112 Chapter 6 Table 3. ICC values for standardized and conventional US procedures. Abbreviations: UTC: Ultrasound Tissue Characterization, US: Ultrasound, ICC: intraclass correlation coefficient, CI: Confidence interval, SEM: standard error of measurement, SRD: smallest real difference Reliability between standardized UTC and conventional US procedures ICC (95% CI) SEM (mm) SRD (mm) Achilles tendinopathy patients (total, n=49) 0.95 (0.91 – 0.98) 0.681 1.89 Midportion (n=25) 0.96 (0.92 – 0.98) 0.577 1.60 Insertion (n=24) 0.81 (0.62 – 0.91) 0.819 2.27 DISCUSSION This is the first large-scale study to evaluate the reliability of Achilles tendon thickness measurements using a standardized US procedure for both the midportion and insertional region in AT patients as well as asymptomatic individuals. Overall, our findings indicate a high level of agreement between and within observers with respect to thickness measurements of the Achilles tendon. This observation holds true for both individuals who suffer from AT and those who are asymptomatic. Lower ICC and higher SRD values were observed for the thickness measurements in the insertional region when compared to the midportion. The reliability of thickness measurements between the standardized UTC procedure and conventional US was excellent for the midportion region and good for the insertional region. Clinical relevance These findings are relevant for the clinical setting, as current guidelines advise performing US as the first imaging modality of choice in the diagnostic process of patients with AT. For this reason, it is important to know the reliability of measuring Achilles tendon thickness in specific regions (the midportion and insertional region) where pathology is frequently observed. It is also relevant to have more knowledge of the reliability when using standardized US procedures, such as UTC, since these are gaining popularity in the clinical setting.16,17 When US is used to monitor change in tendon diameter it is important to verify that changes exceed the SRD to be of relevance. This is illustrated when considering the range of SRD values observed in our study, which lie between 0.8 and 2.5 mm. This range must be interpreted in the context of the absolute mean values of Achilles tendon thickness, which we found to be between 4.3 and 8.3 mm. The SRD values represent a threshold for clinically meaningful changes in tendon thickness. When a change in tendon thickness less than the SRD is observed, it may be considered within the margin of measurement error.
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