6 113 Ultrasonographic Achilles Tendon Thickness: Insertional vs. Midportion Reliability Excellent reliability for measuring Achilles tendon thickness Previous systematic reviews on the reliability of Achilles tendon thickness measurements using conventional US reported wide ranges for intra-rater (0.78-0.99 vs. 0.96 for the current study) and inter-rater (0.68-0.99 vs. 0.93 for the current study) ICC values.14,15 These studies did not distinguish between the midportion and insertional region, did not use a standardized US procedure, such as the UTC, and did not include both AT patients and asymptomatic individuals, which may account for the discrepant findings. Notably, the SRD values observed in the current study were relatively higher (1.22-1.47 mm for midportion AT and 1.51-2.46 mm for insertional AT) than those reported in previous investigations (ranging between 0.007 and 0.84 mm).14,15 This may be attributed to the heterogeneity of the study population evaluated as the previously mentioned studies only reported SRD values for the midportion region and the majority only included asymptomatic individuals. A study by Docking et al (2016) did use UTC and included both AT patients and asymptomatic individuals and reported a minimal detectable change (MDC) of 0.5 mm.11 However, this was only based on intra-observer agreement after scanning eight Achilles tendons and without distinguishing between the midportion and insertional region.11 Lower reliability for measuring thickness of the insertional Achilles tendon region We observed that the ICC values for Achilles tendon thickness measurements were lower for the insertional region compared to the midportion. To our knowledge, this is the first study to comprehensively evaluate thickness measurements in the insertional region, as previous investigations have focused solely on the midportion.9,12,14,15,23,27 The diminished reliability of thickness measurements in the insertional region may be attributed to the unique anatomical properties of this region. Specifically, the insertional part of the tendon has a less straight course compared to the midportion. The appearance of tissuestructures on ultrasound are angle-dependent, a phenomenon referred to as anisotropy, where tissue structure might appear to be hypoechoic due to the positioning of the ultrasound probe.7,28,29 The ‘rotated’ trajectory of the insertional region is more susceptible to anisotropy which may lead to angle-generated artifacts on ultrasonographic images, potentially resulting in reduced measurement reliability.7,29 Our study is the first to show that it is likely that patients with midportion AT will be identified with increased thickness of the Achilles tendon midportion. However, the measurement error in the insertional area may be too large to detect a change in thickness between patients and asymptomatic individuals as the SRD values exceed the mean difference between insertional AT patients and asymptomatic individuals.
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