Posterior shoulder dislocation can be easily missed. In addition to being rare, the main reason is that the posterior shoulder dislocation shows the illusion that the head of the humerus is still in the shoulder pelvis, giving the impression that the shoulder is not dislocated. At this time, it is necessary to remember the typical X-ray manifestation of the ‘bulb sign’ so that the diagnosis will not be missed. On an orthopantomogram of the shoulder joint or scapula, you can see that the humeral head is rotated and dislocated in the same direction as the x-ray, which increases the translucency and forms a round light bulb, which is the typical light bulb sign. When we reset the shoulder joint, the humeral head rotates to its normal position, the translucency decreases and the light bulb sign disappears. Therefore, the bulb sign is a posterior dislocation of the shoulder joint.