3.4 The Bony Pelvis
Key Takeaways
- The bony pelvis is formed by the two hip bones (os coxae), the sacrum, and the coccyx; each hip bone is formed by the fusion of three primary bones — ilium, ischium, and pubis — at the acetabulum (the triradiate cartilage in children).
- The pelvic inlet (superior aperture) is bounded by the sacral promontory, arcuate lines, pecten pubis, and pubic crest; the pelvic outlet (inferior aperture) is bounded by the coccyx, sacrotuberous ligaments, ischial tuberosities, and pubic arch.
- The pelvis is divided by the pelvic inlet into the false (greater) pelvis above (part of the abdomen) and the true (lesser) pelvis below (containing pelvic viscera); the pelvic brim is the boundary.
- Anatomical sex differences include the female pelvis having a wider subpubic angle (~80–85°), a larger pelvic inlet and outlet, a broader greater sciatic notch, and a shorter, less curved sacrum — adaptations for childbirth.
- Major pelvic ligaments (sacrospinous, sacrotuberous, anterior/posterior sacroiliac, inguinal, and lacunar) define foramina and compartments; the pelvic floor is formed principally by the levator ani (pubococcygeus, puborectalis, iliococcygeus) and coccygeus muscles.
Composition of the Bony Pelvis
Quick Answer: The bony pelvis is a ring formed by two hip bones (os coxae) anteriorly and laterally, the sacrum and coccyx posteriorly. Each hip bone is the fusion of three bones — ilium, ischium, and pubis — that meet at the acetabulum (the cup-shaped hip socket) in the triradiate cartilage, which fuses in the mid-teens.
Hip Bone (Os Coxae) Components
| Bone | Key Landmarks |
|---|---|
| Ilium | Iliac crest, anterior/posterior superior and inferior iliac spines (ASIS, AIS, PSIS, PIIS), greater sciatic notch, iliac fossa, arcuate line |
| Ischium | Ischial tuberosity (sitting bone), ischial spine, ischial ramus, lesser sciatic notch |
| Pubis | Superior and inferior pubic rami, pubic crest, pecten pubis (pectineal line), pubic symphysis articulation |
The acetabulum is a cup-shaped socket with contributions from all three bones (ilium ~2/5, ischium ~2/5, pubis ~1/5). It articulates with the head of the femur at the hip joint. The obturator foramen is formed by the pubis and ischium and is largely closed by the obturator membrane (leaving the obturator canal superiorly).
Sacrum and Coccyx
The sacrum is formed by fusion of five sacral vertebrae. It articulates superiorly with L5 via the sacral promontory and laterally with the ilia at the sacroiliac joints. The coccyx is formed by 3–5 fused rudimentary vertebrae and articulates with the sacral apex.
The sacrum has four pairs of sacral foramina (anterior and posterior) transmitting the ventral and dorsal rami of the sacral spinal nerves. The sacral hiatus at the caudal end is the termination of the sacral canal and is the entry point for caudal epidural anesthesia.
Pelvic Inlet and Outlet
| Aperture | Boundary |
|---|---|
| Pelvic inlet (superior aperture) | Sacral promontory → ala of sacrum → arcuate line of ilium → pecten pubis → pubic crest → superior border of pubic symphysis |
| Pelvic outlet (inferior aperture) | Coccyx → sacrotuberous ligaments → ischial tuberosities → ischiopubic rami → inferior border of pubic symphysis |
The pelvic brim is the line of the inlet. Structures above the brim constitute the false (greater) pelvis — part of the abdominal cavity containing the iliac fossae and parts of the ileum and sigmoid colon. Structures below the brim constitute the true (lesser) pelvis, containing the bladder, rectum, prostate/uterus, and vagina.
Sex Differences of the Pelvis
| Feature | Male | Female |
|---|---|---|
| Subpubic angle | Narrow (~50–60°) | Wide (~80–85°) |
| Pelvic inlet shape | Heart-shaped | Round/oval |
| Pelvic outlet | Narrower | Wider |
| Greater sciatic notch | Narrow, deep | Wide, shallow |
| Sacrum | Long, curved | Short, less curved |
| Ischial tuberosities | Closer together | Farther apart (everted) |
| Obturator foramen | Round | Oval |
These differences reflect the female pelvis's adaptation for childbirth (obstetricpelvis). The pelvic inlet, mid-cavity, and outlet are assessed in obstetrics to predict cephalopelvic disproportion.
