The radiocapitellar line is one of the key lines used to assess alignment on the elbow radiograph. It is particularly useful in the paediatric setting.
A line drawn down the neck of the radius should intersect the capitellum. It is important to ensure that you draw the line down the radial neck and not along the shaft because of the slight angulation at the neck of the radius.
If the line does not intersect the capitellum, there is radial head dislocation. This usually occurs in paediatric patients as a result of trauma and the dislocation is usually anterior. If the radial head is rounded, consider congenital radial head dislocation.