The magnification of digital radiographs in the trauma patient: implications for templating

Digital radiographs are increasingly used for planning orthopaedic surgical procedures, despite the fact that they are frequently not calibrated to correct for magnification. The typical magnification of digital radiographs in the trauma patient has not yet been reported. The aims of this study were to assess the magnification of such radiographs, and to discuss if and when accurate calibration is required for trauma templating.

The operative notes and postoperative radiographs of 227 trauma patients were obtained. Each patient had undergone one of the following procedures: hip hemiarthroplasty, femoral nailing, tibial nailing, ankle plating, humeral nailing, humeral plating, or forearm plating. The dimensions of the implants used were measured on the uncalibrated postoperative radiographs using the hospital's Picture Archiving and Communication System software. The actual dimensions of the orthopaedic implants were obtained from the operation notes, and these were compared with the radiographic measurements. The intraobserver and interobserver variability of the radiographic measurements was also assessed.

The radiographic magnification was greatest for the femoral head, and most variable for the femoral shaft. The magnification was least for the forearm. In general the magnitude and variability of magnification was least at the peripheries. There was good correlation between the measured and actual dimensions of the implants. The intraobserver and interobserver variability between the radiographic measurements was extremely small.

Despite the ease and convenience of performing measurements on digital radiographs, these measurements are unreliable if the radiograph has not been calibrated. We believe that careful calibration of digital radiographs is essential for accurate templating in the trauma patient, although is less critical when templating the humeral canal, the tibial canal, the ankle and the forearm.