Effect of rotation on radiographic dimensions of the humerus and femur.

Ten humeri and ten femora were radiographed in nine positions of longitudinal rotation. In each position total, medullary and cortical widths of the diaphyses (TW, MW, CW) were measured to assess the effect of rotation on these dimensions. The results were expressed as differences from the values in the neutral position (delta TW, delta MW, delta CW). In the humerus, medial rotation produced greater changes than lateral rotation. Though rotation in either direction increased CW, the systematic error due to medial rotation to only 5 degrees (approximately +0.0058 cm per degree) was more than double that due to lateral rotation. In the femur, lateral rotation had the greater effect, producing a marked decrease in CW (approximately -0.0072 cm per degree). Medial rotation produced only a slight increase in CW. The ratios CW/TW ("cortical index") and CA/TA ("area ratio") were calculated and expressed as differences from the values in the neutral position (delta CW/TW, delta CA/TA). Rotation affected these variables much less than the linear dimensions. Furthermore, through the variance of delta TW, delta MW and delta CA/TA increased only slightly or not at all. These results suggest that if radiographic morphometry is to be applied to the humerus and femur, these ratios rather than transverse dimensions should be used. Sequential changes in CW may be totally obscured by small rotational errors.