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Radial head arthroplasty is a reliable option when primary repair is unobtainable for radial head trauma. Appropriate sizing of the implant is challenging as there is no standardized method of accurately measuring radial head size. With improper selection, the prosthesis can create undue pressure on the capitellum and lead to ulnohumeral joint degeneration. Previous reports have used computed tomography(CT) to assess the radial head to coronoid length and shown a reliable distance that can be used to guide radial head placement. However, this method is limited by the cost of CT and radiation exposure to a non-injured extremity. We hypothesize that x-ray can be used to define the radioulnar variance at the proximal radioulnar joint (PRUJ) and serve as a landmark for radial head replacement.

Patients older than 18 years with elbow x-rays performed in the last year were eligibile for study. Patients with trauma anywhere from the humerus to wrist were excluded. Patients were excluded if improper radiographic technique was used or the elbow was not at full extension and supination. Upon chart review, radiography indication, age, sex, height, weight, and BMI were abstracted. Two fellowship-trained orthopaedic hand surgery attendings and two residents assessed the radiographs for two measures of PRUJ variance: 1) AP coronoid-radial head height defined as distance from the center of the radial head to the top of the coronoid projection, 2) AP coronoid-radial head angle defined as the angle between a line connecting the proximal projection of the radial head and a line from the lateral radial head to the most proximal point of the coronoid. Data was analyzed for average PRUJ variance and interobserver reliability.

Fifty elbow radiographs were randomly selected from the inclusion group for study. There were 13 females and 37 males in the study group with an average age of 40+16 years. The average coronoid-radial head distance was 2.1+1.5mm and the average coronoid to radial head angle was 3.7+2.6 degrees. Interobserver reliability was 0.95 for both measurements.

Radiographic anatomy of the PRUJ shows a consistent relationship between the radial head and coronoid, with the coronoid on average 2.1 mm proximal to the radial head. This measurement is reliably reproduced between observers and may be used as a marker of radial head placement and sizing in radial head arthroplasty. The lower cost and limited radiation exposure of xray compared to CT make it a preferable alternative to CT for PRUJ anatomy assessment.