Roentgenographic changes in proximal femoral dimensions due to hip rotation.

Three human adult femurs were examined using anteroposterior (AP) roentgenograms in anatomical position and in 15 degrees, 30 degrees, and 45 degrees of internal and external rotation. Relatively small (15 degrees or less) amounts of femoral rotation can cause significant changes in proximal femoral dimensions on supine AP hip roentgenograms, and the magnitude of these changes increases with rotation. Fifteen degrees of rotation can change it by almost 3 mm. These changes are comparable to the medullary canal expansion of a normal adult femur over a five- to ten-year period. These changes in measured proximal femoral dimensions with rotation necessitate either an improved roentgenographic technique or precise rotational matching of standard supine roentgenograms to obtain accurate measurements of their changes over time.