Fig. 1
An illustration about the measurement of STAD. STAD was calculated according to the rule of TAD, while STAD was measured based on the its own FHD (iFHD) instead of Dtrue in caculating TAD. The femoral head was considered as a regular sphere. All the iFHD was assumed the same constant value (in this study, 1iFHD = 6D, or 1D = 1/6 iFHD, no matter how big the actual iFHD was) in order for easier calculation and more convenient comparison. According to the rule of TAD, STAD was the sum of the actually measured distance of the nail tip to the apex of femoral head (X) divided the actually measured FHD (DAP) on the anteroposterior view (X/DAP) and the actually measured distance of the nail tip to the apex of femoral head (Y) divided the actually measured FHD (DLat) on the lateral view (Y/DLat), respectively, then multiplied iFHD. Since STAD was calculated according to the principle of TAD and measured based on the its own FHD (iFHD). When we define “1iFHD = 6D”, the radius of the femur head will be 3D, and 1/12, 2/12, 3/12, 4/12, 5/12, 6/12, 7/12, … and 12/12 times of iFHD will be 0.5D, 1D, 1.5D, 2D, 2.5D, 3D, 3.5D, …, and 6D, respectively. So, STAD = 3D meant that the STAD was a half of its own FHD