Radial head subluxation is also referred to as a nursemaid's elbow, pulled elbow, or annular ligament displacement. It usually occurs primarily in children 1 - 4 years old (peak 2 -3 years), but has been reported both younger and older. Girls have a higher incidence as does the left arm over the right.
The most common cause of the subluxation is from sudden traction placed on the distal radius causing the annular ligament to slip over the head of the radius and trapping it in the radiohumeral joint. This occurs most frequently when axial traction is placed on a pronated forearm as when an adult pulls a child quickly by one arm or is swung by the arm. Although this is the classic story it is only present 50 percent of the time.
As the child develops the annular ligament becomes thicker and is much less likely to be displaced causing the subluxation (around 5 years of age).