INTRODUCTION: Valid length measurements of the different segments of the Achilles tendon are needed in order to investigate if differential elongation of the Achilles tendon takes place after rupture. The purpose of this paper was to present data concerning the accuracy and reliability of an ultrasound measurement of the free part of the Achilles tendon.

METHODS: Both legs of 19 non-injured subjects were examined by magnetic resonance imagining (MRI) and ultrasound. The length from the distal tip of the soleus muscle to the tendon insertion on the calcaneus was measured by three independent ultrasound examiners. Repeated ultrasound measurements were performed and compared with MRI measurements. Intra-rater and inter-rater reliability and the agreement between MRI and ultrasound were determined. Data were evaluated using the intraclass correlation coefficient (ICC), the standard error of the measurement (SEM) and the minimal detectable change (MDC).

RESULTS: The measurement showed excellent intra-rater reliability (ICC = 0.94 (95% confidence interval (CI): 0.91-0.96), SEM = 5 mm and MDC = 13 mm) and inter-rater reliability (ICC = 0.96 (95% CI: 0.93-0.97), SEM = 4 mm and MDC = 11 mm). On average, ultrasound measurements exceeded the MRI measurements by 2 mm (non-significant), resulting in a measurement error of 5%.

CONCLUSIONS: The ultrasound measurement of the free part of the Achilles tendon showed good reliability and accuracy. For comparison between groups of non-injured subjects, differences of > 5 mm can be detected. For repeated assessment of individual subject differences ≥ 13 mm can be detected.

FUNDING: none.

TRIAL REGISTRATION: Institutional Review Board of Zealand, Denmark, Ref. no: SJ-318.