Dr. Mersch received his Bachelor of Arts degree from the University of California, San Diego, and prior to entering the University Of Southern California School Of Medicine, was a graduate student (attaining PhD candidate status) in Experimental Pathology at USC. He attended internship and residency at Children's Hospital Los Angeles.
Melissa Conrad Stöppler, MD, is a U.S. board-certified Anatomic Pathologist with subspecialty training in the fields of Experimental and Molecular Pathology. Dr. Stöppler's educational background includes a BA with Highest Distinction from the University of Virginia and an MD from the University of North Carolina. She completed residency training in Anatomic Pathology at Georgetown University followed by subspecialty fellowship training in molecular diagnostics and experimental pathology.
Nursemaid elbow results from a sudden pulling force applied to the extended arm of the child. Due to the relative strength of the adult in comparison to the weakness of the child's supportive annular ligament, the applied force may not seem strong to the parents and they may not realize an injury has occurred.
Examples of typical situations that can produce the mechanism of force
required to cause this injury are
in lifting the child up from the ground by the hand or wrist,
swinging the child while holding the child by the hands or wrists,
pulling arms through the sleeves of jackets,
catching a child by the hand to prevent a fall, and
pulling a child along when in a hurry or the child suddenly collapsing to the ground in an effort to avoid going with their parent.
The young child is prone to this type of injury largely because of the anatomical features of their bones and ligaments. Understanding the mechanism of this injury is helpful in explaining the cause.
The end of the radius that connects to the elbow joint is known as the radial head. The radial head has a shallow concave shape allowing it to fit over a complementary convex prominence at the end of humerus (the capitellum). As the child matures, the radial head broadens and ultimately becomes wider than the portion of the radius (called the radial neck) adjacent to the head. In the young child, the radial head does not yet have a well-defined lip at its end. As such, in the toddler, the radial neck and radial head are similar in size.
The annular ligament holds the radius alongside the ulna, which is the other bone in the forearm. Besides stabilizing the radial head-humerus joint, the annular ligament permits the radius to twist when the hand changes position from palm down (prone) to palm up (supine). In young childhood, the annular ligament is still relatively loosely attached to the bone and can experience a small tear in some of its fibers.
The combination of these two things (shallow concave radial head and loose-fitting annular ligament which may easily partially tear) allows the loose portions of the ligament to slide over the radial head when a pulling force is applied to the elbow while the forearm is slightly rotated palm down (pronation). When this happens, this annular ligament tissue can become trapped between the radial head and the capitellum, resulting in the subluxation of the radial head, or nursemaid elbow.