7 123 Normative Ultrasound Values for Achilles Tendon Thickness: An International Study INTRODUCTION Achilles tendinopathy (AT) is the preferred term for local tendon pain related to mechanical loading.1 It is frequently occurring (2-3/1,000 individuals)2, longstanding (20-30% persisting symptoms at 10-year follow-up)3,4, has a large impact on quality of life and is associated with substantial costs (840€/patient/year in a western European country).5 Ultrasound is the preferred method for imaging of the Achilles tendon according to the current guidelines.6,7 In the longitudinal plane the Achilles tendon exhibits a pattern of parallel fibrillar lines, while in the transverse plane, it presents as a round-to-ovoid echogenic shape.6 AT is ultrasonographically characterised by tendon thickening in the anterior-posterior direction and a decreased tendon structure.8,9 Imaging could aid in establishing the diagnosis of AT.10 Currently, maximum Achilles tendon thickness is estimated at approximately 6 to 7 mm based on clinical experience and crosssectional studies.6,8,11-15 An important knowledge gap with imaging is that current normative values for Achilles tendon thickness may not be representative of the general population and no studies differentiated between the midportion and insertional region of the tendon.16 Previous studies also showed a considerable deviation surrounding the normative values for Achilles tendon thickness.14 It is likely that tendon thickness is influenced by personal characteristics. Obtaining reference values for Achilles tendon thickness and addressing important personal characteristics will aid clinicians in differentiating between AT and ‘normal’ morphological changes, which will facilitate personalized healthcare. The primary aim of this study aim is to obtain ultrasonographic reference values of the Achilles tendon thickness (maximum anterior-posterior distance) in adults without (previous) Achilles tendinopathy. The secondary aim is to compare these reference values with tendon thickness in patients with clinically diagnosed AT. METHODS Study design The study was designed at the Erasmus MC University Medical Centre (Rotterdam, the Netherlands) in collaboration with the University of Leicester (Leicester, United Kingdom) and conducted at the outpatient departments of these universities from October 2020 to July 2023. The study was temporarily halted between November 2020 and May 2022 because of Covid-19 related restrictions. These restrictions also forced us to adjust the number of participants to 500, which is a decrease by 100 participants compared to the pre-defined protocol. The local Medical Ethics Committee (Southwest-Holland, the Netherlands) approved the study protocol (MEC-2020-0585). The trial was registered before commencement (Netherlands Trial Register, NL9010). We adhered to the Strengthening the
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