Anatomy of ischiorectal fossa and pelvic diameters Flashcards
What is the ischioanal (AKA rectal) fossa?
- It is a wedge-shaped fossa that is fat filled located on either sides of the anal canal in the perineum
- It is an important anatomical area because it allows the anal canal to expand during defecation and contains vessels, nerves and lymphatics
What are the boundaries of the ischioanal/rectal fossa?
1) Base: Skin
2) Apex: The junction between the medial (levator ani) and the lateral (obturator internus)
3) Medial wall: Levator ani and the anal canal
4) Lateral wall: Ischial tuberosity, obturator internus muscle, which is covered with obturator fascia (obturator fascia splits to enclose the pudendal canal, which contains the neurovasculature)
What is the fibrous canal?
- It is a fibrous tunnel formed by the splitting of the obturator fascia
- Located at the lateral wall of the ischio-anal fossa
- It contains the pudendal nerve, internal pudendal vessels, and nerve to obturator internus
What are the contents of the pudendal canal?
- PIN
1) Pudendal nerve
2) Internal pudendal vessels
3) Nerve to obturator internus
What are the recess of the ischioanal/rectal fossa?
we have two recess:
1) Anterior recess found above the urogenital diaphragm and below the inferior fascia of the pelvic diaphragm
- It is important as infections (abscesses) can spread from the anal region to the urogenital triangle through it
2) Posterior recess:
- It is a backward extension of the fossa between the gluteus Maximus and levator ani
- Both recess and fossa communicate with each other and thus abscess can go from one fossa to another
What are the contents of the ischorectal/anal fossa?
1) Pad of fat (supports the anal canal and allows its distentsion)
2) Pudendal nerve and internal pudendal vessels (give rise to the inferior rectal vessels and nerves)
3) Perforating cutaneous nerve (perforates the sacrotuberous ligament)
4) Perineal branch of S4 (it perforates the coccygeus)
What will happen if, during a childbirth the dr damaged the pudendal nerve?
Rectal incontinence (as it supplies the anal canal)
What is the structure of the fat in the superficial part of the ischiorectal/anal fossa?
The fat is split by the fascia into small compartments (loculi), which are fixed to the skin, and the,s if one of these compartments gets infected, there will be no space for the compartment to expand, and thus, it will be very painful
What makes the rectal neurovascular bundle safe from injury during an incision to the fossa (in case of abscess)?
Because it arches up and then curves downwards
- If they were horizontal instead they could be damaged by the needle
Why is it important to measure the size of the pelvis?
Pelvimetry (pelvic dimension) is important to assess the mode of delivery whether C-section or normal vaginal delivery
What is the true pelvis?
- The true pelvis/pelvis minor/obstetric pelvis, is the part that extends between the superior and inferior pelvic aperture (inlet and outlet)
What are the boundaries of the pelvic inlet?
- It is approximately circular
1) Promontory of the sacrum (the upper anterior border of the first sacral vertebra)
2) Ala of the sacrum
- Linea terminalis (Arcuate line + pectineal line + pubic crest)
3) Arcuate line
4) Pectineal line
5) Pubic crest
6) The upper border of the symphysis pubis
What are the boundaries of the pelvic outlet?
- It is diamond-shaped, and wider in females
- Tip of coccyx to the lower border of pubic symphysis
1) Anterior segment: Slopes backwards and downwards
2) Posterior segment: Slopes forward and downwards
3) Lateral sides: Sacrotuberous ligament, ischiopubic rami and tuberosity
What are the different structures when we compare the male to the female pelvis?
1) Sacrum
2) Hip bone
3) Articulated pelvis
What are the causes that make the female pelvis different from the male pelvis?
1) Heavier built and stronger muscles in men, which makes the bones thicker and more prominent in the anatomical markings
2) Adaptation of the female pelvis for childbirth, which makes it wider and shallower
What are the sexual differences in the sacrum?
1) It is less curved in the females (to allow them to accommodate a baby)
2) It is shorter in the females, and the auricular surface occupies the length of only two vertebrae (2.5 in males)
3) It is wider in females as their ala are broader
What are the sexual differences in the hip bone?
1) They have a greater sciatic notch (in females, it is nearly a right angle; however, in males, it is less than 90 degrees)
2) The obturator foramen is triangular in females and oval in males
3) The ischial spine in females is in the plane of the body of the ischium; however, in females, it is inverted towards the pelvic cavity
4) The acetabulum is wider in males
What are the sexual differences in the articulated pelvis?
1) The inlet is transversely oval in females and heart-shaped in males
2) The cavity is shallow and wide in females and deep and narrow in males
3) The subpubic angle is wider in females
How are the pelvic measurements done in obstetrics?
1) AP diameter (11 cm), the true conjugate diameter is the midpoint of the superior border of the symphysis pubis and the midpoint of the sacral promontory
- We can measure it clinically by measuring the diagonal conjugate diameter (per vaginum), where we calculate the length from the promontory to the lower edge of the symphysis pubis, which will be 11.5 cm or more than the true conjugate
diameter
What are the average dimensions of the female pelvis?
1) Inlet:
- AP: 11 cm
- Transverse diameter (maximum diameter): 13.5 cm
- Oblique diameter: 12.5 cm
2) Outler:
- AP (between the lower edge of the symphysis pubis and the sacrum/coccyx): 13.5 cm
- Transverse diameter (between the ischial tuberosities): 11 cm
- FYI: The long diameter of the baby’s head lies in the transverse diameter of the inlet (widest diameter 13.5 cm.)
- Another FYI: if the plane of the least pelvic dimension (the interspinous dimension) are closer than 9.5 cm, it makes the female pelvis similar to the male one and thus fetal head wont pass
Describe the rotation of the fetal head during normal delivery
- First, the long diameter of the baby’s head lies in the transverse diameter of the inlet (13.5 cm)
- Then, as the fetal head descends, it rotates so that the long diameter of the baby’s head faces the AP diameter of the outlet (13.5 cm)
- Inlet 🡪 head of the baby is in transverse
- Outlet 🡪 head of the baby is in AP
What are the different variations in the pelvis of females?
1) Gynaecoid pelvis:
- Typical female pelvis (45%), with the most room obstetrically
2) Android pelvis:
- Typical male pelvis; however, it is present in 35% of females and makes it difficult for labour
3) Anthropoid pelvis (ape-like pelvis):
- The Inlet AP diameter is more than the transverse diameter (AP is normally 11 and Transverse is normally 13.5)
4) Platpelloid (flat) pelvis:
- It is rare where the transverse diameter is longer than normal and the AP diameter is less than normal
