Inability to carry the hand behind the back can be caused by shoulder trauma, rotator cuff tear, frozen shoulder and so on. 1. Shoulder trauma: Shoulder soft tissue injury caused by trauma, or shoulder fracture, can lead to shoulder and upper arm rotational function limitation, it is difficult to carry the hand to the back, and need to consult a doctor for X-ray examination to clarify the injury situation. 2. Rotator cuff tear: When the supraspinatus and anterior rotator cuff muscles are partially torn or completely broken, it can lead to the internal rotation dysfunction of the arm and make it difficult to carry the hand behind the back. Further examination can be done through physical examination and MRI. 3. Frozen shoulder: In severe frozen shoulder, the movement of the shoulder joint in all directions is impaired, which may lead to the inability to carry the hand behind the back, manifesting as severe internal rotation dysfunction of the arm. X-ray and MRI examination of the shoulder joint can be performed to confirm the diagnosis. When the symptoms of inability to carry the hand behind the back appear, timely medical consultation is necessary to help clarify the condition and improve the discomfort as soon as possible.