OUTLET CONTRACTION OF THE PELVIS * By W. I. C. MORRIS, M.B., F.R.C.S.E., M.R.C.O.G. There is no great unanimity in regard to the incidence or even the existence of outlet contraction. Stander (1946) states that contractions of the pelvic outlet occur in about 6 per cent, of all women. De Lee (1938) quoted figures as high as 26 per cent. (Stocker), but others, including Bourne and Williams (1939), are sceptical of the importance of outlet contraction, and emphasise that the head which passes the pelvic brim is unlikely to meet grave difficulty at the outlet. All of us, however, are familiar with the occasional unexpectedly stiff forceps operation, as a result of which we deliver with much soft tissue damage a still-born baby, or, perhaps worse, one which survives to develop signs of grave intra-cranial damage. A tentative diagnosis of outlet contraction in such a case may enable us to lay a flattering unction to our souls, but outlet contraction is a subtle condition which may result from a variety of deformities and abnormalities, and its detection before the occurrence of a disaster is often difficult. I propose to devote the major portion of this lecture to an examination of various diagnostic criteria which may give such forewarning, and to deal but briefly with other aspects of outlet contraction. The Shape and Dimensions of the Fcetal Head in Labour The first approach to this problem should be to obtain an accurate picture of the fcetal head in that stage of labour when it first meets the outlet resistance. As clearly demonstrated by Moir (1929, I932)' the head, moulded in the attitude of full flexion, approximates very closely to a cylinder, with bi-parietal, sub-occipito-bregmatic, and occipito-frontal diameters all more or less equal. As a reasonable be average the diameter of this cylinder in the full-term head may taken as 9-3 centimetres (3! inches), and so long as the head remai*1^ flexed, this is the largest diameter. During the normal mechanism 0 extension of the head an oblique measurement (not a true diamete1' of the cylinder) which we may accept as roughly 10 centimetres (4 inches) has to pass the outlet. The Pelvic Outlet in Relation to the Fcetal Head The fcetal head enters the outlet at the inferior pelvic strait plane of least pelvic dimensions, i.e. the plane which cuts the ischi3 spines, the lower border of the symphysis pubis, and (not infrequently/ * A Honyman Gillespie Lecture delivered in the Royal Infirmary on 27th J?ne 1946. OUTLET CONTRACTION OF THE PELVIS 91 the sacro-coccygeal joint. The axis of advance of the foetal head makes a bend almost equal to a right angle at this plane, changing from a own wards and backwards path to one which is downwards and orwards. This change of direction carries the advance into the canal of the outlet proper, bounded at the sides by the medial surfaces ?f the bodies of the ischia-; bounded posteriorly by the soft tissues the f pelvic floor and by the coccyx (a structure which is of negligible lrnPortance if normally mobile) ; and opening anteriorly through the gap between the conjoint rami of ischium and pubis, referred to as e pubic arch. The apex of the arch is closed the structures ?f pubic by the vestibule. Types of Outlet Contraction B?ny contraction of the outlet may result from :? (0) Absolute narrowing at the plane of least pelvic dimensions. (6) Narrowing below the level of this plane, i.e. narrowing of the pubic arch. 00 Combinations of the above. Narrowing of the Plane of Least Pelvic Dimensions first of these contractions may involve either the coronal or e sagittal measurements of the outlet. Williams (quoted Stander, WaS more imPressed by the frequency of coronal contractions, butut Moir (1941) considers narrowing in the sagittal plane to be at as important. In either case, the narrowing may result from a .5 funnel tendency reducing the pelvic cavity more or less ?rmly from brim to outlet, or on the other hand, the contraction an isolated incident. Certain conditions are a .^e notoriously of the (C ,Cla general funnelling pelvis, e.g. the android pelvis We^> Moloy and D'Esopo, 1933, J934? x935)? high assimilation pe^. s> and spinal deformities such as dorso-lumbar kyphosis and anally spondylolisthesis. It is also recognised that the so-called "^mor contraction of the pelvis is often associated with funnel even more the brim j which reduces the outlet than (Stander, ^^nity, As an isolated incident, the coronal measurements of the plane ^ cav^ reduced by prominent ischial spines, inverted into the pelvic while the antero-posterior diameter may be reduced by unusual Convy>er?ence of jn the lower sacrum towards the symphysis pubis, especially o.