1.What is acromioclavicular impingement? Acromioclavicular impingement is a pathological change in the rotator cuff near the acromion caused by friction and impingement, resulting in inflammation and injury, and clinical symptoms such as localized pain, decreased mobility, and weakness are often manifested. It is common in middle-aged and old people. Degenerative changes (aging) is the most common cause. 2.What are the factors leading to acromioclavicular impingement? (1) Anatomical abnormalities: including narrowing of the subacromial space and increase in the volume of the contents. (2) Exercise mode:Excessive repetitive rotational activities of the shoulder joint cause the rotator cuff and other structures to move back and forth under the rostral acromial arch, leading to inflammatory changes in the relevant structures and producing impingement. (3) Joint instability:induced by increased anterior superior displacement of the humeral head due to shoulder joint instability. 3.What are the clinical manifestations? Shoulder pain is the most important symptom of impingement. Primary subacromial impingement is often associated with nocturnal pain. Subacromial impingement pain is located in the anterior lateral shoulder, shoulder abduction and lifting can cause pain; subacromial impingement pain is located in the anterior medial shoulder, most often occurs in the shoulder anterior flexion and internal rotation; secondary impingement (internal impingement) is located in the posterior aspect of the shoulder, most often occurs in the over-the-top sports, such as throwing athletes. Physical examination reveals abnormal joint mobility, rotator cuff muscle strength, impingement sign and glenohumeral joint stability; positive impingement test test. 4. What type of imaging should be chosen? X-ray: X-ray of the shoulder joint in anterior-posterior position, axillary position and supraspinatus exit position to observe the abnormal shape of glenohumeral joint, acromioclavicular joint and acromion. Magnetic resonance imaging (MRI) of the shoulder joint: In addition to the above, it can clarify whether soft tissue injuries such as rotator cuff injuries are combined. Arthroscopy: It can see the fiber abrasion on the subacromial surface due to impingement or the rotator cuff abrasion or tear on the opposite surface, which has the value of confirming the diagnosis, and at the same time, it can be treated by minimally invasive surgery under arthroscopy. 5.What are the treatment methods? Conservative treatment methods include: non-steroidal anti-inflammatory drugs; closed injections; physical therapy; exercise rehabilitation training. Surgical treatment: For cases that are ineffective after 3-6 months of formal treatment, subacromial decompression surgery can be chosen (there are two kinds of traditional incision surgery and arthroscopic subacromial decompression surgery). Arthroscopic subacromial surgery is the most common choice of surgical method because of its minimally invasive and reliable effect.