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Bony and ligamentous pelvis
The bony pelvis forms a basin which has in inlet that leads into a cavity and also an outlet. The cavity is described using the dimensions of a midcavity. Hence we refer to three main dimensions of the true pelvis as it is called
1. Inlet or superior aperture of lesser pelvis
2. Outlet or inferior aperture of lesser pelvis
Some new terminologies can be identified in the above description
We have lesser pelvis which is also called true pelvis. It describes the cavity which is formed below the pelvic brim
We have the greater pelvis which is also called the false
pelvis. it placed above the pelvic brim and it has no true superior border for it merges with the abdominal cavity and the greater sac of peritoneum. It interconnects with the main peritoneal cavity (other wise known as greater sac) at the pelvic corridor.
The bony pelvis is made up of three bones as follows
1. Hip bone which are paired (os inominatum)
2. Sacrum placed behind
The walls of the bony pelvis are augmented by two important important ligaments aside from the ligaments of the various articular surfaces associated with the pelvic apparatus. They are the
1. Sacrospinous ligament
2. Sacrotuberous ligament
Both ligaments take their origin from the sides of the coccyx and sacrum. The sacrotuberous ligament runs to gain attachment to the ischial tuberosity, while the sacrospinous ligament enters the ischial spine. Their inferior boundaries help to delimit the extent of the outlet of the pelvis or its inferior aperture of lesser pelvis.
Margins of inlet
The inlet margin begins medially from the superior aspect of the symphysis pubis. It then sweeps across the superior border of that bone passing through the pubic tubercle to reach the pectineal line of pubis and the junction between the pubis and the ilium formed by the iliopubic eminence.
It then runs across the ilium laterally to cross the sacroiliac joint. It finally ends at the sacral promontory which is the projection f the the superior aspect of the first sacral vertebra anteriorly.
Margins or level of the midcavity.
The midcavity is placed at the level of the ischial spines
Margins of the outlet:
It again begins medially at the lower border of the symphysis pubis and extends laterally to the inner margins of the sacrotuberous and sacrospinous ligaments. It then turns inwards to the lateral walls of the ligaments and then to the tip of the coccyx.
The importance of the pelvis lies in parturition in the female. It is therefore designed for this function in the female. Hence the female pelvic type which is called gynaecoid pelvis has all the features to make parturition easy for the female. Unfortunately, not all females have this type of pelvis. Some infact have pelvic types similar to that of man called android pelvis.
Caldwell-Molloy classification of pelvic types. More
1. Android pelvis - this is the pelvic type you see in males. It has a wide pelvic brim (inlet) but it tapers to a cone so that the outlet is small compared to inlet. Also, its ischial spines are prominent and may cause arrest of the baby/s head during descent.
This is the bona fide female pelvis. Its walls are parallel so that the inlet dimension is the same as the outlet. It has a roomy inlet and also outlet dimensions. It has an oval inlet and its sacral promontory is not prominent. Also its midcavity diameter is roomy and not constricted. Its ischial spines are not prominent.
This pelvis is also parallel but its AP dimensions are more than the transverse dimensions. Hence they may be said to be contracted transversely.
This is the best example of a flat pelvis. It is contracted at the inlet but wider at the outlet.
So the lateral walls are divergent.
The folowing are the clinical pelvimetric mesurements
1. Diagonal conjugate diameter. This runs from the lower of the symphysis pubis to the sacral promontory. It is measured by pelvic examination
2. Subpubic angle. Whether it is acute or obtuse. The obtuse angle provides more room.
3. Sacrospinous diameter. This diameter runs form the lateral aspect of the sacrum an coccyx to the ischial spine. Normally it should be about 4cm.
4, Intertuberischial diameter. This must take the knuckle and it is more than 8cm
5. Morphology of the ischial spine. A heavy ischial spine is a disadvantage
6. Morphology of the sacral promontory. A heavy sacral promontory may give a contracted inlet dimension.
Professor of Anatomy and Director Human Anatomy Lecture Series.
7 January 2003
University of Jos Human Anatomy Lecture Series http://jogunranti.tripod.com
© 2003 Department of Anatomy, University of Jos. All rights reserved.