Widening of the acromioclavicular joint space refers to the presence of varying degrees of dislocation, osteoarthrosis, and space-occupying diseases in the acromioclavicular joint space. The human anatomy of the shoulder joint is composed of the glenoid of the scapula and the head of the humerus, and the gap between the two is the acromioclavicular joint space. If the acromioclavicular joint is damaged or diseased, widening will occur. 1. Dislocation: when there are different degrees of dislocation of the acromioclavicular joint gap, such as subluxation and dislocation, the gap will be widened. 2. Osteoarticular lesions: such as ligament laxity, muscle laxity in the elderly, etc., which will reduce the pulling force on the bones, thus causing widening of the gap. 3. Occupational diseases: such as inter-articular tuberculosis, cysts, etc., can also cause the gap to widen because they occupy the position of the gap. In addition to the above causes, acromioclavicular joint tumors, trauma, etc. can also cause the above situation. It is recommended to go to the hospital for timely consultation, complete the examination to clarify the cause of the disease, and then give targeted treatment or therapy.