122 Chapter 7 ABSTRACT Objectives: To obtain adjusted ultrasonographic reference values of the Achilles tendon thickness (maximum anterior-posterior distance) in adults without (previous) Achilles tendinopathy (AT) and to compare these reference values with AT patients. Methods: 600 participants were consecutively included, comprising 500 asymptomatic individuals and 100 patients with clinically diagnosed chronic AT. The maximum tendon thickness was assessed using Ultrasound Tissue Characterization. A multiple quantile regression model was developed, incorporating covariates (personal characteristics) that were found to have a significant impact on the maximum anterior-posterior distance of the Achilles tendon. A 95% reference interval (RI) was derived (50th, 2.5th-97.5th percentile) Results: In asymptomatic participants median (95%RI) tendon thickness was 4.9(3.8-6.9) mm for the midportion region and 3.7(2.8-4.8) mm for the insertional region. Age, height, Body Mass Index and sex had a significant correlation with maximum tendon thickness. Median tendon thickness for the midportion region was calculated with the normative equation -2.1+AGE*0.021+HEIGHT*0.032+ BMI*0.028+SEX*0.05. For the insertional region the normative equation was -0.34+AGE*0.010+ HEIGHT*0.018+BMI*0.022+SEX*-0.05. In the equations, SEX is defined as 0 for males and 1 for females. Mean (95%CI) difference in tendon thickness compared to AT patients was 2.7 mm (2.3-3.2, p<0.001) for the midportion and 1.4 mm (1.1-1.7, p<0.001) for the insertional region. Compared to the asymptomatic population 73/100 (73%) AT patients exhibited increased tendon thickening, with values exceeding the 95% RI. Conclusions: This study presents novel reference values for the thickness of midportion and insertional region of the Achilles tendon, which were adjusted for personal characteristics. Our novel web-based openly accessible calculator for determining normative Achilles tendon thickness (www.achillestendontool.com) will be a useful resource in the diagnostic process.
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