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THE CLINICAL VALUE OF X-RAY PELVIMETRYBy HUMPHREY G. E. ARTHURE, M.D., M.R.C.O.G., F.R.C.S.

Assistant Obstetric Physician, Charing Cross Hospital; Surgeon to In-patients, Queen Charlotte's Maternity Hospital,London

IntroductionRoutine radiographic studies of the pelvis in

obstetrics have provided valuable informationabout the variations in shape and size of thefemale pelvis. Improvements in technique and in-creased experience of pelvimetry have made itpossible accurately to predict the outcome oflabour in a high proportion of cases (9i per cent.of 600 cases referred to the radiologist at QueenCharlotte's Hospital). It is an opportune time,therefore, to discuss the ways in which radiographymay assist the clinician, and to assess the value andreliability of pelvimetric methods.

Radiography may be used to confirm thepresence or absence of disproportion, but it mustnever be thought that it can supplant clinicalmethods. X-ray measurements of pelvic diametersmay be more accurate than clinical measurements,but it must clearly be recognized that accurateprediction of the outcome of labour cannot bemade in every case, however precise the measure-ments of the pelvis and foetal head may be. It isimpossible to foretell the strength of the uterinecontractions, the possible expansion of the pelvi-diameters in labour and the mouldability of thsfoetal head. There must, therefore, be a border-line group of cases in which the outcome of labourdepends on factors which are unknown until labouroccurs. In such cases a successful outcome maydepend on the skill of the obstetrician who will bebetter armed to overcome obstetric difficulties ifhe has accurate knowledge of the pelvic architec-ture.. Furthermore, it must be remembered thatdystocia is more often due to imperfect uterineaction than to actual disproportion.

It is convenient to discuss problems of dis-proportion under two headings; disproportion atthe brim and at the outlet. These present differentproblems both in diagnosis and management.Consideration of disproportion in midcavity is un-necessary if, firstly, brim disproportion is taken toinclude those cases which have an antero-posteriordiameter shorter than and slightly below the levelof the obstetric conjugate; and, secondly, if theoutlet is considered not as a single plane, but asthe lower pelvic strait.

Radiological TechniqueThe standard technique at Queen Charlotte's

Hospital (Rohan Wliams, x943) in cases of sus-

pected disproportion includes the following pro-jections:

(I) Antero-posterior projection, with the patientsupine. This film gives a general picture of thepelvis, from which gross contraction, tilting orasymmetry will be apparent, and from which thegeneral convergence or otherwise of the side wallsof the pelvis can be noted.

(2) Supero-inferior projection, with the patientsitting, but reclining backwards so that the brimis parallel to the casette. This film shows the trueshape of the brim and from it the brim diametersare calculated. Difficulty may be experienced indeciding which line on the film should be taken asthe anterior margin of the sacrum, and in suchcases the difficulty must be resolved by checkingthe measurement of the obstetric conjugate on thelateral film. As Chassar Moir (I947) has pointedout, the most prominent point on the sacrum is

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FIG. 2.-Reconstruction chart of brim.

often below the pomontory. A' reconstruction.chart is then drawn, which shows the exact sizeand shape of the brim, and on this chart a circle orellipse with a diameter of 9.5 cm. may be super-imposed, representing an average foetal head atterm.

It will be seen that the greatest transversediameter usually lies behind the mid point of theantero-posterior diameter, and is sometimes quiteclose to the sacrum. For this reason, ChassarMoir has designated the diameter Which intersectsthe conjugate at the mid point as the availabletransverse diameter of the brim.From these films also the prominence of the

ischial spines can be observed, and the ischialbispinous diameter calculated, if the distance fromthe plane of the brim is worked out from the lateral

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256 POST GRADUATE MEDICAL JOURNAL June '949film. This diameter may also be determined fromthe antero-posterior projection.

