”Subacromial impingement syndrome” and “rotator cuff injury” are the most common causes of shoulder pain. A study found that among elderly people over 60 years old who visited the doctor for shoulder pain, the proportion of rotator cuff injury was as high as 60%, and its incidence was much higher than that of frozen shoulder. So, how much do you know about rotator cuff injury and subacromial impingement syndrome? In this issue, I will focus on some general knowledge about rotator cuff injury and subacromial impingement syndrome, one of the most common causes of shoulder pain. What is “rotator cuff”? It is a common phenomenon that we see in our daily outpatient clinics. Patients present to the doctor with shoulder pain and significantly reduced range of motion, especially difficulty lifting the arm, and the doctor will often tell you that you may have frozen shoulder. The doctor will often tell you that you may have frozen shoulder, as if the incidence of frozen shoulder is the highest of all shoulder disorders. But in fact, this is due to the fact that there is still a large misunderstanding of shoulder disorders. According to the American Sports Medicine Association, frozen shoulder is a self-limiting disease that is actually a form of adhesive capsulitis. The incidence of true frozen shoulder is relatively low, and rotator cuff injuries are the most prevalent shoulder disorder, followed by subacromial impingement syndrome. Generally speaking, the rotator cuff consists of the tendons of the supraspinatus, infraspinatus, teres minor and subscapularis muscles, which are wrapped around the shoulder joint, just like the rotator cuffs of the clothes we wear, hence the name. What is a “rotator cuff injury”? A rotator cuff injury is a very common degenerative condition of the shoulder joint that occurs in direct correlation with age. The symptoms of a rotator cuff tear are similar to those of subacromial impingement syndrome, but are accompanied by weakness in shoulder abduction. Athletes, those who lift heavy objects, and those who suffer from traumatic injuries are prone to rotator cuff injuries. Typical symptoms are pain in the neck and shoulder at night and pain in the arm when lifting; sometimes they are afraid to sleep on the affected side and even wake up with pain; the shoulder joint can be weak when abducting, lifting or posterior extension, and sometimes there are difficulties even in personal hygiene, which seriously affects the patient’s life. It is mostly seen in men over 40 years of age, and most of them have a history of serious trauma if they are young. Since the rotator cuff is protected by the rotator cuff, direct violence rarely causes rotator cuff rupture. Indirect violence is mostly due to the degenerative changes of the rotator cuff with age and the abduction of the upper limb and the sudden inversion of the palm of the hand, especially because of the weakness of the supraspinatus muscle, which is subjected to the greatest pulling force and is therefore prone to rupture, accounting for about 50%. Rotator cuff injuries can be divided into two categories, partial rupture and complete rupture, depending on the degree of rupture. If not treated properly, partial rupture can develop into complete rupture. Clinical diagnosis is made by asking whether there is any history of trauma, injury and treatment, examining the active and passive range of motion of the shoulder joint, whether there is shoulder abduction pain and subacromial pressure pain, shoulder X-ray, ultrasound and MRI, etc. What is “subacromial impingement syndrome”? Typical symptoms: Pain when the shoulder is lifted to a certain angle or position, resulting in fear of movement in a certain direction. When the subacromial impingement syndrome is caused by the formation of subacromial bones, inflammation of the bursa, or rotator cuff injury, the gap between the acromion and rotator cuff shrinks further during shoulder abduction and elevation, resulting in impingement. The main causes of subacromial impingement syndrome are: (1) repetitive shoulder lifting activities such as swimming, weight lifting, tennis, and throwing sports; (2) structural changes in the shoulder, such as formation of bony redundancy and thickening of ligaments; (3) bleeding and edema after rotator cuff injury and inflammation of the bursa. Common clinical diagnostic criteria: shoulder pain, which can radiate to the upper arm and worsen at night, and limited shoulder movement in the late stage; certain specific movements can trigger shoulder pain, most commonly during abduction and supination; some patients have obvious pain arcs or pain angles, i.e., pain can be induced when the shoulder joint is abducted at a certain angle; positive impact test on physical examination; X-ray examination can observe shoulder peak morphology and bone superfluous formation, and sometimes can find and MRI and ultrasound can detect rotator cuff injury. On X-ray, there are three types of shoulder crests: type Ι flat, type II curved, and type III hooked; types II and III are more prone to impingement.