Elastic Ruler for Roentgen Pelvimetry

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Elastic rulers have found use in radiology for the determination of the position of the pineal gland. They can also be used to good advantage to simplify roentgen pelvimetry. The figure below shows an elastic ruler consisting of a metal rod bent at one end (A) and carrying a movable piece (B). A rubber band is fastened to A and B. By moving B, the rubber band can be stretched and held at any point along the shaft. The original length of the rubber band is 14 cm. After stretching it to 16 cm., 16 centimeter lines are marked on it. Let us now take six metal rods, all 16 cm. long, and place them on the x-ray table supported by radiolucent material so that one rod will be at table top level, and the others at 5, 10, 15, 20, and 25 cm. above the table top. A film of these six rods is then taken with Bucky and with the film-tube distance that one intends to use for pelvimetry.2 This film is placed on an illuminator and the rubber band is stretched so that its length will correspond to the shadow of the 16 cm. rod placed on the table top. The position of the movable piece (B) is now marked on the metal shaft. Assuming that the shadow of the 16 cm. rod placed on the table top becomes 17.6 cm. when it reaches the film in the Bucky, the rubber band will distribute this increase over its entire length so that the distance between centimeter marks will be 11 mm. instead of 10 mm. The rubber band is then stretched so that its length corresponds to the shadow of the 16 cm. rod placed 5 cm. above the table top and the new position of the movable piece is marked on the metal shaft. This is repeated for the rods placed at 10, 15, 20, 25 cm. above the table top, and new marks are made on the shaft for each position of the movable piece (B). The spaces between marks are then further divided into five equal spaces, each of which represents one centimeter of table top-part distance. Once this is done. pelvimetric measurements become extremely simple. One anteroposterior axial film and one lateral film are taken, and the distance in centimeters between the table top and the inlet and the distance between the table top and the sagittal plane of the patient are noted. The anteroposterior axial film is measured with the ruler set at first for the table top-inlet distance. At this setting, one can measure the true conjugate, the fetal head, the transverse diameter, and the two oblique diameters. A setting of 4 cm. will measure accurately enough the interspinous diameter, and a setting of 1.5 cm. the bi-ischial. On the lateral film, one must set the instrument for the distance between the table top and the sagittal plane of the patient. This setting will serve for the true conjugate, the pubosacral diameter, and the fetal head. As long as the tube-film distance is kept constant, the measurements obtained with the elastic ruler are quite as accurate as those obtained by other methods.