I. Preface The interphalangeal joint of the thumb is deformed in flexion and extension, sometimes with a popping sound. The etiology is unclear. The tendon that flexes the thumb is called the flexor digitorum longus tendon, and there is a circular structure called the A1 carriage that encases this tendon, and the A1 carriage contractures, causing limited flexion of the thumb. Most are thought to be congenital. Incidence: about 3 per 1,000 children under 1 year of age. II. Manifestations Physical examination: Most present with flexion deformity of the thumb and rarely with trigger finger. A hard node can be palpated on the palmar side of the metacarpophalangeal joint of the thumb. Treatment Non-surgical treatment methods: clinical observation Non-surgical treatment indications: children within one year of age have a 30% chance of recovering on their own; children older than one year of age have a less than 10% chance of recovering on their own. Surgical method: A1 slide release Indication for surgery: failure to relieve flexion deformity after 12 months Complications: thumb nerve injury