An Accurate Roentgenologic Method for Determining Pelvic Depth

An Accurate Roentgenologic Method for Determining Pelvic Depth


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(From the Departments of Obstetrics a'flil Gynecology of the School of Medicine of Louisiana State Unit•ers·ity and Chority Hospital of Louisiana, New Orleans)

T long been recognized that the distance from the plane of the inlet to the plane of the ischial spines and the distance from the latter Ipoint to the plane of the outlet, reveal wide individual variations. For HAS

that reason the terms "shallow pelvis" and "deep pelvis" have acquired a definite clinical significance. For the same reason the accurate determination of the distance which the fetus must descend through the bony pelvis is a matter of considerable clinical importance not only in patients who require operative intervention but also in patients who deliver spontaneously. It is curious that, notwithstanding the admitted importance of a knowledge of the depth of the true pelvis, no satisfactory precision method for determining it seems to have been suggested. Textbooks of obstetrics usually state that pelvic depth can be estimated from the impressions derived from vaginal examination, but this is in no sense a satisfactory method. Furthermore, such impressions are fundamentally erroneous because of the introduction of the personal equation, quite aside from the fact that such value as they possess is entirely dependent upon the clinical experience of the examiner. So far as we have been able to determine, Schumanl has published the only method on record in the literature for obtaining the measurement of pelvic depth. By his metho.d the perpendicular distance from the inferior surface of the ischial tuberosity to the superior border of the ramus of the pubis is determined by means of an ordinary pelvimeter. In his cases the average clinical measurement of the pelvic depth was 11.5 em. and the average bony measurement, after due allowance for pubic and gluteal soft tissues, was 10.5 em. Schuman emphasized the importance of this measurement and stated that increased pelvic depth was the mDst important characteristic of the male, funnel, or high as~imilation pelvis.

The method for determining pelvic depth which we are describing in this communication was devised in the course of an investigation of the clinical value of the Ball technique of pelvicephalography. Following preliminary studies, 2 we analyzed 503 selected cases3 studied by this method in regard to pelvic architecture, pelvimetry, fetometry and fetopelvic relations. Our conclusion was that when due regard was paid to clinical considerations and to those intangible. factors which, even more than mechanical considerations, determine the course of labor, Ball pelvicephalography is a very useful procedure, in that it warns the obstetrician, accurately in most instances, of possible mechanical risks and difficulties to be expected during labor and at delivery. 467



The method for determining pelvic depth, which is proposed herewith, should be accepted with precisely the same reservations and qualifications. That is, we consider it an accurate method which supplies valuable data. We are not proposing it as a substitute for clinical observation and judgment. TECHNIQUE

Pelvic depth can be determined from the pelvicephalograms made for the purposes listed above, no special additional exposures being required. Three sites were used for the determinations, the forepelvis, the posterior-pelvis, and the pelvic canal, and

Fl~. 1.-Pelvlcephalogram of i}l'lmlpara. with. shaUow pelvis. ;Note. the shallowness of the pelvis (line (}) and the difference In distance from the inlet to tile spines ( Ot) and from the spines to the outlet ( 0;.}. The patient delivered a child weighing 7%, pounds after a two-hour labor.

it was determined by comparative studies that measurements of the pelvic canal

furnished the most accurate .and valuable data. ·whatever site is used, the me~1lre­ ments are made from the lateral film .. The. ~terior film .furnishes the correetion factor; since all measurements on the lateral :film are in .the. same plane, the satne correction is necessary as is necessary in the estimation ·Of the true conjugate. 1. When the forepeJ.vis is used as the site of measurement, the pelvic depth is represented by a line perpendicular to the true conjugate and drawn from the upper margin of the symphysis pubis to the plane of the pelvic outlet (Figs. 1 and 2, line A). This measUrement closely J~.pproxima.tes the ~b



is lessened by the fact that the fetal head does not descend in this far anterior location. The measurement also does not make any correction for the changes in depth which usually occur in the posterior aspects of the pelvis and which occur more commonly in the male type of pelvis. 2. When the posterior pelvis is used as the site of measurement, the pelvic depth is represented by a line drawn from the sacral promontory to the sacrococcygeal articulation (Figs. 1 and 2, line B). This measurement is dependent upon the length and curvature of the sacrum, and therefore is the chord of the arc described by the sacrum rather than the true pelvic depth. The measurement is of even less value, furthermore, because the inclination of the sacrum is of more importance than its height. Two pelves, for example, may have sacra of equal length and curvature. In one the sacrum is directed backward and the pelvis is therefore adequate. In the other, the sacrum points sharply forward, encroaching on the outlet and thus producing an inadequate pelvis.