(3) Lateralprojection, with the patient erect. Inthis position the femoral shafts do not obscure thesymphysis pubis, and the postural tone of theabdominal muscles tends to engage the foetal headinto the brim. From this film measurements aremade of the antero-posterior diameter of the brim(from the upper posterior margin of the symphysisto the most prominent anterior margin of the.uppersacrum), and of the antero-posterior diameter ofthe outlet (from the lower posterior margin of thesymphysis to the anterior margin of the lowest partof the sacrum, or to the coccyx if the sacro-coccygeal junction is fixed).The lateral film also gives information on the

inclination of the pelvic brim to the lumbar spine.This can be measured as the angle between thetrue conjugate and the anterior margin of the fifthlumbar vertebra. A very large angle may accountfor delayed engagement of the foetal head andperhaps asynclitism, but the angle will be con-siderably reduced by drawing up the knees, apoint to be remembered in the management of thefirst stage of )abour.,A reconstruction chart is made from the lateral

film of the median or sagittal plane of the pelvis,and from this the curve of the pelvic canal can be

FIG. 4.--Lateral reconstruction chart.

studied, using a disc representing the averagefoetal head at term. It must, however, be re-membered that part of the antero-posteriordiameters may not be available to the foetal head,and it is best to observe the general shape and lieof the symphysis and descending pubic rami infront, and of the sacrum behind.

(a) The symphysis tends to slope backwardsroughly parallel with the backward inclination ofthe sacrum; if it has a greater backward slopethan. usual, the antero-posterior diameter of theoutlet will be shortened. The descending pubicrami normally slope backwards more than thesymphysis, but this is variable, and a markedbackward slope is particularly unfavourable when

the subpubic arch is narrow. In such a case thefoetal head will be forced to emerge further backthan usual, and much of the antero-posteriordiameter is wasted.

(b) The anterior border of the sacrum isnormally concave, but variations are common, sothat it may be roughly straight or even slightlyconvex; in this case it is usually the second sacralsegment which projects more than the promontory,forming a conjugate which must be considered in-stead of the true conjugate of the brim. There aretwo sacral variations which may shorten theantero-posterior ,diameter of the outlet; firstly,the deeply hollowed sacrum in which the sacro-coccygeal shelf projects forward to lie almostunder the ischial spines ; secondly, a long sacrumhaving perhaps six segments instead of five, whichmay be comparatively straight without a prominentsacro-coccygeal shelf, yet because it is often morevertical than normal, the antero-posterior diameterof the outlet may be seriously shortened.

(4) Subpubic arch projection, taken with thepatient sitting well forwards, so that the arch isparallel with the casette. The manner in whichthe foetal head will fit into the subpubic arch de-pends on the angle of the arch and on the curvatureof the descending pubic rami. These factors arebest assessed by using a disc with a diameter of

FIG. 6.-Subpubic arch reconstruction chart.

9.5 cm. representing the average foetal head atterm: When the arch is wider than normal (90goor more) the disc occupies the whole of the arch,with an average arch (about 850) there is a gap of0.5 to I cm. between the circumference of the discand the symphysis; when the arch is narrow (lessthan 80o) there will be a still wider gap. Thelength of this gap is measured on the film andreduced to actual size, which will represent thedistance an average-sized unmoulded foetal headwill lie from the lower border of the symphysisduring delivery.

This distance can be marked off on the lateral

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June I949 ARTHURE : The Clinical Value of X-Ray Pelvimetry

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FIG. I.-Supero-inferior projection.

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FIG. 3.-Lateral projection, patient erect.

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FIG. 5.-Subpubic arch projection; narrow arch

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POST GRADUATE MEDICAL JOURNAL June i949

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FIG. 8.-Lateral X-ray. Head now engaged.

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FIG. 9.-Lateral X-ray. Head free and occiput slightlyposterior; no disproportion.

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J.une 1949 ARTHURE : The Clinical Value of X-Ray Pelvimetry 259,

reconstruction chart on the line of the descendingpubic rami, and the available antero-posteriordiameter is measured from the lowest anterior

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FIG. 7.--Lateral reconstruction chart in same case asFig. 6, showing available A.P. diameter.

margin of the sacrum to this point. Recently,Morris (I947), Allen (I947) and Rohan Williams(I949) have given considerable attention to thisavailable antero-posterior diameter of the lowerpelvic strait, and Morris considers the outlet iscontracted when this diameter is less than io cm.

