Accuracy of C-arm measurements in assessment of paediatric femoral fracture shortening

PURPOSE: Spica casting of femoral shaft fractures is standard practice for young children. In 1983, Dr. Rang published an illustration demonstrating that radiographs can be misleading when measuring femoral fracture shortening. Today, fluoroscopy is used to determine shortening after closed reduction and spica casting. This paper aims to determine the accuracy of fluoroscopy when measuring 25 mm of shortening in a femur fracture sawbone model. METHODS: Magnification, elevation and angulation were evaluated using a femur fracture sawbone model with a fixed overlap of 25 mm. Two C-arm machines used in clinical practice were studied: the OEC 9900 Elite GE C-arm and the Fluoroscan InSight mini C-arm. RESULTS: The OEC 9900 Elite GE C-arm demonstrated a 1.75 × magnification. The Fluoroscan InSight mini C-arm demonstrated a 2.5 × magnification. An 11% and a 17% magnification with elevation of the femur to 8 cm occurred using the OEC 9900 Elite GE and the Fluoroscan InSight mini C-arm respectively. Changing the tube angulation from 0° to 60° resulted in a 36% and 16% change in measured femur fracture shortening on the OEC C-arm and Fluoroscan mini C-arm respectively. CONCLUSION: C-arm images are inaccurate. When measuring 25 mm of shortening of a paediatric femur fracture, each machine has a unique amount of magnification that should be determined and accounted for in clinical practice. Elevation of the femur above the C-arm receiver and angulation of the femur further increase the inaccuracies when measuring 25 mm of fracture shortening. Images perpendicular to the fracture are most accurate.