Bony Pelvis and Pelvic Joints

Each formed by the fusion of ilium, ischium, and pubis at the acetabulum (see Lower Limb content for full individual bone detail, this section focuses on their contribution to the pelvic ring).

• Articulate with the sacrum (sacroiliac joints, posteriorly) and with each other (pubic symphysis, anteriorly), together with the sacrum and coccyx forming the closed bony ring of the pelvis.

• A triangular bone formed by the fusion of five sacral vertebrae, wedged between the two hip bones posteriorly.
• Base (superior) articulates with L5 (lumbosacral joint); apex (inferior) articulates with the coccyx; sacral promontory: the prominent anterior lip of the base, a key pelvic inlet landmark and obstetric reference point.
• Ala (wings): lateral masses flanking the body of S1, articulating with the ilium at the sacroiliac joints.
• Sacral canal: continuation of the vertebral canal, containing the cauda equina/filum terminale, terminating at the sacral hiatus (a common site for caudal epidural anaesthesia access).
• Anterior and posterior sacral foramina: paired openings transmitting the ventral and dorsal rami of the sacral spinal nerves respectively.

• Small, triangular bone formed by the fusion of (typically) four rudimentary coccygeal vertebrae, articulating with the sacral apex.
• Provides attachment for parts of levator ani, coccygeus, and the anococcygeal ligament/body, a functionally important attachment point for the pelvic floor despite its small size, and a site of pain (coccydynia) after direct trauma (e.g., a fall onto the buttocks, or difficult vaginal delivery).

• The bony ridge marking the boundary between the greater and lesser pelvis, formed by the sacral promontory (posteriorly), the arcuate line of the ilium, pecten pubis, and pubic crest (anterolaterally), synonymous with the pelvic inlet's bony margin.

• The superior pelvic aperture, of major obstetric significance for its diameters (anteroposterior/conjugate and transverse) which determine the pattern of fetal head engagement and descent.

• The inferior pelvic aperture, diamond-shaped, closed by the perineal/pelvic floor structures.

• Iliac crest: superior border of the ilium, palpable, extends from ASIS to PSIS.
• ASIS (anterior superior iliac spine): anterior termination of the iliac crest; sartorius origin, inguinal ligament attachment, key surface/obstetric landmark.
• AIIS (anterior inferior iliac spine): rectus femoris (straight head) origin.
• Ischial spine: separates the greater and lesser sciatic notches; key landmark for pudendal nerve block, and for assessing fetal station during labour (the ischial spines define "station 0").
• Ischial tuberosity: weight-bearing in sitting, hamstring origin; forms the lateral boundary of the pelvic outlet and the ischioanal fossa.
• Pubic crest: anterior border of the pubic bone, medial to the pubic tubercle; rectus abdominis attachment.
• Pubic tubercle: lateral end of the pubic crest, inguinal ligament attachment.
• Pecten pubis (pectineal line): sharp ridge along the superior ramus of the pubis, continuous with the arcuate line of the ilium, forming part of the pelvic brim.
• Sacral promontory: anterior projecting lip of the sacral base, the most posterior point of the pelvic inlet, and the key obstetric reference for measuring the true (obstetric) conjugate diameter (sacral promontory to the closest point on the posterior pubic symphysis).

Sacroiliac joints:

• Synovial (anteriorly) and syndesmotic/ligamentous (posteriorly) joints between the sacral ala and the iliac auricular surface, a hybrid joint type, reflecting the need for substantial load transmission (body weight to the lower limbs) with minimal but essential mobility.
• Heavily reinforced by strong ligaments (anterior, posterior, and interosseous sacroiliac ligaments), given the enormous mechanical stress transmitted through this joint.
• Relaxes slightly during pregnancy (under hormonal influence: relaxin, progesterone), permitting minor increased pelvic mobility to facilitate childbirth.

Pubic symphysis:

• A secondary cartilaginous joint (symphysis) between the two pubic bones, with an interposed fibrocartilaginous disc.
• Normally allows minimal movement, but: like the sacroiliac joints, undergoes hormonally-mediated relaxation/widening during pregnancy, and can become pathologically unstable/painful (symphysis pubis dysfunction) in some pregnancies.

Lumbosacral joint:

• Between the L5 vertebral body and the sacral base, structurally similar to other intervertebral joints (intervertebral disc, facet joints), but distinguished by the marked angulation at this junction (the lumbosacral angle), reflecting the transition from the lumbar lordosis to the sacral curve.

Sacrococcygeal joint:

• Between the sacral apex and the coccygeal base, a symphyseal joint (with an interposed fibrocartilaginous disc), permitting slight posterior movement of the coccyx during defecation and childbirth.

Sacrotuberous ligament:

• From the sacrum/coccyx (and PSIS) to the ischial tuberosity; converts the greater sciatic notch into the greater sciatic foramen (with the sacrospinous ligament); resists forward tilting of the sacral base (nutation) at the sacroiliac joint.

Sacrospinous ligament:

• From the sacrum/coccyx to the ischial spine; converts the greater sciatic notch into the greater sciatic foramen (superiorly to the sacrotuberous ligament's contribution) and the lesser sciatic notch into the lesser sciatic foramen (together with the sacrotuberous ligament).

Iliolumbar ligament:

• From the transverse process of L5 (and sometimes L4) to the iliac crest; stabilises the lumbosacral junction, resisting excessive movement at this mechanically vulnerable transition point.

• Formed by the greater sciatic notch, converted to a foramen by the sacrotuberous (inferior boundary) and sacrospinous (contributes to defining the foramen's inferior margin alongside the ischial spine) ligaments.
• Divided by piriformis into suprapiriform and infrapiriform parts (see Lower Limb content, Gluteal Region, for the full structure list).
• Structures passing through: Superior gluteal nerve/vessels (suprapiriform); sciatic nerve, inferior gluteal nerve/vessels, pudendal nerve, internal pudendal vessels, posterior cutaneous nerve of thigh, nerve to obturator internus, nerve to quadratus femoris (infrapiriform).

• Formed by the lesser sciatic notch, converted to a foramen by the sacrospinous ligament (superior boundary) and sacrotuberous ligament (inferior boundary).
• Structures passing through: Tendon of obturator internus, nerve to obturator internus, pudendal nerve and internal pudendal vessels (re-entering the pelvis/perineum here after having exited via the greater sciatic foramen, the characteristic "figure-of-eight" path around the ischial spine, see Lower Limb content).

• These differences reflect the female pelvis's adaptation to accommodate childbirth, and are a classic basis for forensic/anthropological sex determination from skeletal remains.