- .at extremely common abnormality where the sacrum consists of vertebra:. S'*^d of alte aVrowi^ ?f the Pubic arch is usually independent any general ?f the pelvic shape. The effect of narrowing of the pubic arc^a.tl0nls *? deny the anterior of the bony hiatus to the head, It js ^ portion the symphysis pubis were prolonged downwards and back ards to an extent depending on the degree of narrowing. The 92 W. I. C. MORRIS influence of this intangible barrier (Fig. i) produces a reduction in the available antero-posterior diameter of the outlet canal dependent upon two factors :? (?) The extent of the pubic arch denied to the head. (?) The angulation between the plane of the pubic arch and the axis of the symphysis pubis. The more nearly this approaches i8o?, the less the influence of pubic arch narrowing, and vice versa. afC^' Fig. i.?Showing effect of varying angle between axis of symphysis and plane of &TC^' Shaded area = portion of arch denied to head. Besides reducing the available antero-posterior diameter of the outlet, the closure of the anterior portion of the arch alters the genefa &e axis of the advance of the head, partially undoing the curve of birth canal, and compelling the head to emerge more posteriorly than usual. This has two important effects to be referred to later, viz. (a) The stimulus to internal rotation of the head is diminish^ (Moir, 1929, 1932). (b) Perineal trauma is more common and more severe. Measurements of Pelvic Outlet in the Dried Pelvis 0) Measurements of Least Pelvic Plane.?Exact definition of ^ measurements is simple. The antero-posterior diameter is measure between the lower border of the symphysis pubis and the tip of OUTLET CONTRACTION OF THE PELVIS 93 are fused with the sacrum. When one or more pieces of the coccyx fixed the diameter requires to be measured to the lowest point. of the ischial ^crum,e transverse diameter is measured between the tips spines. Stander (1946) defines these as :? . 11 centimetres Antero-posterior *5 . centimetres Transverse . .10-5 Nicholson variations occur. It is suggested by (1938, 1943) area of this as a at *t is more profitable to calculate the plane are the axes, geometrical ellipse of which the foregoing measurements er measurements alone. Radiological than to rely on the linear women les by Ince and Young (1940) in 500 consecutive yielded can be with the average figure of 93-7 sq. cm. This compared is ea of a cross-section of a 9*3 cm. cylinder, which approximately 08 sq. cm. to obtain a (P) Measurement of Pubic Arch.?It is difficult of the arch. The method of stating the measurement J* lsfactory that is the USUa^ is to measure the base of the arch, technique this with dist ketween the ischial tuberosities, and to correlate e and the sacral thean,CGdistance between the mid-point of this diameter measurement of the This is referred to as the posterior sagittal diameter and e ' The distance between the inter-tuber-ischial ^ to as the anterior can also be measured and is referred sa .SymPhysis the ^^ameter of the outlet. By applied trigonometry, quotient of V diameter anter*or sagittal diameter by the inter-tuber-ischial can h at the to determine the angle subtended symphysis pubis bvt, e.used Nicholson lnter"tuber_ischial diameter (Ince and Young, 1940 ; and T a referred to as the sub-pubic er> !946), and this is sometimes an 1 of course. Figures & e- It can be measured by more direct means ^oted are . 11 cm. Inter-tuber-ischial . 1946) cm. (Stander, sagittal . 7 cm.-10 posterior 8? 1943) Sub-pubic angle . (Smout, index o~ a satisfactory These measurements do not provide very e of the arch. For example : Capacity pubic betweenuclwccxi is to beue measuredmcdsuicu (*) The inter-tuber-ischialluier-rUDer-iscniai diameter quari'mt defined and, ral masses, not between easily points, ? bony even are 1945). material, wide errors possible (Heyns, S^etal of the foetal head passing Moreoy ^ We consider the cylindrical body throu that major obstruction Ier> Pu^c arch it will be apparent any Win to the cylinder, 0g w^ere the converging rami become tangential gener iiUr the arch were ^ distance from the tuberosities. If pubic a ?0me uiamcici andanu strict lnier-iuuer-istnidi diameter angle> a<1 knowledgeKnowieage of01 thetne inter-tuber-ischial the S?K these with would enable us to predict points angle ischia and are accuSub-Pubicracy- However, the conjoint rami of the pubis 94 W. I. C. MORRIS seldom straight, but show a slight concavity directed medially near the symphysis, and in some cases a convexity towards the middle line in their ischial portions so that each conjoint ramus comes to have a- sigmoid curve.