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Sexual Dimorphism – Pelvic Wall and Floor

by Craig Canby, PhD
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    00:00 muscle. There is sexual dimorphism of the pelvis or gender differences in how the pelvis is constructed. Here, we see two pelves which would be plural of pelvis. The upper one is a woman. The lower one is associated with a typical male pelvis and there are going to be differences between men and women with respect to the dimensions of the inlet and the dimensions of the outlet. And if we take a look, first, at the inlet, here is the pelvis of a woman. And if we follow the landmarks or boundaries of the inlet, we see that they are fairly round in appearance. If we shift our attention down below to a male pelvis and follow the boundaries, these boundaries form a more heart-shaped structure.

    01:05 Thus, this male pelvis has a narrower inlet. The outlet in a woman is going to be larger than in a man. The ischial spines are going to have greater separation. The dimension between the coccyx and the pubic symphysis will be longer in a woman than in a male.

    01:35 And you can appreciate the dimensional differences here from coccyx to pubic symphysis, shorter distance than above and the dimension between the ischial spines is shorter in a male pelvis than in a woman's pelvis. And the purposes of the sexual dimorphism relates to childbearing.

    01:59 A woman needs a larger inlet and a larger outlet for the birth of a child.

    02:07 There is also a difference in a region called the subpubic angle between a male pelvis and a female pelvis. The subpubic angle is shown right in through here and you can appreciate the angle in this example versus the angle that's formed over here. The pelvis with the wider subpubic angle belongs to a woman and then the subpubic angle that is narrow in appearance would belong to a man. However, the sexual dimorphism that we just went through is not always black and white. In some cases, those lines can become somewhat blurred.

    03:00 Now, we'll shift our focus to the joints of the pelvis. And when we think about the pelvis and its articulations, we're going to highlight three of those, a lumbosacral articulation, a sacroiliac articulation and the pubic symphysis. All three of those articulations are demonstrated in these two slides that we see here. We have a male pelvis and a female pelvis.

    03:36 And if you take a look at the subpubic angle here versus the subpubic angle here, the pelvis below may has a much wider angle. So, that would be a female pelvis with the male pelvis above.

    03:52 The joints that are demonstrated here would be between the 5th lumbar vertebra and we see the transverse processes of it at this level and then the 5th lumbar vertebra will articulate with the sacrum. The sacroiliac joints are here and here, or here and here, and then the pubic symphysis is where the two pubic bones will meet in the anterior midline as well, as here in the female pelvis. These are very very strong articulations because of the tension, stress and weight that's placed upon them.

    04:34 To help with structural integrity of these articulations, there are several ligaments of the pelvis. Some of these are much more important in providing for structural integrity of the articulations. Others, we've mentioned before, in creating notches into apertures.

    04:52 But, some of the ligaments to highlight here that would be found in the pelvis would be the lumbosacral ligament. This one is really not well identified, but if we look at the transverse process here of the 5th lumbar vertebra and look at this ligament that's attached to it and then attached to the superior aspect of the ala of the sacrum, this is your lumbosacral ligament. The iliolumbar ligament would run from this same transverse process.

    05:30 It would course more horizontally and then run on the superior aspect to the posterior iliac crest. That is also not well shown here. The sacroiliac joints are very very strong ligamentous structures because of the forces placed upon them. We have the anterior sacroiliac ligament here and then on the posterior aspect, we would have the posterior sacroiliac ligament.

    06:00 Anteriorly, here, we have the inguinal ligament. This is not really supporting any of the articulations, but represents the inferior attachment of the aponeurosis of the external abdominal oblique. Underneath it, you have some aisles and that will allow for the passage of muscles into the thigh and will allow for the passage of neurovascular structures in and out of the pelvis.

    06:32 The sacrotuberous ligament is seen here. Sacrospinous ligament is shown there.

    06:45 That brings us to our summary slide and the key take-home messages from this lecture are as follows. The pelvis is composed of the sacrum, coccyx and the pelvic bones. The false pelvis is the inferior part of the abdominal cavity whereas the true pelvis defines the pelvic cavity itself.

    07:03 The pelvis is bounded by lateral walls and a floor.

    07:07 The pelvis wall is formed by bony structures, ligaments and muscles and presents three apertures.

    07:15 The pelvic diaphragm and the deep perineal pouch and the perineal membrane constitute the floor. Sexual dimorphism of the pelves is necessary to facilitate childbirth in women. The lumbosacral, sacroiliac and pubic symphysis represent joints of the pelvis and these joints are strengthened or reinforced by strong ligaments.

    07:42 Thank you for joining me on this lecture on “The Pelvis”.


    About the Lecture

    The lecture Sexual Dimorphism – Pelvic Wall and Floor by Craig Canby, PhD is from the course Abdominal Wall.


    Included Quiz Questions

    1. Sacroiliac ligament
    2. Sacrotuberous ligament
    3. Iliolumbar ligament
    4. Ischiotuberous ligament
    5. Lumbosacral ligament
    1. A wide angle.
    2. A narrow angle.
    3. An acute angle.
    4. A right angle.
    5. A circular shape.
    1. Larger inlet and larger outlet.
    2. Narrow inlet and narrow outlet.
    3. Heart-shaped inlet and outlet.
    4. There is no difference.
    5. Difference is minute and not significant.
    1. Inguinal ligament.
    2. Anterior Sacroiliac ligament.
    3. Posterior sacroiliac ligament.
    4. Lumbosacral ligament.
    5. Iliolumbar ligament.
    1. Short.
    2. Large.
    3. Same as women.
    4. Greater than women.
    5. No gender based difference.

    Author of lecture Sexual Dimorphism – Pelvic Wall and Floor

     Craig Canby, PhD

    Craig Canby, PhD


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