Behind the scapula there is a transverse ridge of bone called the scapular gonad. Along the scapular gonad outward and slightly upward to the highest point of the shoulder, a large and firm prominence can be felt, which is the scapular crest. Below the acromion is a subacromial joint that resembles a joint. Since some people refer to the acromioclavicular joint, which is made up of the humeral head and the shoulder pelvis, as the first shoulder joint, the subacromial joint is also referred to as the second shoulder joint. The subacromial joint (second shoulder joint) consists of the rostrocarpal arch (including the acromion, rostral process, and the rostrocarpal ligament between them) in the upper part and the rotator cuff and humeral tuberosity in the lower part, which is composed of the greater humeral tuberosity, rotator cuff, subacromial bursa, and rostrocarpal arch. Although the second shoulder joint does not have a typical joint structure, there are many anatomical similarities to the joint structure, such as the acromion, rostrocarpal ligament and rostral process are equivalent to the components of the articular socket, the greater tuberosity of the humerus is equivalent to the mortar and pestle portion of the joint, while the rotator cuff can be compared to the meniscus and the synovial cavity of the subacromial bursa can be compared to the joint cavity. It plays a very important role in shoulder abduction, supination and rotational activities similar to the function of the joint. In normal shoulder activities the rotator cuff muscles (especially the supraspinatus) and the long head of the biceps tendon are the main power structures that maintain the motor function of the subacromial joint. The subacromial bursa assists the muscles on the one hand and is the main lubricating, heat dissipating and stress absorbing device of the subacromial joint on the other hand, thus ensuring the smooth passage of the greater tuberosity of the humerus during abduction. The rostral arch, which is composed of the acromion, rostral process and rostral shoulder ligament, has a stabilizing function to prevent the humeral head from dislocating superiorly or posteriorly. When the above mentioned structures of the second shoulder joint become functional or structurally abnormal, it will affect the motor function of the shoulder joint, for example, bone spurs in the rostral arch area will easily cause rotator cuff tears and affect the function of the shoulder joint, and inflammation of the subacromial bursa will also produce shoulder pain and limited movement.