The validity and reliability of preoperative radiographic canal diameter measurements of the femur.

Background: The femoral canal is frequently measured preoperatively in cases where an intramedullary device is planned for operative fixation of a fracture. To our knowledge, a formal assessment of validity and reliability of preoperative canal measurements has not been previously performed.

Questions/purposes: This study aims to determine the validity and reliability of preoperative canal measurements of the femur made on plain radiographs using comparison with curved planar reformation software as the gold standard.

Methods: Fifty-six patients were identified based on availability of anterior-posterior (AP) and lateral radiographs of the femur and computed tomography (CT) of the lower extremity. Four "raters" measured the canal diameter at its narrowest point and the distance from the lesser trochanter to the isthmus on the AP, lateral radiograph, and CT. The width of the femoral nail on AP radiographs was also measured to determine magnification error. Curved planar reformation (CPR) was used to provide the most accurate calculation of the canal diameter.

Results: Compared to the isthmus position determined by CPR, the measurement was most accurate on an AP and the diameter of the canal was most accurate using coronal CT, followed by AP radiographs. The measured canal diameter of the fractured femur on APs was compared to that of the used implant and varied by 1 mm.

Discussion/conclusion: The AP plain radiographic measurement was found to be more accurate for determination of the canal diameter compared to the lateral radiograph. These findings confirm the utility of preoperative canal measurements in predicting the feasibility of placing a specific size intramedullary implant.