Pelvic arteries

Common iliac arteries

As previously mentioned, the common iliac arteries are branches of the abdominal aorta. The left common iliac artery is shorter than the contralateral vessel owing to the fact that the abdominal aorta is displaced to the left by the inferior vena cava. The vessel is anterior to the obturator nerve, iliolumbar artery, lumbosacral trunk, the sympathetic trunk and the fourth and fifth lumbar vertebrae and intervening discs. The vessel is posterior to the superior rectal artery and the ureter (at its bifurcation). It is bordered posteromedially by the left common iliac vein and laterally by the left psoas major muscle.

The longer right common iliac artery travels in an inferolateral direction anterior to the fourth and fifth lumbar vertebrae, the junction of the common iliac vein and inferior vena cava, as well as the ipsilateral sympathetic trunk, iliolumbar artery and lumbosacral trunk. It is posterior to the pelvic sympathetic plexus and the ipsilateral ureter (at its bifurcation). The parietal peritoneum provides an anterior barrier between the vessel and the small intestine. The right common iliac vein and the right psoas major muscle lie superior and inferior (respectively) to the artery. The left common iliac vein forms the medial boundary of the right common iliac artery, while the inferior vena cava forms its lateral boundary.

The common iliac arteries bifurcate at the sacroiliac joint to give off an internal and a larger external iliac artery. The external iliac artery is the main blood supply to the lower limb, while the internal iliac artery perfuses the pelvic viscera, perineal and gluteal regions. The common iliac arteries may also give off:

• Vasa nervorum (small arteries that supply nerves)
• Branches to the ureter
• Direct branches to the peritoneum.

External iliac arteries

The external iliac artery continues in an inferolateral course along the medial aspect of psoas major before passing across the mid inguinal point (midpoint between an imaginary line drawn from the anterior superior iliac spine to the symphysis pubis), posterior to the inguinal ligament and enters the lower limb.

The external iliac arteries are covered by the parietal peritoneum, and conveniently protected from abrasive contact with the terminal ileum and appendix on the right hand side, and the sigmoid colon and loops of the small intestine on the left hand side. The external iliac artery is crossed by the following structures anteriorly:

• Gonadal vessels
• Deep circumflex iliac veins
• Genital component of the genitofemoral nerve (L1, 2)
• Vasa deferentia (males)
• Round ligament (females)
• Ureter

Psoas major muscle offers a lateral border to the vessel, while the external iliac vein is located, first posteriorly (to the superior segment of the artery) then medially (to the inferior part of the artery).

Before the external iliac artery becomes the femoral artery (at the mid inguinal point), it gives off two branches that support the contents of the pelvis. These are the inferior epigastric artery and the deep circumflex iliac artery. The inferior epigastric artery is an artery of the anterior abdominal wall that leaves the external iliac artery opposite to the origin of the deep circumflex iliac artery. It takes a superomedial course adjacent to the medial wall of the deep inguinal ring before continuing through the substance of rectus abdominis, where it anastomoses with the superior epigastric artery.

The deep circumflex iliac artery emerges from the lateral surface of the external iliac artery and takes a superolateral course towards the anterior superior iliac spine. It forms several anastomoses directly with the lateral circumflex femoral, superior gluteal and iliolumbar arteries, and indirectly with the inferior epigastric and lumbar arteries. It provides arterial supply to the transversus abdominis and internal oblique muscles.

Internal iliac arteries

The smaller branch of the common iliac artery is the internal iliac artery. It commences anterior to the lumbosacral and sacroiliac joints. At the roof of the greater sciatic foramen, the internal iliac artery divides to give off an anterior and a posterior trunk. The posterior trunk courses through the greater sciatic foramen to enter the gluteal region, while the anterior trunk continues in line with its parent branch.

