Summary
Objectives: To describe the radiographic length distortion associated with femoral tilt during
cranio-caudal positioning. To describe a modified bisecting angle technique to alleviate
image foreshortening.
Methods: Five pairs of femurs were imaged. The femurs were tilted from 0 to 50 degrees and
the X-ray anode was tilted from 0 to 40 degrees. Femoral length was measured on the
resulting images. A geometric model depicting sagittal femoral positioning was described
with two trigonometric formulas. Trigonometric formula 1 (TF1) was designed to predict apparent femoral length. Data generated by TF1 were compared to the cadaveric study results. Trigonometric formula 2 (TF2), based on the same model, was used to predict the ideal anode angle (iAA) to alleviate
foreshortening. Unrelated to the model, a simplified linear formula (SF) was created
to approximate the TF2. Data generated with the SF and the TF2 were compared.
Results: When the femoral specimens were tilted to 20 degrees or greater, significant foreshortening
was appreciated (p <0.001). Increasing the anode angle increased apparent length,
eliminating foreshortening. Data generated by the TF1 agreed closely with the results of the cadaveric study (R2 = 0.999). The SF and the TF2 demonstrated close agreement and were able to predict an iAA consistent with the
results of the cadaveric study.
Clinical significance: A femoral tilt of 20 degrees or greater results in significant image foreshortening
and may warrant alternate radiographic techniques. If the femur is tilted, adjusting
the anode angle can alleviate femoral foreshortening.
Keywords
Femur - radiographic - artifact - foreshortening - bisecting