Forecast graphs. Chassar Moir (I947) hasadvocated the use of graphs to assess the capacityof the pelvis at three planes. He has preparedgraphs for various biparietal diameters of the foetalhead, determined by postnatal measurement, andhas shown that a line may be drawn on each graphfor different sizes of the foetal head, on the lowerside of which a great preponderance of difficultdeliveries will be plotted.

Cephalometry. As yet no series of forecasts havebeen published using antenatal cephalometry inconjunction with pelvimetry, and there are twodifficulties. Firstly, many patients are X-rayedearly in pregnancy before cephalometry is feasible;and, secondly, in patients X-rayed at or after the36th week of pregnancy, measurements of thefoetal diameters can only be made in about halfthe cases because of unfavourable positions of thehead. Cephalometric methods are then onlyapproximate, as is estimation of further growth inthe last month.

Nevertheless, cephalometry should be practisedwhenever possible, and provided its limitations arerecognized, the additional knowledge will furthernarrow down the borderline group. The clinicalaphorism that 'the foetal head is the bestpelvimeter' must be applied to radiology, if thebest results are to be achieved. -Even without

accurate cephalometry, the clinician can oftencompare cephalic diameters with the pelvis in thelateral film, provided the head is central, and moreuse might be made of radiography'in the' course of'trial labour, to determine the progress of the headthrough the brim, and the degree of moulding.Indications for RadiographyAlthough Munro Kerr (1942) has advocated

routine radiography in all primigravidae, mostobstetricians believe they can exclude dispropor-tion with certainty in a large majority of cases by-clinical methods. Yet most of them would admit-that they have occasionally missed disproportion,even though they have applied their usual methodsof examination, and some are ready to look for thecause of unexpectedly difficult labour in postnatalradiography.

X-rays may be used to confirm gross pelvicabnormality, to provide accurate information incases of suspected disproportion, or to excludedisproportion with certainty when the pelvis isthought to be normal. In the latter group may bementioned patients who are difficult to examine,primigravidae with breech presentations, andpatients who have previously had a difficult orprolonged labour 'or a Caesarean section. It iscertainly helpful to both obstetrician and patientto have X-ray evidence that disproportion will notoccur, and this evidence will probably lessen un--necessary surgical procedures. A number ofpatients have now been delivered at Queen Char-lotte's Hospital, as at other maternity hospitals,without undue difficulty, having previously had aCaesarean section for disproportion diagnosedwithout the benefit of X-rays.

Confirmation of obvious pelvic abnormality isnowadays almost a routine before deciding onCaesarean section, and in not a few of these casesX-rays show that normal delivery is feasible inspite of a flattened brim, an asymmetric pelvis ora narrow outlet.One of the common indications for radiography

is the high head that cannot be pushed into thebrim. It is well known that this is often due. tobad flexion and that engagement will readily occurwhen the patient is examined sitting up. But itmay be difficult to feel the head in this position,.and the obstetrician may be uncertain whether itwill or will not quite engage. It.is best to resolvethis difficulty with a lateral X-ray in the erectposition and a full pelvimetry if the head is notfound to be engaged. -If the biparietal diameter issmaller than the obstetric conjugate it may usuallybe stated that there is no brim disproportion.SuCh a procedure will often obviate the necessity'for a trial labour, which is not infrequently accom-panied by defective uterine action, when both

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26o POST GRADUATE MEDICAL JOURNAL ' June I949

patient and obstetrician are wondering about thepossibility of Caesarean section.

'The Value of Radiography in BorderlineCasesMinor degrees of disproportion may be dis-

covered by careful pelvic examination, because ofa high head that cannot be pushed into the brim,or simply on routine radiographic examination.Evidence of such disproportion was found in aboutI8 per cent. of cases referred to the radiologist atQueen Charlotte's Hospital, and in these thechances of normal or difficult delivery are aboutequal. Problems at the brim must be consideredseparately from problems at the outlet, but bothmay give rise to uncertainty in the generallycontracted pelvis.

