Real-time sonoelastography using an external reference material: test–retest reliability of healthy Achilles tendons

Objective: To establish the test–retest reliability of sonoelastography (SE) on healthy Achilles tendons in contracted and relaxed states using an external reference system. Materials and Methods: Forty-eight Achilles tendons from 24 healthy volunteers were assessed using ultrasound and real-time SE with an external reference material. Tendons were analyzed under relaxed and contracted conditions. Strain ratios between the tendons and the reference material were calculated. The intraclass correlation coefficient (ICC_2.k) and Bland–Altman plot were used to assess test–retest reliability. Results: The reliability of SE measurements under relaxed conditions ranged from high to very high, with an ICC_2.k of 0.84 (95 % CI: 0.64–0.92) for reference material, 0.91 (95 % CI: 0.83–0.95) for Achilles tendons and 0.95 (95 % CI: 0.91–0.97) for Kager fat pads (KFP). The ICC_2.k value for skin was 0.30 (95 % CI: –0.26 to 0.61). Reliability for measurements in the contracted state ranged from high to very high, with an ICC_2.k of 0.93 (95 % CI: 0.87–0.96) for reference material, 0.72 (95 % CI: 0.50–0.84) for skin, 0.93 (95 % CI: 0.87–0.96) for Achilles tendons, and 0.81 (95 % CI: 0.66–0.89) for KFP. Reliability of the strain ratio (tendon/reference) under relaxed conditions was high with an ICC_2.k of 0.87 (95 % CI: 0.75–0.93), and in the contracted state, it was very high with an ICC_2.k of 0.94 (95 % CI: 0.90–0.97). Conclusion: Sonoelastography using an external reference material is a reliable and simple technique for the assessment of the elasticity of healthy Achilles tendons. The use of an external material as a reference, along with strain ratios, could provide a quantitative measure of elasticity.