Roentgen Pelvimetry and Fetal Cephalometry: A New Technic

Abstract

IN THE effort to increase the accuracy of obstetrical examination, especially in marginal size cases in which the need for increased accuracy may be vital, we are using the technic described herein. The technics developed and advocated by others, while satisfactory in their own hands, do involve the use of special film markers, the personal equation of the examiner, and rather wide variation in results. Of these methods advocated, we obtained the most information by the use of teleoroentgenograms (S. S. M.). A study of a series of these teleoroentgenograms suggested the value and importance of the visualization of labor, accurate mensuration, the construction of a passenger-passage ratio, and the establishment of a normal fetal increment of increase. Accordingly we present this simplified technic of pelvimetry and fetal cephalometry. It embodies the use of a circular chart, placed upon the dial of an instrument which corrects for the size of the fetal head and object-film distance. The fetal skull is measured by tracing the instrument around the circumference of the skull on the roentgenogram, and from the chart a corrected circumference measurement in centimeters is read directly. This is translated into terms of volume in milliliters. The pelvic measurements determined are the anteroposterior diameter (true conjugate) and the ischial interspinous diameter in centimeters, and are translated into terms of volume capacity of a sphere of this diameter. A description of the chart and technic employed, with results, is presented. For practical purposes, the fetal head and maternal pelvis must be measurable in relative terms. In the past, it has been customary to compare the diameters of the fetal head with the diameters of the pelvis of the mother. By this means the size of the fetal head was compared with diameter measurements of the pelvis. However, it has been found that the mean circumference of the fetal head is always obtainable from the anteroposterior and lateral views and is a more dependable measure of the size of the fetus than are diameters. The percentage of error in the reading is reduced in more ways than one. To begin with, it makes no difference how obliquely the head is presented and hereby a larger percentage of films is readable. The units of measure are increased by more than three times the diameter units. In estimating the body weight of a fetus, the mean circumference has been reliable to within one-half pound in over 85 per cent of the cases and to within one pound in the remaining 15 per cent. The following technic, chart, instrument, etc., were conceived and designed by one of us (R. P. B.) and will be presented as they are used in an examination. Technic of Exposure A film of the pelvis and lower abdomen in anteroposterior view is taken with the patient in the supine position.