[Rippstein and Müller roentgenologic determination of the actual femoral neck-shaft and antetorsion angle. 1: Correction of the conversion table and study of the effects of positioning errors].

The femoral neck-shaft angle (CCD) and the angle of torsion (AT) are angles in space; in commonly used radiographs the angles are not portrayed in their actual true size (rCCD, rAT) but in their projected size, which deviates somewhat (pCCD, pAT). The formulas required for the conversion are explained in detail and the previous conversion tables have been corrected. The effects of minor deviations (5 degrees, 10 degrees and 20 degrees, increased/decreased abduction or flexion, exterior/interior rotation) from the prescribed position of the patient have been calculated and are displayed in diagrams. It is evident that determination of the angles according to Rippstein and Müller can be influenced to a considerable extend by minor discrepancies (+/- 5 degrees -10 degrees) in the positioning of the patient. If there are high AT-angle values or positioning of the patient causes problems, and where determination of the angle would involve major therapeutic measurements, another procedure will have to be used that should be almost independent from discrepancies in the positioning of the patient.