Fig. 2.-Roentgenogram of the pelvis of a primipara. Note that the pelvic depth (line 0) is considerably greater than the pelvic depth in Fig. 1. Note also the increased depth from the forepelvis to the posterior pelvis. Fourteen months before this study was made the patient delivered a child weighing seven pounds after a sixteenhour labor. 3. When the pelvic canal is used as the site of measurement, two determinations must be made. A perpendicular line is drawn from the true conjugate to the mid· point of the diameter between the ischial spines (Figs. 1 and 2, line 01). A second line is drawn from the midpoint of the biischial spinous diameter to the midpoint of the transverse diameter of the outlet (Figs. 1 and 2, line 02). The total depth of the pelvic canal (C) is the sum of line 01, which is the distance from the inlet to the midplane of the pelvis, and line 02, which is the distance from the midplane to the pelvic outlet. The third of these measurements, in our opinion, is a more accurate index of pelvic depth than either of the first two because it represents the approximate distance the fetal head must descend through the true pelvis. This measurement also furnishes useful information concerning the relationship of the midplane (spines) to the inlet and outlet of the pelvis.





In 100 consecutive cases studieu by i.Jw method described, the avt>rage pelvi·· depth was 11.97 em., with 15 em. and R.2 em. t1w upper an;] lower limits. The difference between the deepest and shallowest pelves, 6.8 em., is surprisingly widt•. 'l'he great majority of eases, 72, measured from 10.1 to 13.1 em. The pelvic depth was between 8.1. and 9 em. in 4 cases; lwtween 9.1 and 10 em. in 5; between 10.1 !l.Jld 11 in 19; between 11.1 and 12 in :n; between 12.1 and 13 in 22; h<'tween 13.1 and 14 in 12; and between 14.1 and l 5 em. in 7 instances. In addition to making information available as to total pelvic depth, the methml we have propose;] supplies data eoncerning the relationship of the ischial spines to the inlet and outlet of the pelvis. This relationship is subject to marked variations. 1u the 100 eases studied, the average distance of the spines from the Jlelvk inlet was 7.3 em., and the average distance from the spines to the pel de outlet was 4.67 ern. Greater variations oecmre•·l in the relation~hip of the spines to the inlet than to the1 outlet. This relationship is important becrmse tlw station (degree of deseent) of the pre· senting part is determined from these lanol marks. Generalizations as to station are not satisfactory, because individual variations make it necessary to individualize in each case the degree of descent of the presenting part. If the ischial spines are a relatively short distance from the inlet, for instance, as when Cl is less than usual (Fig. 1), the presenting part may not actually be engaged when it reaches the lewl of the spines. On the other hand, when Cl is greater than usual (Fig. 2) and the ischial spines are relatively far from the inlet, the presenting part, when it rf'aehes this plane, would be deep in the pelvi~ and well engaged. The accuracy of the method has been ehecked on cadaveric material, and the errorH revealed have been consistently less than ±0.15 em. \Ve are presently engaged in correlating the type of labor and delivery with the faetor nf pelvic depth in the 100 cases studied and shall report these data elsewhere. SUMMARY AKD CONCLUSIONS

1. An accurate determination of pelvic depth is a valuable adjunct to pelvic measurements as they are now taken. Data concerning the relationship of the ischial spines to the planes of the inlet and outlet of the pelvis are also of value. 2. A precision method of securing these data is afforded by the use of Ball pelvicephalograms. The method of securing the measurements is described. 3. The average depth of the pelvie canal in 100 eonsecutive cases was 11.97 em. The variation was considerable, the shallowest and deepest pelves being, respectively, 8.2 em. and 15 em. in depth. 4. In the same 100 cases the average distance from the pelvic inlet to the ischial spines was 7.3 em., and from the ischial spines to the outlet 4.67 em. Considerable variations occut' in the distance from the inlet to the spines, but the variations in the distanre from the spines to the outlet are less marked. REFERENCES

(1) Schuman, JV.: A;~-1. .T. OnsT. & GYNEC. 28: 497, 1934. (2) GtJ,erriera, JV. l!'., and Smith, W. I'": New Orleans M. & S. J. 91: 299, 1938. (3) Guerriero, W ..F., Arnell, Rupert E., and. Irwin, J. B.: South. M. J. (In press.)