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//SKELETAL PELVIS AS A WHOLE

SKELETAL PELVIS AS A WHOLE

The term pelvis (‘basin’) is applied variously to the skeletal ring formed by the hip bones and the sacrum, the cavity therein, and even the entire region where the trunk and lower limbs meet. It is used here in the skeletal sense, to describe the irregular osseous girdle between the femoral heads and fifth lumbar vertebra. It is large because its primary function is to withstand the forces of body weight and musculature. In this section, its obstetric, forensic and anthropological significance will be considered.

The pelvis can be regarded as having greater and lesser segments, the true and false pelves. The segments are arbitrarily divided by an oblique plane passing through the sacral promontory posteriorly and the lineae terminales elsewhere. Each linea terminalis includes the iliac arcuate line, pectineal line (pecten), and pubic crest. The segments are continuous, and the parts of the body cavity that they enclose are also continuous through the pelvic inlet (pelvic inlet).

Greater pelvis

The greater pelvis consists of the ilium and pubis above the lineae terminales and the sacral base. This junctional zone is structurally massive and forms powerful arches from the acetabular fossae to the vertebral column around the visceral cavity, which is part of the abdomen. It has little anterior wall because of the pelvic inclination.

Pelvic inlet (superior pelvic aperture)

The pelvic inlet or brim may be round or oval, and is indented posteriorly by the sacral promontory. The pelvic brim is obstetrically important and has also long been measured for anthropological reasons, as has the pelvic cavity.

By convention, the pelvic inlet is described in three dimensions. The anteroposterior diameter (true conjugate) is measured between the midpoints of the sacral promontory and upper border of the symphysis pubis and on average is 10 cm in the adult male and 11.2 cm in the adult female. The transverse diameter is the maximum distance between similar points (assessed by eye) on opposite sides of the pelvic brim and is on average 12.5 cm in the male and 13.1 cm in the adult female. The oblique diameter is measured from the iliopubic ramus to the opposite sacroiliac joint and is on average 12 cm in the adult male and 12.5 cm in the adult female. These measurements vary with the individual and racial group.

Articulated bony pelvis

The lesser pelvis encloses a true basin when soft tissues of the pelvic floor are in place. Skeletally it is a narrower continuation of the greater pelvis, with irregular but more complete walls around its cavity. Of obstetric importance, it has a curved median axis, and superior and inferior openings. The superior opening is occupied by viscera. The pelvic floor, viscera and subjacent perineal sphincters close the inferior opening.

Cavity of the lesser pelvis

The cavity of the lesser pelvis is short, curved, and markedly longer in its posterior wall. Anteroinferiorly it is bounded by pubic bones, their rami and symphysis. Posteriorly it is bounded by the concave anterior sacral surface and coccyx. Laterally on each side its margins are the smooth quadrangular pelvic aspect of the fused ilium and ischium. The region so enclosed is the pelvic cavity proper, through which pass the rectum, bladder and parts of the reproductive organs. The cavity in females must also permit passage of the fetal head.

The pelvic cavity diameters are measured at approximately the mid level. The anteroposterior diameter is measured between the midpoints of the third sacral segment and posterior surface of the symphysis pubis and is about 10.5 cm in the male and 13 cm in the adult female. The transverse diameter is the widest transverse distance between the side walls of the cavity, and often the greatest transverse dimension in the whole cavity. It measures about 12 cm in the adult male and 12.5 cm in the adult female. The oblique diameter is the distance from the lowest point of one sacroiliac joint to the midpoint of the contralateral obturator membrane and measures about 11 cm in the male and 13.1 cm in the adult female. All measurements vary with the individual and racial group.

Pelvic outlet (inferior pelvic aperture)

Less regular in outline than the pelvic inlet, the pelvic outlet is indented behind by the coccyx and sacrum and bilaterally by the ischial tuberosities. Its perimeter thus consists of three wide arcs. Anteriorly is the pubic arch, between the converging ischiopubic rami. Posteriorly and laterally on both sides are the sciatic notches between the sacrum and ischial tuberosities. These are divided by the sacrotuberous and sacrospinous ligaments into greater and lesser sciatic foramina.

With ligaments included, the pelvic outlet is rhomboidal. Its anterior limbs are the ischiopubic rami (joined by the inferior pubic ligament) and its posterior margins are the sacrotuberous ligaments, with the coccyx in the midline. The outlet is thus not rigid in its posterior half, being limited by ligaments and the coccyx, all slightly yielding. Even with the sacrum taken as the posterior midline limit (more reliable for measurement), there may be slight mobility at the sacroiliac joints. Note also that a plane of the pelvic outlet is merely conceptual. The anterior, ischiopubic part has a plane which is inclined down and back to a transverse line between the lower limits of the ischial tuberosities, and the posterior half has a plane approximating to the sacrotuberous ligaments, sloping down and forwards to the same line.

Three measurements are made for the pelvic outlet. The anteroposterior diameter is usually measured from the coccygeal apex to the midpoint of the lower rim of the symphysis. The lowest sacral point may also be used (on average male 8 cm, female 12.5 cm). The transverse (bituberous) diameter is measured between the ischial tuberosities at the lower borders of their medial surfaces (on average male 8.5 cm, female 11.8 cm). The oblique diameter extends from the midpoint of the sacrotuberous ligament on one side to the contralateral ischiopubic junction (on average male 10 cm, female 11.8 cm). All measurements vary with the individual and racial group.

