There are many reasons for the protruding bone at the bend of the arm, such as tendon sheath cyst of elbow joint, posterior dislocation of elbow joint, elbow fracture and so on. 1. Tendon sheath cyst of elbow joint: tendon sheath cyst can be formed when the elbow joint is overworked or suffered from trauma, so it will lead to protruding bone at the bend of the arm, accompanied by slight pressure and pain. 2. Posterior dislocation of the elbow joint: the elbow joint can be dislocated due to trauma, which will lead to the protruding of the bone at the bend of the arm. 3. Elbow joint fracture: the elbow joint suffers from impact and other traumatic injuries, which can lead to localized fracture, resulting in the elbow deltoid joint disorders, and therefore the arm bend bone protrudes. There are many reasons for the protruding bones at the bend of the arm, and after the symptoms appear, it is recommended to go to the hospital in time to improve the examination and take standardized treatment according to the doctor’s instructions, and not to treat the problem privately, so as to avoid the adverse consequences.