Square shoulder deformity refers to a deformity in which the shoulder joint, which is originally curved in shape, becomes square. There are several reasons for square shoulder deformity: 1. The most common cause of square shoulder deformity is dislocation of the shoulder joint. This refers to an injury to the shoulder joint where the humeral head has been displaced and separated from the scapula. Due to the dislocation of the humeral head, the entire lateral side of the shoulder joint is supported by the scapula and clavicle. The deltoid muscle on the lateral side of the shoulder joint was also distorted by the dislocated arm, making the scapula and clavicle more prominent. Visually, the shoulder joint changes from a rounded arc to a square shape.2. Local nerve lesions can also lead to a square shoulder deformity. Nerve damage or even paralysis can cause painful symptoms in the soft tissues around the shoulder joint. If the nerve damage is not corrected in time, the muscles around the shoulder joint will change from pain to paralysis, and eventually muscle atrophy will occur, causing the shoulder joint to form a square shape.