Ligaments of the Pelvis
- Anterior and posterior sacroiliac ligaments — bind sacrum to ilium; the posterior are the strongest in the body and resist forward displacement of the sacrum.
- Sacrospinous ligament — from sacrum/coccyx to ischial spine; converts the greater sciatic notch into the greater sciatic foramen.
- Sacrotuberous ligament — from sacrum/coccyx to ischial tuberosity; converts the lesser sciatic notch into the lesser sciatic foramen.
- Inguinal ligament — from ASIS to pubic tubercle; forms the floor of the inguinal canal.
- Lacunar ligament — medial extension of the inguinal ligament to the pubic pecten; forms the medial boundary of the femoral ring.
The greater sciatic foramen transmits the piriformis, superior/inferior gluteal nerves and vessels, the sciatic nerve, posterior femoral cutaneous nerve, internal pudendal vessels and pudendal nerve, and the nerve to obturator internus. The lesser sciatic foramen transmits the obturator internus tendon, the pudendal nerve, and the internal pudendal vessels (re-entering the pelvis after exiting through the greater foramen).
Pelvic Floor Muscles
The pelvic floor (pelvic diaphragm) is formed by the levator ani and coccygeus:
| Muscle | Components / Action |
|---|---|
| Levator ani | Pubococcygeus, puborectalis (forms U-shaped sling around the anorectal junction — maintains fecal continence), iliococcygeus |
| Coccygeus | From ischial spine to lower sacrum/coccyx; supports pelvic floor |
The levator ani is innervated by branches from S4 (and sometimes the pudendal nerve). Injury to the pelvic floor — common in childbirth — leads to pelvic organ prolapse (cystocele, rectocele, uterine prolapse) and stress urinary incontinence.
Pelvic Ring Stability and Weight-Bearing Arches
The bony pelvis is not just a container for viscera — it is a mechanical ring that transmits body weight from the spine to the lower limbs and resists the pull of the hip abductors, hamstrings, and pelvic floor muscles. Understanding how the ring resists deforming forces is the key to classifying pelvic fractures, a common PA-CAT anatomy application.
Conceptually the pelvis is two linked arches:
- Posterior arch — the sacroiliac joints together with the posterior sacroiliac, sacrotuberous, and sacrospinous ligaments. This is the primary weight-bearing strut and the dominant stabilizer of the ring.
- Anterior arch — the pubic symphysis and the superior and inferior pubic rami. It functions as a tension strut and is only a secondary stabilizer.
A practical clinical rule follows directly from ring geometry: a ring broken in only one location is generally stable, while a ring broken in two or more locations is unstable. This principle underlies the Young-Burgess classification of pelvic fractures:
| Mechanism | Fracture Pattern | Stability |
|---|---|---|
| Anteroposterior compression (APC) | Diastasis of the pubic symphysis with possible sacroiliac widening | APC I stable, APC II partially unstable, APC III unstable |
| Lateral compression (LC) | Pubic rami fractures with sacral impaction | LC I and LC II stable, LC III (opposite-side injury) unstable |
| Vertical shear | Vertical displacement through symphysis and sacroiliac joint | Unstable |
Weight is transmitted along two distinct arcs depending on posture. In the standing arch, load passes from the sacroiliac joints through the ilia to the acetabula and onto the femoral heads. In the sitting arch, load passes from the sacroiliac joints to the ischial tuberosities. The sacroiliac joint itself is a synovial joint anteriorly that becomes a syndesmosis posteriorly, and its interosseous ligament is among the strongest in the body. When the sacrospinous and sacrotuberous ligaments avulse in an unstable fracture, the hemipelvis is free to rotate and translate, endangering the internal pudendal and superior gluteal vessels — the reason unstable pelvic fractures carry high mortality from retroperitoneal hemorrhage.
Which structure forms the boundary that converts the greater sciatic notch into the greater sciatic foramen, and which key nerve passes through that foramen most laterally?
A 31-year-old woman reports stress urinary incontinence and a sensation of pelvic heaviness after a prolonged vaginal delivery. Weakness of which muscle group is most directly responsible for the loss of pelvic floor support?