Bony Pelvic Anatomy
The bony pelvis is the fixed operative reference frame of every reconstructive pelvic procedure. Each major reconstructive landmark — sacral promontory for sacrocolpopexy, ischial spine for sacrospinous fixation, pubic tubercle and Cooper's ligament for Burch and hernia repair, ischiopubic ramus for bulbar urethroplasty and clitoral NVB preservation — is a named bony point. This article focuses on those landmarks, the joints and pelvic-diameter geometry that drive access and obstetric mechanics, and the sex-specific features that influence surgical planning.
See also Pelvic Vascular Anatomy, Pelvic Neuroanatomy, The Perineum, and The Presacral Space.
Components
The bony pelvis is a ring formed by:
| Bone | Parts |
|---|---|
| Paired innominate (hip) bones | Ilium + ischium + pubis — fused at the acetabulum |
| Sacrum | Five fused sacral vertebrae (S1–S5) |
| Coccyx | 3–5 fused rudimentary vertebrae |
Three paired joints complete the ring: pubic symphysis (anterior, fibrocartilaginous), paired sacroiliac (SI) joints (posterolateral, synovial anteriorly and syndesmotic posteriorly), and sacrococcygeal (mobile hinge).
The Ilium
- Iliac crest — palpable landmark; donor site for autologous bone graft; level of L4 for imaging.
- Anterior superior iliac spine (ASIS) — attachment of the inguinal ligament and sartorius; landmark for ilioinguinal / iliohypogastric nerve block, Pfannenstiel planning, and donor-site design.
- Anterior inferior iliac spine (AIIS) — attachment of rectus femoris.
- Posterior superior iliac spine (PSIS) — marks the sacroiliac joint.
- Iliac fossa — anterior concavity covered by iliacus; operative working surface during retroperitoneal access.
- Arcuate line — part of the linea terminalis (pelvic inlet).
- Greater sciatic notch — transmits the piriformis, superior/inferior gluteal vessels and nerves, sciatic nerve, pudendal nerve / vessels (which then loop back through the lesser sciatic foramen).
The Ischium
- Ischial tuberosity — sitting weight-bearing; attachment of the sacrotuberous ligament and long head of biceps femoris. Lateral boundary of the UG and anal triangles of the perineum.
- Ischial spine — the reconstructive surgeon's landmark:
- Attachment of the sacrospinous ligament (SSL) — the target of sacrospinous ligament fixation (SSLF) for apical prolapse.
- Medial reference for pudendal nerve block ("hook the pudendal around the ischial spine").
- Divides the greater from the lesser sciatic foramen.
- Lies on the course of the pudendal nerve as it crosses from greater to lesser sciatic foramen.
- Lesser sciatic foramen — pudendal nerve and internal pudendal vessels re-enter the pelvis here into Alcock's canal.
- Ischiopubic ramus — forms the ischiopubic arch; perineal-membrane attachment; bulbar-urethra / clitoral-NVB neighbor.
The Pubis
- Pubic body — articulates in the midline at the pubic symphysis.
- Pubic tubercle — medial attachment of the inguinal ligament; landmark for hernia repair, Pfannenstiel, and male-sling fixation.
- Superior pubic ramus — site of Cooper's (pectineal) ligament (attachment target for Burch colposuspension and Stoppa-type hernia repair).
- Inferior pubic ramus — fuses with the ischial ramus to form the ischiopubic arch.
- Obturator foramen — nearly closed by the obturator membrane; the obturator nerve, artery, and vein exit through the obturator canal at its anterosuperior corner. Target of the transobturator (TOT) sling trajectory.
The pubic symphysis widens ~2–3 mm physiologically in late pregnancy; excessive diastasis >10 mm causes symphyseal pain. Forceful pelvic-ring disruption (straddle trauma, pelvic fracture) correlates with pelvic-fracture urethral injury (PFUI) — the risk rises approximately 10% for every 1 mm of pubic symphysis diastasis. See Male Urethra.
