Must Know

Pelvic brim runs from pubic crest along pectineal and arcuate lines to the sacral ala and promontory.
Brim plane slopes at about 60 degrees to the horizontal when standing.
Standing, the anterior superior iliac spines and the top of the symphysis share one vertical plane.
One horizontal plane joins the top of the symphysis, ischial spine, coccyx tip, femoral head centre and greater trochanter tip.
Brim divides the false pelvis above, part of the abdomen, from the true pelvic cavity below.
Side wall is hip bone lined by obturator internus. Back wall is sacrum with piriformis heading for the greater sciatic foramen.
Obturator canal is the gap above the upper edge of the obturator membrane, carrying obturator nerve and vessels.
Sacroiliac joint is synovial with jagged, interlocking surfaces and little movement.
Pubic symphysis is a secondary cartilaginous joint and its central cleft never has a synovial lining.

Sex Differences

Feature	Male	Female
Subpubic angle	Acute, like a pointed arch	Wide, like a rounded arch
Brim outline	Heart shaped, indented by the promontory	Transversely oval, widest further forward
Greater sciatic notch	Narrower	Near a right angle
Obturator foramen	Oval	Triangular
Ischial spine	Turned in toward the cavity	In line with the ischial body
Preauricular groove	Absent	Often present below the arcuate line
Symphysis to acetabulum distance	Equal to or less than acetabular diameter	Greater than acetabular diameter

Fibrous adhesions slowly obliterate the sacroiliac cavity, earlier in men and after menopause in women.
Anterior sacroiliac ligament is a thin band, stronger in women where it grooves the ilium.
Interosseous sacroiliac ligament is a very strong mass filling pits on the back of the sacral lateral mass.
Posterior rami and vessels run between the interosseous and posterior sacroiliac ligaments.
Joint stability is wholly ligamentous. No muscle crosses to hold it.
Interosseous sacroiliac and iliolumbar ligaments resist the sacrum gliding down on the ilium.
Sacrotuberous and sacrospinous ligaments resist forward tilt of the sacral promontory under body weight.
Ligaments soften late in pregnancy, allowing slight sacral rotation in labour.

Ligament	Attachments	Exam point
Sacrotuberous	Posterior ilium, PSIS and PIIS, lower sacrum and coccyx to medial ischial tuberosity	Gluteus maximus arises from it. Pierced by perforating cutaneous nerve. Falciform process lies below pudendal canal
Sacrospinous	Lower sacrum and coccyx to ischial spine	Gluteal surface of coccygeus. Separates greater from lesser sciatic foramen
Iliolumbar	L5 transverse process to iliac crest and front of anterior sacroiliac ligament	Upper band gives partial origin to quadratus lumborum
Lateral sacrococcygeal	Coccygeal transverse process to sacral inferolateral angle	Completes the foramen for the S5 anterior ramus
Superior and arcuate pubic	Above and below the symphysis	Deep dorsal vein of penis or clitoris lies below the arcuate ligament

Lesser sciatic foramen transmits obturator internus laterally and leads medially into the pudendal canal.
Obturator internus tendon turns 90 degrees on cartilage covered bone at the lesser sciatic notch, cushioned by a bursa.

Pelvic diaphragm is a gutter of levator ani and coccygeus sloping down and forward around the midline viscera.
Levator ani arises from the body of pubis, the tendinous arch over obturator fascia and the ischial spine.
Above the tendinous arch is the pelvic cavity. Below it the obturator fascia walls the ischioanal fossa.
Pubococcygeus arises from the front half of the arch and the back of the pubic body.
Puborectalis slings around the anorectal junction, holding it angled forward, and blends with external anal sphincter.
Iliococcygeus arises from the back half of the arch and ischial spine, inserting into coccyx and anococcygeal raphe.
Coccygeus runs from ischial spine to coccyx and lowest sacrum. Its gluteal surface is the sacrospinous ligament.
Levator ani takes S3 and S4 twigs on its pelvic surface.
Puborectalis and the sling fibres also receive the perineal branch of S4 and inferior rectal nerve from below.
Coccygeus is supplied by S3 and S4.
Floor contracts against rises in abdominal pressure, as in coughing and lifting, and relaxes for defaecation.

Fascia over piriformis fuses with periosteum at the anterior sacral foramina.
Sacral anterior rami and the sacral plexus lie behind this fascia.
Internal iliac vessels and their branches lie in front of the fascia.
Sacral plexus is a flat triangle on piriformis built from the lumbosacral trunk (L4, 5) and S1 to S4.
Perforating cutaneous nerve (S2, 3) pierces the sacrotuberous ligament to supply skin of the lower medial buttock.
Pudendal nerve (S2 to 4) curls over the gluteal surface of the sacrospinous ligament into the pudendal canal.
Pelvic splanchnic nerves (S2 to 4) carry sacral parasympathetic outflow to bladder, hindgut and erectile tissue.
Bladder and rectal pain can be referred along the posterior femoral cutaneous nerve.

Item	Value
Pelvic brim to horizontal	About 60 degrees
Levator ani and coccygeus roots	S3 and S4
Pudendal and pelvic splanchnic roots	S2 to S4
Perforating cutaneous nerve	S2, 3
Sacral plexus contributions	L4, 5 trunk and S1 to S4
Obturator internus tendon turn	About 90 degrees at the lesser notch

Gothic versus Roman arch. Male subpubic angle is pointed, female rounded.
Rami behind, vessels in front. Sacral nerves sit behind piriformis fascia, internal iliac vessels in front.
Two pairs of restraints. Interosseous and iliolumbar stop gliding. Sacrotuberous and sacrospinous stop forward tilt.

The symphysis is never a synovial joint, even when a central cleft forms.
Male obturator foramen is oval and female triangular, not the reverse.
Posterior rami pass between interosseous and posterior sacroiliac ligaments, not in front of the joint.
Coccygeus takes direct S3 and S4 branches, not the pudendal nerve.
Sacral plexus lies behind the fascia over piriformis, while internal iliac vessels lie in front.

The sacroiliac joint is the largest axial joint. Its anteroinferior C-shaped part is synovial and its dorsal part fibrocartilaginous.
The interosseous ligament is the strongest. Iliolumbar and long dorsal ligaments are the extrinsic stabilisers.
Lateral branches of L5 to S4 dorsal rami supply the posterior joint. L4 and L5 ventral rami supply the anterior.
Motion is small, about 3 degrees flexion extension, 1.5 degrees rotation and 0.8 degrees lateral bending. Women move more.
Nutation is the sacrum nodding forward on the ilium. Counternutation is backward tilt.
Shear is resisted by form closure (joint shape and ridges) and force closure (compression from surrounding muscles).
The joint causes 10 to 38 percent of low back pain.
Degenerative change appears in up to 83 percent of asymptomatic people.
For dysfunction, CT is 58 percent sensitive and 69 percent specific, MRI 65 and 79 percent.
Lumbosacral fusion speeds sacroiliac degeneration. Sacroiliac fusion speeds hip degeneration and doubles THA instability risk.

Sinnatamby CS. Last’s Anatomy. Regional and Applied. 12th ed. Edinburgh, Churchill Livingstone Elsevier, 2011. Chapter 5, Abdomen. p. 288-91, 323-7.

Reviewed by Professor Phong Tran, Head of Orthopaedic Surgery, Western Health. Last updated 17 October 2026.