In our daily life, shoulder pain is often thought of as frozen shoulder, and we are familiar with it. So what exactly is frozen shoulder? There are three answers to this question: one is frozen shoulder, also known as “fifty shoulder” (abbreviated as FS), which is the most correct answer. The second is widespread pain around the shoulder joint, which 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, is diagnosed as “frozen shoulder”. The diagnostic criteria for frozen shoulder: basically, the five criteria of Codman in 1934: slow onset; pain around the deltoid stop of the shoulder joint, nocturnal pain; limitation of 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 comprehensive, active and passive activity restriction. It mainly occurs in the age group of 40-50 years. According to its causes, it can be divided into idiopathic frozen shoulder, diabetic frozen shoulder, traumatic frozen shoulder, and post-surgical shoulder stiffness, which strictly speaking does not belong to the category of frozen shoulder. The relationship between frozen shoulder and diabetes mellitus 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. Idiopathic frozen shoulder is thought to heal spontaneously in most cases in about 1 year, while diabetic frozen shoulder is 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”, and some patients have delayed treatment, which may even lead to shoulder joint function. Some patients have delayed treatment, which may even lead to functional disability of the shoulder joint and seriously affect their daily life. Below we describe the differential diagnosis of each lesion and frozen shoulder by site. 1.Injury, sclerosis and tumor of the deltoid muscle: three main characteristics: superficiality and change in shape; painful points are clear and positive; and limitation of movement is obvious in adduction and abduction. 2.Shoulder lock joint: including injury, osteoarthrosis, calcification and inflammation (such as strong spine), etc. The main characteristics are superficial, painful points are clear and positive; limitation of movement is obvious by horizontal inversion and abduction of more than 150 degrees. 3, calcific tendonitis: different sites, different performance. Calcification of the infraspinatus tendon, which shows normal external rotation and limited internal rotation, often requires Y-position film for observation; calcification of the supraspinatus tendon, which shows limited abduction and forward flexion and 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 in plain film due to overlap. However, the patient often has an acute attack with significant anterior elevation and internal restriction, which is different from frozen shoulder and should be further differentiated by CT or MRI. 4. Glenohumeral joint lesions can be identical to frozen shoulder, but each has its own characteristics. There are changes in the plain films of osteoarthrosis. In the early stage of RA, it is difficult to differentiate. Swelling of the shoulder joint is characteristic, often bilateral, with morning stiffness, and laboratory tests help to differentiate. The diagnosis of tuberculous arthritis is difficult, with fast blood sedimentation or positive antibody helps to differentiate, but the performance is often atypical, and cold pustules are a feature. 5, GIRD is mainly athletes, external booth internal rotation and horizontal internal rotation is limited, external rotation is basically normal. 6, LHB (long head tendonitis of biceps tendon, or partial tear), external rotation is mostly normal. 7, Posterior dislocation of shoulder joint, no matter children or adults, there is an overall decrease in mobility, abduction and external rotation is obvious, similar to frozen shoulder, but the anterior hollow posterior convexity of shoulder is characteristic, axillary axial position and CT can help to differentiate. 8. Ligamentous fibroma of the shoulder joint is a difficult point of differentiation, showing an overall decrease in mobility, negative pain and plain films, and normal laboratory tests. However, it often has a long history, and the muscles around the shoulder joint, especially the subscapularis, are characterized by painful and hard masses, which often occur in young people. 9. Rotator cuff tears are often confused with frozen shoulder because they are both common after the age of 50 and are characterized by pain, nighttime pain, and pain or difficulty in lifting the shoulder, although both can exist at the same time. A huge rotator cuff tear, on the other hand, can also present as a decrease in range of motion due to contracture of the joint capsule, making the diagnosis somewhat difficult. 10. In cervical spondylosis, the sensory nerves of the skin of the shoulder come from C3 and C4 nerve roots, and the lateral cutaneous nerves of the upper arm come from C5 and C6, while the deep sensory, including the joint capsule, and the sensory nerves distributed by the ligaments come from C5 to C8 nerve roots. Therefore, nerve root damage caused by cervical degeneration or cervical disc herniation, symptoms can involve the shoulder. The main manifestations are neck pain, neck stiffness, with one side of the shoulder, upper limb pain or radiating pain in the upper arm and forearm. 11.Pulmonary tumor, such as Pancoast tumor, also known as apical lung tumor syndrome, refers to a group of symptoms of persistent pain in the upper limbs and ipsilateral Horner’s syndrome caused by tumor infiltration and compression in the apical part of the lung. 12.Thoracic outlet syndrome: It refers to the comprehensive symptoms caused by the compression of brachial plexus nerve and subclavian artery and vein at the thoracic outlet and the rostral attachment of pectoralis minor muscle. It can be caused by cervical rib, congenital hypertrophy of the attachment of the anterior oblique muscle, congenital incomplete separation of the anterior and middle oblique muscles, which will reduce the outlet and squeeze the subclavian artery and brachial plexus nerve. This includes what used to be called cervical rib syndrome, anterior rhomboid syndrome, clavicular rib syndrome, and hyperextension syndrome. A positive Adson’s test (head rotation to the posterior or simultaneous upper extremity elevation with diminished to absent radial artery pulsation) is usually complained of unilateral shoulder and arm pain, numbness and weakness in the arm, sometimes with the presence of cervical ribs. Special physical symptoms can be differentiated from frozen shoulder.