.001) and knee extended (0.501, p=0.001) positions. In individuals greater than 1 year following rupture, relative ATRA with the knee flexed related to shear modulus (rho=0.800, p=0.01) and total work on the heel-rise test (rho=-0.782, p=0.008) 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 one 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

III, case-control study