Evaluation of Achilles tendon stiffness during contraction using strain sonoelastography

Purpose: To evaluate Achilles tendon stiffness using strain sonoelastography with an external reference material during an isometric ramp contraction. Methods and Materials: Ultrasound and strain sonoelastography examinations were performed on 37 healthy participants using a commercially available ultrasound system (MyLab ClassC, Esaote, Genoa, Italy) with a linear transducer (LA533) in the frequency range 3–13 MHz. Strain sonoelastography, using an external reference material made with Zerdine® (CIRS, Norfolk, VA, USA) with known elasticity properties, was performed in the longitudinal plane at the level of the medial malleolus. The experimental protocol was tested and validated in previously published work. The external reference material was used to provide a comparison between the examined tissue and a material with a known elasticity. Stiffness was computed according to the sonoelastography color scale as the strain ratios between the tendon and reference material. Sonoelastography images were captured during rest and during a series of isometric contractions of the gastrocnemius muscle at different force levels (0.5kg, 1kg, 2kg, 5kg, 10kg.) The mean and standard deviation of the strain ratios for all subjects across the different conditions were calculated and the Friedman test with post hoc pairwise comparison were used to determine the correlation between the different contraction levels. Results: No significant differences were found for Achilles tendon stiffness between men and women (p=0.6) or between the left and right Achilles tendon (p=0.5), therefore the data were pooled for further analysis. The mean (±SD) values for the strain ratio was 1.98 (±1.09) in a relaxed state and 1.32 (±0.63), 1.07 (±0.55), 0.83 (±0.37), 0.49 (±0.19) and 0.33 (±0.10) for the contractions with 0.5 kg, 1 kg, 2 kg, 5 kg and 10 kg respectively. Friedman test for related samples showed significance (p=0.000) and the post hoc pairwise comparison revealed significant differences (p