Subacromial impingement syndrome: Caused by the morphology of the acromion, subacromial impingement syndrome, also called shoulder impingement syndrome, is most commonly caused by the soft tissues located between the acromion, rostroscapular ligament, and humeral head bumping against the acromion and rostroscapular ligament, resulting in aseptic inflammation of these soft tissues and causing pain and sometimes even impingement. The diseases that make up this syndrome include subacromial impingement bursitis, supraspinatus tendinitis, calcific tendinitis of the supraspinatus, tenosynovitis of the long head of the biceps, and degenerative rotator cuff tears, among many other pathological changes. Subacromial impingement syndrome is a common disorder, and a large proportion of previous frozen shoulders are shoulder impingement syndromes. The subacromial joint, also known as the second shoulder joint, is the main mobile joint in the shoulder joint. The acromion, rostrocarpal ligament and part of the rostral process form the rostrocarpal dome, below which is the humeral head, and between them is the subacromial space, through which the rotator cuff and the long head of the biceps tendon pass. The cause of impingement syndrome can be either a morphological problem of the acromion or a narrowing of the subacromial space due to subacromial bone growth. It is also believed that due to excessive shoulder abduction or long-term cumulative injury, the tissues within the gap become worn, and the repeated wear exacerbates the inflammatory response of the tissues, increasing the pressure within the gap and aggravating the impingement, ultimately leading to shoulder impingement syndrome. In conclusion, regardless of the narrowing of the subacromial space or the increase in the content of the subacromial space, as long as there is not enough space in the subacromial space, impingement will occur, resulting in impingement syndrome.