(a) Brim disproportion. There is no doubt thatclinical estimation of the shape and size of thebrim is less accurate than pelvimetry. X-raysmay be unnecessary when the head is engaged orcan.be pushed into the brim, but when this cannotbe done radiography will provide more informationthan can be obtained by examination under ananaesthetic at the 36th week, with less discomfortto the patient. This procedure should be don-demned as strongly as indiscriminate surgicalinduction.

Although the pelvic joints may expand some-what during pregnancy, there is little evidencethat appreciable expansion occurs during labour,and the mouldability of the foetal head cannot beassessed until labour occurs. Trial labour is,therefore, the best procedure for borderline brimdisproportion, unless the age or history of thepatient causes a bias in favour of elective Caesareansection. The outcome of trial labour must bedetermined mainly by clinical examination, butsometimes a head will mould to such an extentthat the vertex reaches the pelvic floor before thegreatest diameter has passed the brim. X-rays inlabour may still be of value in assessing theprogress of the head.

(b) Outlet disproportion. It is perhaps sur-prising that accurate measurements of the pelvicotitlet are difficult to achieve by clinical methods,but X-rays sometimes show that this is so.Furthermore, the foetal head cannot be used as apelvimeter except during delivery, and for thisreason trial labour, as usually understood, has noplace in outlet disproportion. Much greater ex-pansion of the pelvic diameters is possible at theoutlet than at the brim, and cases of absolute dis-proportion necessitating Caesarean section arerarely seen except in osteomalacic pelves or inachondroplasiac dwarfs. Caesarean section may,however, be the best treatment for a patient who-has -previously had a difficult forceps delivery with

perhaps a stillborn child. Labour may be surgic-ally induced about the 38th week in cases ofmarked outlet contraction confirmed by X-rays,but in most cases of minor contraction this is un-necessary and it may reasonably be expected thatforceps aid will overcome difficulty althoughoccasionally at the expense of a dislocated coccyx.When a persistent occipito-posterior occurs in

labour it is an advantage to know if the pelvis isanthropoid, as then a face to pubes delivery may besafely achieved, whereas with an android pelvis it isbetter to rotate the occiput. The accepted teachingand practice in cases of deep transverse arrest is torotate the head manually and deliver it throughthe lower pelvic strait with the long axis antero-posterior. Yet X-rays have shown that in 30per cent. of these cases the ischial bispinousdiameter is actually greater than the antero-posterior diameter of the outlet. This is a forcibleargument in favour of drawing the head throughthis plane in the transverse diameter in theseparticular cases, allowing rotation to occur afterthe head has passed the ischial spines. Suchpractice might well reduce the high foetal mortalityassociated with deep transverse arrest.

ConclusionsThe use of radiography in borderline cases has

been criticized on the grounds that unnecessaryinterference will increase. This should not be soif X-rays are done well and are interpreted withintelligence. It is true that unnecessary Caesarean·section has sometimes been done when theobstetrician has been misled by bad X-rays, butthis should make us improve our X-ray diagnosticmethods and learn from our mistakes. The chiefvalue of radiology may be the exclusion of dis-proportion, but valuable information can also begained in the borderline case, and forewarning ofpossible dystocia should at least ensure that thesepatients are confined in hospital, where facilitiesare available for a- Caesarean section or difficultforceps delivery, and for resuscitation of the babyif the need arises.

ACKNOWLEDGMENTSI wish to record my thanks to Dr. Rohan

Williams for his kind assistance and helpfulcriticism, and for providing the illustrativematerial.

BIBLIOGRAPHY

ALLEN, P. (I947), Brit. 7. Radiol., 20, 45 and 205.CHASSAR MOIR, J. (I947), Y. Obst. and Gyn. Brit. Emp., x, 20.MORRIS, W. I. C. (I947), Edin. Med. J., 54, 90.MUNRO-KERR, J. M. (I948), .7. Obs t.and Gyn. Brit. Emp., 4, 401.ROHAN WILLIAMS, E. (I943), Brit. J. Radiol., x6, 173.

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