Pelvis

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Male pelvis.

The pelvis (aka pelvic cavity) is continuous with the abdominal cavity at the pelvic inlet. The pelvis consists of the true pelvis [1] and the false pelvis - the part of the bony pelvis above the level of the pelvic inlet, formed mainly by the iliac fossae.

Superiorly, the pelvic inlet lets stuff in, while inferiorly, the pelvic outlet lets stuff out. The perineum is a region below the pelvic diaphragm containing the anal canal among other fun things.

Osteology

The bony pelvis consists of

  1. the two hip (pelvic) bones, which meet anteriorly at the pubic symphysis
  2. the sacrum, which articulates with the two hip bones, posteriorly, at the synovial, sacro-iliac joints

Gender differences

The differences are related to the function of the female pelvis as a birth canal, which should ideally be large enough for a fully formed fetus to pass through it The mid-pelvic plane, between the two ischial spines, is important, since it is the plane of least dimensions in the female pelvis. The following are the main differences between the "typical" female and male pelvis:

feature female male
Sub-pubic angle rounded and greater than 90° pointed and less than 90°
Pelvic inlet oval in shape heart-shaped
Sacrum short, broad, relatively flat long, narrow, curved
Lateral walls almost vertical convergent from inlet to outlet (pelvis more cone-shaped)
Ischial spines almost in the plane of the ischium inverted with respect to ischium
Greater sciatic notch approximately 90° acute angle
Inlet-outlet distance shorter longer
Articulations of pelvis. Anterior view.

Articulations

  • Sacro-iliac joints
    • bilateral, synovial joints
    • located between the auricular surfaces of the ilia and the sacrum
    • Relatively immobile due, primarily, to the presence of strong ligaments

Interosseous sacro-iliac ligaments

  • surround the joint
  • hold the sacrum in place, between the two hip bones
  • sacro-tuberous and sacro-spinous ligaments prevent the upper end of the sacrum rotating downwards and forwards between the hip bones. The presence of the sacro-tuberous and sacro-spinous ligaments also converts the greater and lesser sciatic notches into the greater and lesser sciatic foramina. The sacro-iliac joint is further stabilized on each side by an ilio-lumbar ligament, passing from the tip of the transverse process of the L5 vertebra to the iliac crest.
Left Levator ani from within.

Muscles

Most muscles associated with the bony pelvis are related to movements of the lower limb. Two pairs of these are located inside the pelvic cavity and contribute to its walls:

  • Posteriorly - the piriformis arises from the anterior surface of the sacrum and exits through the greater sciatic foramen
  • Laterally - the obturator internus muscle attaches to the obturator membrane and to the bone around the periphery of the obturator foramen
  • Floor - Pelvic diaphragm
Coronal section of pelvis, showing arrangement of fascia.

Fascia

The walls and floor of the pelvic cavity are lined by (parietal) fascia, and may be thickened in some areas (e.g., obturator internus fascia forming the tendinous arch for the levator ani). The pelvic viscera are covered by a loose (visceral) fascia, also thickened in some areas to form either "ligaments" that support the viscera or visceral sheaths.

Peritoneal reflections

The peritoneum lining the anterior abdominal wall reflects on to the upper surface of the bladder and over its posterior surface, dipping down towards the pelvic diaphragm. In the male, it then reflects on to the anterior wall of the rectum and leaves a recess of the peritoneal cavity, the recto-vesical pouch (recess), between the rectum and bladder. In the female, the uterus intervenes between the bladder and rectum, and it, too, is covered by visceral peritoneum. This results in two recesses:

  1. utero-vesical pouch (recess) - between the uterus and bladder
  2. recto-uterine pouch (recess) - between the rectum and uterus
Deep and superficial dissection of the lumbar plexus.

Nerves

  • define the term "venous plexus", as it applies to the pelvis
  • state the sites and functional significance of the communication between pelvic venous plexuses and other veins
  • name the groups of lymph nodes to which pelvic structures drain
  • state the location of the sacral plexus and name its components and main branches
  • define the term "hypogastric plexus", state its components, its location and general functions.

Somatic

  • derived from branches of the lumbar and sacral plexuses
  • Most of the nerves derived from these plexuses supply structures outside the pelvis, notably the lower limb

Sacral plexus

  • formed by the anterior primary rami of sacral spinal nerves (primarily S1-S4), which exit the anterior sacral foramina
  • Here they are joined by the lumbo-sacral trunk (L4, L5), derived from the lumbar plexus
  • The superior and inferior gluteal and sciatic branches of the plexus exit the pelvis through the greater sciatic foramen to enter the lower limb
  • gives motor branches to the muscles of the pelvic wall (piriformis, obturator internus)
  • gives rise to the important pudendal nerve

Autonomic

  • sympathetic and parasympathetic efferent, as well as visceral afferent fibres all contribute to plexuses that supply the pelvic viscera
  • The sympathetic fibres are derived partly from the superior hypogastric plexus, an offshoot of the aortic plexus in the abdomen
  • Fibres from the hypogastric plexuses supply the pelvic viscera, sometimes forming additional plexuses on the surfaces of the organs they supply (e.g., rectal plexus, vesical plexus)

Vessels

  • state the locations of the common, internal and external iliac vessels
  • name the main branches of the internal iliac artery and state their areas of supply
  • name sites of anastomotic communications with other arteries

Arteries

Other arteries also contribute to the supply of pelvic structures:

Veins

  • drainage is mainly to the paired internal iliac veins
  • Exceptions include:
    1. ovarian veins - drain into the inferior vena cava (via the left renal vein, on the left)
    2. superior rectal vein - drains into the inferior mesenteric vein
  • A significant feature of the visceral veins of the pelvis is that they form interconnecting venous plexuses in the fascia surrounding the viscera
  • The veins of the plexuses are valveless and communicate freely with each other and with other veins
  • Infections and tumours can therefore spread in either direction through the veins

Lymphatics

  • In general, the lymphatics follow vessels to regional groups of lymph nodes located in the pelvis
  • They include:
    1. internal iliac nodes
      • found along branches of the internal iliac artery
      • receive lymphatics from most of the pelvic organs
    2. Sacral nodes
      • located in front of the sacrum
      • Male: receive afferent vessels from the prostate gland
      • Female: receive afferent vessens from the uterus and vagina
    3. Some external iliac nodes
      • related to the external iliac artery
      • closely related to the pelvic brim
      • receive some lymphatics from the uterus and bladder
  • Efferent lymphatics from these groups of nodes converge on the common iliac nodes, which, in turn, drain to lumbar (paraaortic nodes)
  • Some superficial structures of the perineum drain to superficial inguinal nodes, while the ovaries and testes drain directly to para-aortic nodes

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