Other measurements

Apart from these main measurements, by consensus the basis of pelvic osteometry, other planes and measurements are used in obstetric practice. The plane of greatest pelvic dimensions is an obstetric concept. It represents the most capacious pelvic level, between the pelvic brim and midlevel plane, and corresponds with the latter anteriorly at the middle part of the symphysis pubis and posteriorly at the level of the second and third sacral segments.

The plane of least dimensions is said to be at about midpelvic level. Its transverse diameter is between the apices of the ischial spines. This measurement is about 9.5 cm in an adult female and is just wide enough to allow passage of the biparietal diameter of a fetal head (about 9 cm). Not surprisingly, most difficulty in parturition occurs here.

The above measurements are sometimes made in clinical practice using X-ray or MRI pelvimetry. Precise measurement is not possible without radiological techniques, and even these do not take into account the soft tissues. In the past, measurements were made at physical and vaginal examination. However, these manual measurements have proved to be of little clinical value and are now obsolete.

Morphological classification of pelves

Interest in the dimensions described above is primarily obstetric and, less frequently, forensic. All pelvic measurements display individual variation and the values quoted are means from limited surveys. Sexual and racial differences also occur. These measurements have been analysed by many anatomists, anthropologists, obstetricians and radiologists in attempts to classify human pelves, especially female. The four most common terms used today are gynaecoid, anthropoid, platypelloid and android. The gynaecoid pelvis is the traditional Western female pelvis with a heart-shaped brim and the range of measurements quoted above. An anthropoid pelvis has a larger midcavity and a wide anteroposterior inlet which is oval in shape: it is more common in women of African origin and may be associated with a ‘high assimilation’ pelvis where there is an additional lumbar vertebra. A platypelloid pelvis is flat and oval from side to side at the brim: it is a contracted pelvis that is rarely seen nowadays, having previously been associated with rickets. An android pelvis has a triangular brim and is the shape of a male pelvis.

Pelvic axes and inclination

The axis of the superior pelvic aperture traverses its centre at right angles to its plane, directed down and backwards (Fig. 80.9). When prolonged (projected) it passes through the umbilicus and midcoccyx. An axis is similarly established for the inferior aperture: projected upwards it impinges on the sacral promontory. Axes can likewise be constructed for any plane, and one for the whole cavity is a concatenation of an infinite series of such lines (Fig. 80.9). The fetal head, however, descends in the axis of the inlet as far as the level of the ischial spines; it is then directed forwards into the axis of the vagina at right angles to that axis. The form of this pelvic axis and the disparity in depth between the anterior and posterior contours of the cavity are prime factors in the mechanism of fetal transit in the pelvic canal.

  

Fig. 80.9  Median sagittal section through the female pelvis, showing the planes of the inlet and outlet and the axis of the pelvic cavity.

In the standing position the pelvic canal curves obliquely backwards relative to the trunk and abdominal cavity. The whole pelvis is tilted forwards, the plane of the pelvic brim making an angle of 50–60° with the horizontal. The plane of the pelvic outlet is tilted to about 15°. Strictly, the pelvic outlet has two planes, an anterior passing backwards from the pubic symphysis and a posterior passing forwards from the coccyx, both descending to meet at the intertuberous line. In standing, the pelvic aspect of the symphysis pubis faces nearly as much upwards as backwards and the sacral concavity is directed anteroinferiorly. The front of the symphysis and anterior superior iliac spines are in the same vertical plane. In sitting, body weight is transmitted through inferomedial parts of the ischial tuberosities, with variable soft tissues intervening. The anterior superior iliac spines are in a vertical plane through the acetabular centres, and the whole pelvis is tilted back with the lumbosacral angle somewhat diminished at the sacral promontory.

Pelvic mechanism

The skeletal pelvis supports and protects the contained viscera, but is primarily part of the lower limbs, affording wide attachment for leg and trunk muscles. It constitutes the major mechanism for transmitting the weight of the head, trunk and upper limbs to the lower limbs. It may be considered as two arches divided by a coronal transacetabular plane. The posterior arch, chiefly concerned in transmitting weight, consists of the upper three sacral vertebrae and strong pillars of bone from the sacroiliac joints to the acetabular fossae. The anterior arch, formed by the pubic bones and their superior rami, connects these lateral pillars as a tie beam to prevent separation; it also acts as a compression strut against medial femoral thrust. The sacrum, as the summit of the posterior arch, is loaded at the lumbosacral joint. Theoretically this force has two components, one thrusting the sacrum downwards and backwards between the iliac bones, the other thrusting its upper end downwards and forwards. Sacral movements are regulated by osseous shape and massive ligaments. The first component therefore acts against the wedge, its tendency to separate iliac bones resisted by the sacroiliac and iliolumbar ligaments and symphysis pubis.

Vertical coronal sections through the sacroiliac joints suggest division of the (synovial) articular region of the sacrum into three segments. In the anterosuperior segment, involving the first sacral vertebra, the articular surfaces are slightly sinuous and almost parallel. In the middle segment the posterior width between the articular markings is greater than the anterior, and centrally a sacral concavity fits a corresponding iliac convexity, an interlocking mechanism relieving the strain on the ligaments produced by body weight. In the posteroinferior segment the anterior sacral width is greater than the posterior and here its sacral surfaces are slightly concave. Anteroinferior sacral dislocation by the second component (of force) is prevented, therefore, mainly by the middle segment, owing to its cuneiform shape and interlocking mechanism. However, some rotation occurs, in which the anterosuperior segment tilts down and the posteroinferior segment up. ‘Superior’ segmental movement is limited to a small degree by wedging but primarily by tension in the sacrotuberous and sacrospinous ligaments. In all movements the sacroiliac and iliolumbar ligaments and symphysis pubis resist separation of the iliac bones.

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