The elbow x-ray shows a dislocated radial head; the radial head is not in alignment with the capitellum leading to disruption of the radio-capitellar line.
Isolated radial head dislocation can be difficult to appreciate on the frontal view but easier on the lateral as in this case. Radio-capitellar line helps in the detection of these injuries. This line is drawn along the long axis of the proximal radius . The line should pass through capitellum on both AP and lateral view. If not, the radial head is dislocated.
Reference: Grainger & Allison’s Diagnostic Radiology, 6th Edition.