Comparative Accuracy of the Chassard-Lapiné and Recumbent Positions in Roentgen Measurement of the Pelvic Outlet

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Abstract
Allen (1) in 1947 outlined a technic of roentgen pelvimetry, one of the advantages of which was said to be accurate measurement of the pelvic outlet. We began to use this method in November 1947, because it seemed to us that it offered some definite advantages over that which we had employed and which previously had been described by one of us (Good, 2). Essential for measurement of the pelvic outlet is an accurate roentgenographic reproduction of the subpubic arch. To achieve this, Allen (3) had previously recommended the position of Chassard-Lapiné over the more widely used recumbent position. In the method of Chassard-Lapiné the patient straddles a cassette, then bends forward at the hips until the undersurface of the symphysis pubis and the ischial tuberosities are equidistant and horizontal to the film. The x-ray tube is centered perpendicular to the film above, or slightly posterior to, the ischial tuberosities (Fig. 1). For the recumbent position, the patient lies supine on the table and the x-ray tube is angled 33 degrees toward the head in an effort to have the central rays strike the pubic rami perpendicularly. It has been apparent to us for some time that the reproduction of the subpubic arch as obtained with the patient in the recumbent position is frequently inaccurate. If the central rays are not perpendicular to the plane of the subpubic arch, considerable distortion of the arch and its angle will result. The distortion is exaggerated by the fact that the central rays strike the film at an angle of 33 degrees. In an effort to determine whether the position of Chassard-Lapiné is more accurate, we decided to perform a series of experiments with both positions, including in each series the variations in position which are actually encountered during roentgen pelvimetry. Nature of Present Study We obtained from the Section on Anatomy a disarticulated bony pelvis with sacrum attached, the subpubic angle of which measured 57 degrees. We constructed an adjustable wooden stand to hold the pelvis. By rotating the stand, the pelvis could be rotated forward or backward; it could also be raised or lowered. Multiple roentgenograms were made using both the recumbent position and the position of Chassard-Lapiné under the following conditions: (1) forward-backward rotation of pelvis. (2) variations in the angle of the x-ray tube, and (3) variations in the distance between pelvis and film. For each roentgenogram of the subpubic arch two measurements were made. First, the subpubic angle was measured by drawing lines from the lower edge of the symphysis tangentially to the inner surface of the tuberosities and measuring the angle thus formed. Secondly, the symphysis-biparietal distance was measured, as described by Allen (1). This measurement comprises one step in Allen's method of computing the pelvic outlet.
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