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In view of two recent articles5,6 in the literature in which measurements of a series of subpubic angles are reported, and in which the importance of a narrow subpubic angle as a cause of delay at the pelvic outlet is stressed, it seems pertinent at this time to offer some comments upon this angle as it concerns the radiologist. We have during the past year been making an investigation into radiological pelvimetry, and during our study we have been impressed with the difficulty of measuring this angle accurately and with the paucity of references in the literature to the subject. In many articles the radiological technique is not sufficiently detailed to allow comparison of one series with another, and we have been unable to find any discussion of the possible errors in the methods commonly employed in the measurement of the angle. Criticism is sometimes levelled at radiological pelvimetry as being an inaccurate method, and although we cannot subscribe in general to this opinion, it does appear that there...

The constitution of the pelvic outlet is examined as a functional unit, and a method of determination of the true antero-posterior plane of the outlet is described. From the foregoing is developed a more satisfactory method of outlet pelvimetry; its application to a series of 62 cases is given. The figure obtained by this method is termed the “free space at the outlet”, or F.S.O. The significance of the F.S.O. in a particular case is indicated in the accompanying graph and follow-up table. It is concluded that a radiological estimation of the pelvic outlet can be a useful part of a pelvimetric examination.

A method of pelvimetry is described which is recommended for standardisation. Two radiographic views are required, three if the pelvic outlet is to be measured. Only two measurements are recorded by the radiographer. No calculations are necessary. Procedure.—Metal calipers are applied to the pelvis in the manner suggested by Hederman (1940): (a) At the level of the pelvic brim in Thoms' position (a special manoeuvre ensures that the obstetric conjugate is horizontal). (b) In the sagittal plane. The anode-film distance is not critical. The caliper measurement is recorded in each instance. The “pelvimetry protractor” is constructed from a rectangular sheet of perspex. It bears a centimetre scale from each division of which lines are engraved which radiate uniformly. A cursor thread with sliding hooks may be moved up and down parallel to the scale. Mode of use.—Two radiating lines are chosen according to the recorded caliper measurement by reference to the basal centimetre scale. The protractor is placed ove...