Standardised Radiological Pelvimetry (Part III)

Abstract

Measurement of the outlet is a relatively neglected aspect of pelvimetry. The reason seems to be that the outlet is accessible to clinical examination, and the general feeling is that clinical methods are sufficiently accurate. Clinical measurement, however, is usually rough and subject to major errors, and there are few who use special outlet pelvimeters (Nicholson and Tauber, 1945; Thoms, 1946). It will not be denied that knowledge of the capacity of the outlet is important for the practice of good obstetrics. Disproportion at the inlet can often be gauged with reasonable accuracy by using the head as a pelvimeter, and if found can be dealt with by the appropriate elective procedure. Disproportion at the outlet, however, cannot be recognised by using the head in this manner before labour, and, if encountered unexpectedly late in labour, it may well prove disastrous to both mother and fœtus. Some have even decried outlet measurements on the dangerous and fallacious assumption that outlet contraction does not occur except in association with inlet contraction. No system of pelvimetry could be termed complete which failed to take the outlet into consideration. In a previous communication (Allen, 1943), the radiological aspects of the subpubic angle were discussed at length, and it was suggested that measurement of the angle might be replaced by measurement of what was termed “the symphysis-biparietal distance”. Since then, the method has been further investigated, modified, and extended.