Tenosynovitis of the thumb in children is stenosing tenosynovitis of the thumb, also known as trigger thumb, which manifests as flexion and deformity of the interphalangeal joint of the thumb and is caused by thickening and narrowing of the tendon sheath of the thumb long flexor muscle in the metacarpophalangeal joint area and local acquired nodular enlargement of the flexor tendon. Sometimes the interphalangeal joint can be forced to straighten temporarily, but it soon returns to the flexed position, and there can be local pain when the joint is passively extended. For stenosing tenosynovitis of the thumb, only observation is needed in the early stages. If the localized swelling and limitation of joint movement persist, surgical treatment is required.