Biomechanical simulation of radial head subluxation in cadaveric pediatric elbows.

BACKGROUND

Radial head subluxation, known as pulled elbow or nursemaid's elbow, is a common pediatric condition that occurs when a longitudinal traction force is applied to an extended elbow. The purpose of our study was to demonstrate and quantify the axial traction force required to produce a nursemaid's elbow in a pediatric cadaver specimen and to review the current relevant literature.

METHODS

We performed a literature search to identify relevant articles describing the nursemaid's elbow injury, as well as relevant biomechanical studies of the radiocapitellar joint. For the biomechanical model, 2 fresh-frozen cadaveric elbows from a single 3-year-old male donor were dissected by a fellowship-trained orthopedic surgeon. An Instron 5,944 testing machine with a 2 kN load cell was used to perform uniaxial testing. The radius and humerus were mounted to the Instron machine and loaded in the axial direction with the elbow in full extension. Loading occurred at a rate of 10 mm/second for 4 seconds, during which the force and actuator displacement were continuously recorded. The local instantaneous load and extension displacement at the time of subluxation were recorded.

RESULTS

The load to failure required to produce the nursemaid's elbow injury in our first specimen was 31 N, with a failure displacement of 4.6 mm. The second specimen demonstrated a load to failure of 26 N, with a failure displacement of 4.6 mm. After subluxation, we reduced the annular ligament from the first specimen. The elbow was then retested and demonstrated a load to failure of 20 N, with a failure displacement of 2.6 mm.

CONCLUSIONS

Axial traction applied to a pediatric elbow results in subluxation of the annular ligament into the radiocapitellar joint, at relatively low loads. We demonstrated a mean load to failure in a pediatric cadaveric model of 28.5 N, and a lower load to failure was required to produce a recurrent subluxation in a previously injured specimen.

KEY CONCEPTS

(1)Radial head subluxation, known as pulled elbow or nursemaid's elbow, is a common pediatric condition that occurs when a longitudinal traction force is applied to an extended elbow.(2)This study is the first to demonstrate and quantify the axial traction force required to produce a nursemaid's elbow in a pediatric cadaver specimen.(3)Axial traction applied to a pediatric cadaver specimen results in subluxation of the annular ligament into the radiocapitellar joint.(4)The mean load to failure is 28.5 N, and a lower load to failure is required to produce a recurrent subluxation in a previously injured specimen.(5)Literature supports that the nursemaid's elbow injury is a result of subluxation of the annular ligament into the radiocapitellar joint, though this may involve a spectrum of injury to the annular ligament itself.