Dislocation is a joint dislocation, and the joint most likely to be dislocated is the shoulder joint, which accounts for 45% of all joint dislocations. Moreover, it is still possible to have a recurrence after resetting and become a habitual dislocation. In some cases, the dislocation may recur during exercise, and in some cases, it may recur in sleep, leading to a long-term fear of “boots that never hit the ground” in the mind. Once an athlete suffers from this disease, it is the end of his or her athletic career. To understand why the shoulder joint is habitually dislocated, it is important to understand how it maintains stability. The shoulder joint is characterized by a “large head with a small glenoid and a shallow base”, which allows the humeral head to move over a wide range of motion, but freedom always comes at a price, and this is the “original sin” of potential instability in the shoulder joint. Because of the lack of bony accommodation of the humeral head by the scapular glenoid, its stability can only take the path of “softness over strength”, relying more on soft tissues such as muscles, glenoid lips and ligaments. The stabilization mechanism is more like tai chi, which is all about balance and coordination. First of all, the four muscles of the rotator cuff hold the humeral head in three directions: front, top and back, and their combined force forms the net force of the humeral joint. In addition, the scapular glenoid is surrounded by a circle of glenoid labrum and joint capsule ligament complex, forming a bowl-like structure with a hard center and soft periphery, deepening the glenoid fossa, in which there is a small amount of joint fluid bonded to produce negative pressure to suck the humeral head, which is known as the glenohumeral suction cup mechanism. In addition, the glenoid labrum also has proprioception, which tells you when the range of motion of the shoulder joint is too great to exceed the limit: Dude, take it easy! When the shoulder joint is first dislocated, the glenoid labrum and rotator cuff often tear (tearing of the glenoid labrum is also called Bankart injury), and even tear off a piece of the bony scapular glenoid to which they are attached, forming a bony Bankart injury. The torn glenoid labrum joint capsule complex is often difficult to heal because of contracture, forming a permanent fissure in the front, or a deformed healing below the anterior scapular glenoid, when the shoulder joint is abducted and externally rotated. When the shoulder joint is abducted and rotated, the “net glenohumeral joint force” falls to the position of the anterior and inferior weak points, then anterior dislocation of the shoulder joint occurs. After dislocation, the humeral head is stuck in the anterior inferior part of the scapular glenoid, and the bones of the posterior superior humeral head and the anterior inferior part of the scapular glenoid squeeze each other, forming a “kissing injury”. This is a typical case of “love and hurt each other”, called “bipolar bone defect”, and patients with this bipolar bone defect are more likely to have the head of the humerus come out of the motion track on the scapular glenoid during shoulder movement, which is called “off track” of the humeral head. “The patient is more prone to dislocation of the shoulder joint. Studies have shown that the younger the patient is when the initial dislocation occurs, the more likely it is to recur. For example, after an anterior shoulder dislocation at <20 years of age, the likelihood of recurrent dislocation is 90%, and the risk of recurrent dislocation is 12.7 times higher than in patients >20 years of age! So you can say this: young sports loving people, once a shoulder dislocation occurs, have a very high chance of becoming habitually dislocated if they do not receive timely treatment! Sometimes, the instability is not secondary to trauma or is caused by only a minor injury, and the patient often cannot say exactly why the first dislocation occurred. In such patients, the scapular pelvis tends to be flatter, the joint capsule more flaccid, and the neuromuscular control poor, often producing instability in multiple directions, and once dislocated, often entering a cycle of instability. In addition, impaired neuromuscular control such as encephalitis, cerebral palsy, upper brachial plexus birth palsy, stroke, and epilepsy can also easily lead to shoulder dislocation. Other patients have a willingness to perform shoulder dislocations due to psychological factors, which is known as random dislocation. Non-traumatic dislocations are more difficult to manage and more likely to recur than trauma-induced dislocations. Schematic diagram of a bipolar shoulder injury causing derangement of the humeral head: a, where the Hill-sachs injury posterior to the humeral head is within the scaphoid glenoid track, b, where there is also a large scaphoid glenoid defect, where the Hill-sachs injury is outside the scaphoid glenoid track causing derangement of the humeral head. When you have similar shoulder discomfort, do not believe in “prescriptions” or “experience”, but go to a regular hospital and ask a specialist.