Femoral imaging artifacts associated with dorsal recumbency craniocaudal radiographic positioning. Description of a modified bisecting angle technique.

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.