The Sacrum
- Sacral promontory — anterior projection of S1; the key landmark for sacrocolpopexy (mesh anchored to the anterior longitudinal ligament just below the promontory).
- Anterior sacral foramina (S1–S4) — transmit ventral rami; exit points for the presacral venous plexus when torn.
- Anterior longitudinal ligament covers the anterior sacral surface; median thickness at the promontory ~1.9 mm (range 1.2–2.5) — adequate for sacrocolpopexy suture purchase.
- Posterior sacral foramina — transmit dorsal rami.
- Ala of the sacrum — lateral expansion alongside the body of S1; anchor of the sacroiliac joint.
- Sacral hiatus — inferior midline opening; target of caudal epidural anesthesia.
The median sacral artery descends the midline; the left common iliac vein crosses the promontory from right to left — both relevant to sacrocolpopexy vessel geometry (see The Presacral Space).
The Coccyx
Three to five fused rudimentary vertebrae articulating with the sacrum via the sacrococcygeal joint. Insertion point for the anococcygeal ligament, levator ani (via anococcygeal raphe), and sacrotuberous ligament. Target of coccygectomy for refractory coccydynia; dislocation is a common obstetric injury.
Pelvic Joints
| Joint | Type | Function / clinical note |
|---|---|---|
| Pubic symphysis | Fibrocartilaginous | Widens in pregnancy; diastasis linked to PFUI; target of symphyseal plating for pelvic-ring disruption |
| Sacroiliac (SI) | Synovial anteriorly, syndesmotic posteriorly with interosseous + posterior SI ligaments | Transmits load from spine to pelvis; chronic SI pain is a common mimic of hip and lumbar pathology |
| Sacrococcygeal | Symphyseal / fibrocartilaginous | Mobile hinge; dislocated in obstetric coccyx injury |
Pelvic Inlet and Outlet — the Operative Landmarks
Pelvic inlet (linea terminalis)
Boundary between the false (greater) pelvis (above) and true (lesser) pelvis (below). Course: sacral promontory → anterior ala of sacrum → arcuate line of ilium → pectineal line of pubis → pubic crest → pubic symphysis.
Pelvic outlet
Diamond-shaped, boundaries:
- Anterior: pubic symphysis and inferior pubic rami
- Posterior: coccyx and sacrotuberous ligaments
- Lateral: ischial tuberosities and ischiopubic rami
Divided by the interischial line into the urogenital triangle (anterior) and anal triangle (posterior) — the anatomic scaffold of The Perineum.
Pelvic diameters (obstetric mechanics)
| Plane | Typical female AP diameter | Typical female transverse |
|---|---|---|
| Inlet | 11 cm (diagonal conjugate ~12.5 cm) | 13 cm |
| Midpelvis | 12 cm | 10 cm (ischial-spine level — narrowest transverse) |
| Outlet | 9.5–11 cm (increases with coccyx hinge) | 11 cm (intertuberous) |
The ischial spines define the midpelvis — the tightest cross-section obstetrically, and also the reference for fetal station (0 station).
Sex Differences
Female pelvis is evolutionarily shaped for parturition. Clinically relevant differences:
| Feature | Female | Male |
|---|---|---|
| Subpubic (pubic arch) angle | >80° (often ~90°) — rounded | <70° — acute, V-shaped |
| Pelvic inlet | Round / oval | Heart-shaped |
| Sacrum | Shorter, wider, less curved | Longer, narrower, more curved |
| Ischial spines | Less protrusive (wider midpelvis) | More protrusive |
| Ischial tuberosities | Everted, wider apart | Inverted, closer together |
| Greater sciatic notch | Wide (~70–90°) | Narrow (~50–70°) |
| Obturator foramen | Oval | Round |
These differences matter for pessary fit, robotic-port placement, deep pelvic exposure, and the anatomic feasibility of vaginal approaches.
