Pain in the lateral side of the arm is caused by the following: First, frozen shoulder: especially in patients who are around 50 years old, it is highly suspected to be such a disease, because frozen shoulder occurs in people around 50 years old, so it is also known as the 50th shoulder. Its main manifestation can be pain around the shoulder joint, typically in the outer part of the upper arm, but also in the armpit, in the front of the shoulder joint and in the scapular area. When frozen shoulder develops to a later stage, it can cause adhesions in the joint capsule, which can lead to limited active and passive movement of the joint. Second, shoulder impingement syndrome: It is mainly due to degeneration of the shoulder joint and subacromial osteophytes. When the patient abducts the shoulder, it will cause the bones below the greater tuberosity of the humerus and the acromion to hit each other, so it can manifest as pain on the lateral side of the shoulder, that is, the lateral side of the big arm.