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„Enthesitis-Related Arthritis
(...) Entheses are the attachments of the tendons, ligaments, or joint capsules to bone. These can be tender even in healthy children, but usually ≥3 tender entheses are associated with disease (see Table 53.1). ERA often occurs in boys 8 years of age and older.1,8 They typically complain of joint pain related to playing sports, but there is also morning stiffness and pain that gets better during the day and worsens toward the end of the day or after engaging in a lot of activities.”

https://www.sciencedirect.com/topics/medicine-and-dentistry/...

tender adjective (PAINFUL)

(of part of the body) painful, sore, or uncomfortable when touched:
My arm was very tender after the injection.”

https://dictionary.cambridge.org/dictionary/english/tender


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Note added at 1 hr (2022-09-15 19:18:35 GMT)
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„In some of the tender entheses without ultrasound signs of enthesitis, we could identify other origins of the pain than enthesitis (subtalar synovitis, tenosynovitis and retrocalcaneal bursitis). Thus, this study underlines the value of ultrasound for establishing the origin of pain around the heel in patients with SpA...”

https://www.google.com/search?q=Ultrasound of the Heel Impro...