The shoulder joint is the most flexible joint in the human body, but also the most unstable, it consists of the glenoid and the humeral head, a typical ball and socket joint, the shallow and flat glenoid only holds 1/3, 1/4 of the bulbous humeral head, even if there is a fibrocartilaginous ring to deepen the joint, the osseous structure of the shoulder joint is still very unstable, coupled with the thin and flaccid joints around the shoulder joint and the joint ligaments are few and weak, the stability of the shoulder joint mainly relies on the rotator cuff. The stability of the shoulder joint depends mainly on the rotator cuff. The rotator cuff, also called rotator cuff, is a group of muscles with similar functions, consisting of supraspinatus, infraspinatus, subscapularis, and teres minor, which originate from the scapula and are attached to the humeral head, and the muscle keys of these four muscles form a cuff-like structure at the anatomical neck of the animal bone. The rotator cuff is a cuff-like structure formed at the neck and mixed with the shoulder capsule to encircle the anterior, superior and posterior aspects of the shoulder joint. The rotator cuff plays an important role in maintaining the stability of the shoulder joint and the function of the shoulder joint. The rotator cuff also cooperates with other shoulder muscles to accomplish adduction and rotation in the same direction. The supraspinatus muscle in the rotator cuff plays an upward stabilizing role for the head of the animal bone. It is closely integrated with the joint capsule to form the most crucial part of the rotator cuff, which plays an initiating role in the abduction of the upper arm and assists the deltoid muscle in stabilizing the head of the humerus within the glenoid during the entire external rotation and flexion of the upper arm. Therefore, the supraspinatus has a special significance in the active movement of the shoulder joint. In addition, the infraspinatus and teres minor muscles stabilize the humerus posteriorly and externally rotate the humerus, whereas the subscapularis muscle internally rotates the humerus. The rotator cuff is located between two rigid structures, the rostrocapillary arch and the humeral head, consisting of the acromion, the rostrocapillary ligament, and the rostral eminence, and has an articular capsule on the deep side and a subacromial bursa on the superficial side. The space between the rotator cuff and the surrounding tissues is very narrow. When the shoulder joint is abducted, especially in an abducted position with slight internal rotation, the rotator cuff muscles and the subacromial bursa are subjected to constant compression, friction and tension by the humeral head and the acromioclavicular or rostroclavicular ligaments, which can easily lead to rotator cuff injuries. The dual role of rotator cuff and its special position constitute the anatomical and physiological reasons for rotator cuff injury. 2.The etiology of rotator cuff Rotator cuff disease is a kind of multi-source disorder caused by various etiologic factors. The etiologic factors associated with rotator cuff injury include: vascular factors, degeneration and aging, impingement syndrome, and traumatic factors, which combine in some way to cause rotator cuff tears. Clinical manifestations Pain: The most typical pain is night pain in the neck and shoulder area and pain aggravated when trying to lift the upper arm. Often located in the shoulder joint anterior and superior, sometimes to the neck and deltoid muscle under the upper limb radiation, the affected side lying position pain aggravated. Nighttime pain radiates to the neck and lower deltoid and upper extremity, and is worse when the affected side lies down. Weakness: abduction weakness, supination weakness, posterior extension weakness or inability to resist resistance. Restriction of active activities: pain and weakness limit the active activities of the shoulder joint. However, there is no obvious limitation of passive activities. Accompanied by supraspinatus and deltoid atrophy. Differential diagnosis of rotator cuff injury rotator cuff injury is easy to be misdiagnosed, especially the elderly, pay attention to differentiate with special cases. Frozen shoulder: generally around 50 years old, poor passive activity of the shoulder joint and extensive pressure points around the shoulder; while rotator cuff injury is generally passive activity. The pressure point is limited to supraspinatus and infraspinatus. Cervical spondylosis: the pressure pain generally radiates from the neck to the chest, and there are abnormalities in neck imaging; while the pressure pain of rotator cuff injury is at the supraspinatus terminus, and the pain is limited to the vicinity of the deltoid muscle. (iii) Biceps long head tendonitis: the pressure point is mainly in the biceps interosseous groove, and the pain is mainly worse when the upper limb is extended; while rotator cuff injury has typical pain, and the pain point is in the lifting and external rotation. 5, rotator cuff injury treatment method selection Conservative treatment is suitable for non-giant tear, especially less than 3 months after the injury, if the patient is older on the shoulder joint function requirements are not high, also applies to non-surgical treatment. The purpose of surgical treatment is to stop the pathological process, relieve pain and restore the function of shoulder joint. Common methods: ① Simple rotator cuff repair can repair the ruptured rotator cuff and joint capsule at the same time of internal fixation of fracture. ② Acromioplasty is the most common method to treat rotator cuff injury. ③ Arthroscopic surgery is an effective minimally invasive method. Care should be taken to prevent postoperative complications, such as re-tear, anterior dislocation of the shoulder joint, non-healing of the rotator cuff, axillary nerve injury, and anchor nail extraction.