[A new measurement method for the radiographic assessment of the proximal femur: the center-trochanter distance].

OBJECTIVES: We developed a new method --measuring the perpendicular distance between the center of the femur head and the tip of the trochanter-- for radiographic assessment of the proximal femur.
METHODS: The "center-trochanter distance" (CTD) refers to the perpendicular line drawn to the femoral shaft axis between two parallel lines that pass through the center of the femoral head (C) and the tip of the greater trochanter (T). The measured distance in millimeters is expressed as a positive or negative value depending on the location of the point C, that is, above or below the point T, respectively. Measurements were made on anteroposterior plain radiographs of 200 skeletally immature and 600 skeletally mature hips, all of which had been assessed as normal. Values that were found to be below or above the standard deviation of 1 and 2 of the mathematical means were regarded as "normal" and "pathological", respectively. The reliability of the method was tested in 43 hips that had been surgically treated for developmental dysplasia of the hip and had a long-term follow-up.
RESULTS: The center-trochanter distance measured below -5 mm and above 15 mm in the skeletally immature proximal femora, and below -17 mm and above 7 mm in the skeletally mature proximal femora were regarded as "pathologic". The intraobserver (kappa coefficient, 0.92 and 0.81) and interobserver (kappa coefficient 0.88) reliability of the method was found to be excellent.
CONCLUSION: The center-trochanter distance may prove to be useful in radiographic assessment of the proximal femur in several hip disorders such as developmental dysplasia of the hip and Legg-Calvé-Perthes disease.