9 163 General discussion increasingly important in clinical practice to enhance the consistency of tendon geometry measurements.32-34 Ultrasound Tissue Characterization (UTC) is a customized tracking and ultrasonographic data-collection device designed to facilitate these standardized measurements.31,35 In a comprehensive reliability study, we included 50 patients with Achilles tendinopathy and 50 asymptomatic individuals, demonstrating excellent reliability in measuring tendon thickness using UTC. Additionally, we showed that measurements of tendon thickness obtained through UTC can be reliably translated to conventional ultrasound. However, we observed significantly lower intra-class correlation coefficients when assessing the reliability of thickness measurements in the insertional region compared to the midportion. This study is the first to comprehensively evaluate thickness measurements in the insertional region, as previous research had primarily focused on the midportion. Until recently, a value of 6 mm was being accepted as reference standard for maximum anterior posterior distance of the midportion region.29,35,36 However, a significant gap in our understanding of imaging lay in the absence of reference values for Achilles tendon thickness within the general population. In a large international cross-sectional study we presented normative values for Achilles tendon thickness on ultrasound examination, adjusted with personal characteristics. Our findings reveal that age and height have the most substantial impact on the maximum anterior-posterior distance. The mean difference in tendon thickness between asymptomatic persons and patients with Achilles tendinopathy was 2.7 mm for the midportion region and 1.4 mm for the insertional region. Notably, this difference of 1.4 mm in thickness in the insertional region is smaller than the Smallest Real Difference (SRD) of 1.5 mm for intra-rater reliability we observed in insertional Achilles tendinopathy patients in our reliability study. This questions the use of ultrasound when evaluating tendon thickness in patients with insertional Achilles tendinopathy. Previous studies reported lower SRD values (ranging between 0.007 and 0.84 mm), but these studies solely evaluated the midportion region and the majority only included asymptomatic individuals.32,37 Thus, this study raises the question whether characteristic changes indicative of tendinopathy can be observed in all Achilles tendinopathy patients, especially for patients with insertional Achilles tendinopathy. In patients with midportion Achilles tendinopathy, however, the SRD was smaller than the observed difference between patients and asymptomatic individuals. Using the normative equations for the median, lower (2.5th) and upper (97.5th) values of tendon thickness for each Achilles tendinopathy patient in our large international study, we found that 27% of the patients that were clinically diagnosed as having Achilles tendinopathy did not have increased tendon thickening outside the 95% reference interval. In addition, ‘‘morphological changes’ (changes characteristic for tendinopathy) are present in up to 25% of asymptomatic Achilles tendons.38 While imaging, such as ultrasound or UTC,
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