Bony Pelvis and the Pelvic Floor (female)

Bony pelvis

• Consists of two hip bones (ileum and ischium), joined by the sacrum posteriorly and symphysis pubis anteriorly
• The pelvic inlet can be described as the plane between the sacral promontory and superior aspect of the the symphysis pubis
  • Longest diameter is transverse (13cm); shortest diameter is anteroposteriorly (11cm); oblique (12cm)
• The pelvic outlet can be described as the plane between the inferior tip of S5 to the inferior aspect of the symphysis pubis
  • Longest diameter is anteroposterior (13cm); shortest diameter is transverse (11cm); oblique (12)
• NB The diameters in the mid-cavity are all equal (12cm- i.e. ‘perfect’ circle)
• The ‘rotation’ of diameters is mimicked during labour
• Clinically, the ischial spine is important as it can be felt during labour in relation to the descending foetal part to measure descent
  • NB the ischial spine is also the attachment point for the sacrospinous ligament (to sacrum) which, together with the sacrotubrous ligament, forms the greater and lesser sciatic foramen.

Pelvic Floor muscles

• The obturator internus muscle sits on the medial side of the ischial bone and, with the body of the pubis, forms a wall that supports the pelvic floor muscles
  • The pyriformis muscle also run from the anterior sacrum to the greater troachanter (part of the external rotators).  They also line the pelvis and are important because they reduce the diameter of the inlet and mid-cavities (which, during labour, will push the foetus forward).
• Pelvic floor (/diaphragm or levator ani)
  • Sling of various muscles through which the anus, vagina and urethra are supported and pass through
    • Puborectalis forms a sling around the junction of the anus and rectum
    • Pubococcygeus lies posteriorly to this, forming the bulk of the levator ani
    • The illeococcygeus is often poorly developed and lies most posteriorly