What is “frozen shoulder”? There are three answers: one is frozen shoulder, also known as “frozen shoulder”, which is the most correct answer. The second is widespread pain around the shoulder joint. This is being replaced by a more accurate diagnosis. The third type is the “wastepaper basket” diagnosis: pain around the shoulder joint, which is not clearly diagnosed and is labeled as “frozen shoulder”. The basic diagnostic criteria for frozen shoulder are still based on Codman’s five criteria of 1934: slow onset; pain around the deltoid stops of the shoulder joint, nocturnal pain; limited active and passive movement of the shoulder joint in all directions; negative x-ray; except for other known causes of shoulder pain, such as rheumatoid. The main criterion is a full range of active and passive activity restriction. It mainly occurs in 40-50 years old. According to its causes, it can be classified as idiopathic frozen shoulder, diabetic frozen shoulder, traumatic frozen shoulder, and post-surgical shoulder stiffness. Strictly speaking, the latter does not belong to the category of frozen shoulder. The relationship between frozen shoulder (FS) and diabetes mellitus (DM) has attracted a lot of attention. Many studies have reported that frozen shoulder is more prevalent and more severe in diabetic patients. The prevalence of frozen shoulder in the general population ranges from 2.3% to 5%, but increases to 10.8% to 36% in the diabetic population. Most idiopathic frozen shoulders are thought to heal spontaneously in about 1 year, while diabetic frozen shoulders are more persistent. Other endocrine disorders, such as hyper- or hypothyroidism and hypoadrenalism, can also trigger the development of frozen shoulder. In conclusion, frozen shoulder mainly presents with pain and limited movement, especially limited external rotation. However, clinically, frozen shoulder is often misdiagnosed as “subacromial impingement”, while rotator cuff tears, calcific tendonitis, arthritis, and even various tumors are often misdiagnosed as “frozen shoulder”, which seriously affects the work of the shoulder joint. Below we describe the differential diagnosis of each lesion and frozen shoulder by site. Injury, sclerosis and tumor of the deltoid muscle: there are three main characteristics: superficiality and change in shape; painful points and positive finger sign; and limitation of movement with pronation and abduction. Acromioclavicular joint: including injury, osteoarthrosis, calcification and inflammation (such as strong spine), etc. The main features are superficiality, clear pain points, positive finger sign; limitation of movement is obvious with horizontal adduction and abduction above 150 degrees. Calcific tendonitis: different sites, different manifestations. Calcification of the infraspinatus tendon, which shows normal external rotation and limited internal rotation, often requires Y-slice observation; calcification of the supraspinatus tendon, which shows limited abduction and forward flexion, but normal external rotation; the most difficult is calcification of the subscapularis tendon, which often shows completely similar to the frozen shoulder, with an overall decrease in range of motion, especially external rotation; and it is difficult to see the calcification on plain film due to overlap. However, the patient often has acute attacks with significant anterior elevation and internal restriction, which is different from frozen shoulder and should be further differentiated by CT or MRI. The glenohumeral joint lesion can appear exactly the same as the frozen shoulder, but each has its own characteristics. In the early stages of RA, it is difficult to differentiate. The diagnosis of tuberculous arthritis is difficult, with fast sedimentation or positive antibodies help to differentiate, but often the performance is atypical, cold pustules are a feature. gIRD is mainly in athletes, with limited internal rotation and horizontal internal rotation of the external booth, and basically normal external rotation. In contrast, in LHB (long head tendinitis, or partial tear of the biceps tendon), external rotation is mostly normal. Posterior shoulder dislocation, in both children and adults, is characterized by a general decrease in mobility, which is obvious in abduction and external rotation, resembling a frozen shoulder, but the anterior hollow posterior convexity of the shoulder is characteristic, and axillary axial position and CT can help to differentiate it. Ligamentous fibroma of the shoulder joint is a difficult task to differentiate, showing a general decrease in mobility, negative pain and plain films, and normal laboratory tests. However, it often occurs in young people with a long medical history and is characterized by painful, hard masses in the muscles surrounding the shoulder joint, especially in the subscapularis. Rotator cuff tears are often confused with frozen shoulder because they are both common after the age of 50 and both present with pain, nighttime pain, and pain or difficulty in lifting the shoulder, although they can coexist. In the case of large rotator cuff tears, there can be a decrease in the range of motion due to contracture of the joint capsule, which can make the diagnosis difficult.