Anterior trunk

The anterior trunk of the internal iliac artery has several branches supplying the pelvic viscera. These include the:

• Superior vesical
• Inferior vesical or vaginal (in females)
• Middle rectal
• Obturator
• Uterine (in females)
• Internal pudendal
• Inferior gluteal arteries

The superior vesical artery is a large vessel that takes an anteroinferior course, caudal to the pelvic brim along the lateral wall of the pelvis. In males, it perfuses the proximal ductus deferens and seminal vesicles; and in both genders, it brings arterial blood to the distal ureter and bladder.

The inferior vesical and middle rectal arteries may share origins as they branch from the anterior internal iliac artery. The inferior vesical artery supplies the bladder, and in males it also supplies the prostate, vasa deferentia, and seminal vesicles. In females, the artery may be replaced by the vaginal artery, which otherwise would arise from the uterine artery (a large artery running inferomedial to the uterine broad ligament). There may be multiple branches of the middle rectal artery. It courses the lateral border of the mesentery of the rectum (mesorectum).

The obturator artery is limited superiorly by its eponymous nerve and inferiorly by its vein. It is posterior to the ureter and ductus deferens (in males). It gives off an iliac branch to the iliac fossa (supplying the bone and associated muscle), a vesical branch to the bladder and a pubic branch that forms an anastomosis with the pubic branch of the inferior epigastric artery and the contralateral pubic artery.

The internal pudendal artery arises caudal to the origin of the obturator artery and travels inferolaterally towards the lower margin of the greater sciatic foramen. It enters the perineum – after leaving between ischiococcygeus and piriformis – via the lesser sciatic foramen. It gives off branches to the muscles and nerves in the pelvic and gluteal regions.

Finally, the inferior gluteal artery provides arterial supply to piriformis, iliococcygeus and ischiococcygeus muscles. It may also support the middle rectal arterial network. This artery is a large terminal vessel that takes a posteroinferior route between piriformis and the sacral plexus, posterior to the internal pudendal artery. It enters the gluteal region after passing through the greater sciatic foramen, where it runs beneath gluteus maximus.

Mnemonic: struggling to remember all these branches? We have something to help! Just memorise the phrase ' Often My Sexy Underwear Is Inside Pants', which stands for:

• Obturator a.
• Middle rectal a.
• Superior vesical a.
• Uterine a. (female only)
• Inferior gluteal a.
• Inferior vesical a. (often vaginal a. in females)
• Internal Pudendal a.

Posterior trunk

The posterior trunk of the internal iliac artery gives off the:

• superior gluteal
• lateral sacral
• iliolumbar arteries

The largest branch of the internal iliac artery is the superior gluteal artery. It travels posteroinferiorly between the sacral rami before leaving the pelvis via the greater sciatic foramen, cranial to the piriformis muscle. It then bifurcates to give off a superficial branch, which supplies gluteus maximus and sacral skin as well as a deep branch, which supplies gluteus medius and gluteus minimus.

The lateral sacral arteries may emerge from the posterior trunk either as a paired artery or as a single branch. If the vessel arises as a single branch, it subsequently divides to give off an inferior and a superior branch. The anterior sacral foramina provide conduits for the superior vessels to enter the sacral canal and supply the canal and its contents. They subsequently leave the canal via the dorsal sacral foramina to supply the musculature and skin of the dorsal sacrum. The inferior branches anastomose with the median sacral artery and its contralateral counterpart on the ventral surface of the coccyx. The subsequent branches utilize the same route as the superior lateral sacral artery to gain access to the sacral canal and the dorsal sacrum.

Thirdly, the iliolumbar artery is the first to emerge from the posterior trunk of the internal iliac artery. It travels superolateral toward the sacroiliac joint behind the obturator nerve on its way toward the medial aspect of psoas major. Here it divides into an iliac and a lumbar branch. The iliac branch brings arterial supply to iliacus muscle before anastomosing with the iliac branches of the obturator artery. The lumbar branch supplies quadratus lumborum, psoas major and gives off a spinal branch that supplies the cauda equina.

There's a mnemonic to help here too! Just remember 'PILS', which stands for:

• Posterior branch:
• Iliolumbar a.
• Lateral sacral a.
• Superior gluteal a.