Key Bony / Ligamentous Landmarks by Procedure
Sacrocolpopexy / sacrohysteropexy
- Sacral promontory — mesh anchor point
- Anterior longitudinal ligament — suture purchase (~1.9 mm thick at promontory)
- Avoid: left common iliac vein (~27 mm cephalad), median sacral vessels, presacral plexus, S1 foramen (~2.5 cm caudal / 2 cm lateral)
Sacrospinous ligament fixation (SSLF)
- Ischial spine — two-finger-breadth medial and inferior target
- Sutures placed ~2 cm medial to the spine to avoid the pudendal NVB
Burch colposuspension
- Cooper's (pectineal) ligament on superior pubic ramus — suture anchor for paravaginal / bladder-neck elevation
Inguinal hernia repair (Stoppa, Lichtenstein)
- Inguinal ligament (ASIS → pubic tubercle), pubic tubercle, Cooper's ligament, iliopubic tract
Transobturator sling (TOT)
- Obturator foramen entry / exit; the obturator neurovascular bundle sits at the anterosuperior corner (safer to pass inferolateral)
Pelvic fracture reduction / pelvic-ring disruption
- Symphyseal plating for open-book injuries
- SI screws for posterior-ring disruption
- Associated with PFUI and bladder rupture
Pudendal nerve block
- Ischial spine as transvaginal or transperineal palpable landmark — needle placed medial to the spine through the sacrospinous ligament
Bulbar urethroplasty / AUS
- Ischiopubic ramus — perineal-membrane attachment; plane of crural dissection
Clinical Correlations for the Reconstructive Surgeon
- PFUI risk scales with pubic diastasis — every 1 mm of symphyseal widening on pelvic-fracture imaging correlates with ~10% increase in PFUI risk. Anticipate the injury at the film.
- Sacrospinous proximity to the pudendal NVB. Suture placement too lateral or through the SSL itself can injure the pudendal nerve; 2 cm medial to the ischial spine is the rule.
- Obturator-canal injury during TOT / paravaginal / LN dissection. Obturator nerve gives medial-thigh sensation + adductor motor; injury produces medial-thigh numbness and adductor weakness.
- SI-joint mobility and age. Obstetric ligamentous laxity and degenerative SI arthritis both change pelvic-floor biomechanics and may influence prolapse-repair outcomes.
- Coccygectomy. Reserved for refractory coccydynia; releases anococcygeal attachments; post-op care includes doughnut cushion and pelvic-floor PT.
- Narrow androgen-type pelvis. A narrow subpubic angle, closer ischial tuberosities, and deeper sacral curve increase the difficulty of deep pelvic exposure (vaginal route, transperineal prostatectomy, radical cystectomy) — plan accordingly.
- Cooper's ligament and the Burch. Knowing the pectineal-line anatomy allows a transverse two-suture bite that elevates the paravaginal fascia at the bladder neck without capturing the obturator NVB.
- Sacral curvature and sacrocolpopexy mesh length. Deeper sacral curves mandate longer mesh; inadequate length produces vaginal foreshortening; excessive length causes bowstringing that can erode into the rectum or sigmoid.
References
- Ashton-Miller JA, DeLancey JO. "Functional Anatomy of the Female Pelvic Floor." Ann N Y Acad Sci. 2007;1101:266–296. doi:10.1196/annals.1389.034
- Florian-Rodriguez ME, Hamner JJ, Corton MM. "First Sacral Nerve and Anterior Longitudinal Ligament Anatomy: Clinical Applications During Sacrocolpopexy." Am J Obstet Gynecol. 2017;217(5):607.e1–607.e4. doi:10.1016/j.ajog.2017.07.008
- Giraudet G, Protat A, Cosson M. "The Anatomy of the Sacral Promontory." Am J Obstet Gynecol. 2018;218(4):457.e1–457.e3. doi:10.1016/j.ajog.2017.12.236
- Johnsen N, Wessells H, Archer-Arroyo K, et al. "Best Practices Guidelines — Management of Genitourinary Injuries." American College of Surgeons. 2025.
- Coccolini F, Moore EE, Kluger Y, et al. "Kidney and Uro-Trauma: WSES-AAST Guidelines." World J Emerg Surg. 2019;14:54. doi:10.1186/s13017-019-0274-x