Bony Pelvic Anatomy
The bony pelvis is the reference frame for reconstructive pelvic surgery. The promontory guides sacrocolpopexy, the ischial spine guides sacrospinous fixation and pudendal block, Cooper's ligament guides Burch colposuspension, and the ischiopubic ramus guides perineal urethral and crural dissection.
See Pelvic Vascular Anatomy, Pelvic Neuroanatomy, The Perineum, and The Presacral Space.
Components and joints
The pelvic ring consists of the paired hip bones and sacrum. Each hip bone develops from the ilium, ischium and pubis, which meet at the acetabulum. The sacrum usually comprises five fused vertebrae. The coccyx is the terminal segment, usually with three to five rudimentary vertebrae and variable fusion.
| Joint | Relationship | Operative relevance |
|---|---|---|
| Pubic symphysis | Single anterior fibrocartilaginous joint between the pubic bodies | Landmark for retropubic access; disruption can accompany lower urinary tract trauma |
| Sacroiliac joints | Paired posterior articulations, with strong ligamentous support | Transfer load between spine and hip bones; posterior ring injury affects pelvic stability |
| Sacrococcygeal joint | Articulation between sacrum and coccyx, outside the principal load-bearing ring | Variable mobility and pelvic-floor attachments |
The principal pelvic ring has two sacroiliac joints and one pubic symphysis.
Ilium
- Iliac crest: palpable upper border and a bone-graft donor site. Its projection onto the lumbar spine varies, so a fixed L4 relationship is not used to select a neuraxial level.
- Anterior superior iliac spine (ASIS): attachment of the inguinal ligament and sartorius. It is a reference for Pfannenstiel planning, ilioinguinal and iliohypogastric nerve blocks, and donor-site design.
- Anterior inferior iliac spine: attachment of the direct head of rectus femoris.
- Posterior superior iliac spine: surface landmark that marks the level of the sacroiliac joint.
- Iliac fossa: internal concavity occupied by iliacus; the working surface in retroperitoneal access.
- Arcuate line: part of the pelvic brim (linea terminalis).
- Greater sciatic notch: becomes the greater sciatic foramen with the sacrospinous and sacrotuberous ligaments. Piriformis divides its passageways for gluteal vessels and nerves, the sciatic nerve, and structures travelling toward the perineum.
Ischium and the sciatic foramina
The ischial tuberosities bear weight in sitting and mark the lateral corners of the perineum. They provide attachment to the hamstrings and sacrotuberous ligaments.
The ischial spine separates the greater and lesser sciatic notches and anchors the sacrospinous ligament. The pudendal nerve and internal pudendal vessels leave the pelvis through the greater sciatic foramen, pass close to the spine and posterior to the sacrospinous ligament, and enter the perineum through the lesser sciatic foramen toward Alcock's canal. The obturator internus tendon also traverses the lesser sciatic foramen.
The ischiopubic rami support the perineal membrane and the penile or clitoral crura. They lie close to the bulbar urethra and the clitoral or penile neurovascular tissue, so they define the plane of crural dissection during urethroplasty and artificial urinary sphincter placement.
Pubis and obturator region
| Landmark | Relationship |
|---|---|
| Pubic body and symphysis | Anterior boundary of the retropubic space |
| Pubic tubercle | Medial attachment of the inguinal ligament |
| Superior pubic ramus / pectineal line | Site of Cooper's ligament, used in Burch colposuspension and selected hernia repairs |
| Inferior pubic ramus | Joins the ischial ramus to form the ischiopubic arch |
| Obturator foramen | Largely closed by the obturator membrane; the obturator nerve and vessels pass through the obturator canal at its superior margin |
A transobturator sling traverses the obturator region, and the obturator canal at the anterosuperior margin of the foramen is the structure to protect. Trajectory follows the specific device technique. Corona mortis vessels may cross the superior pubic ramus; see the vascular anatomy page.
Sacrum and coccyx
The sacral promontory is the anterior projection of the upper S1 body. The anterior longitudinal ligament is a fixation structure used in sacrocolpopexy. In an 18-cadaver female study, its median thickness at the promontory was 1.9 mm (range 1.2–2.5 mm). The same study found the first sacral nerve about 2.5 cm below the promontory midpoint and 2 cm to the right of midline (median vertical distance from the promontory to the first sacral foramen 26 mm, range 22–37 mm), outside the presacral space behind the piriformis fascia. The thin ligament calls for controlled suture depth and attention to the adjacent L5–S1 disc (median height 16 mm), because fixation above the promontory risks discitis and osteomyelitis.[1]
The anterior sacral foramina transmit ventral rami of the sacral nerves; the posterior foramina transmit dorsal rami. The sacral hiatus opens into the caudal sacral canal. Small venous openings in the anterior sacral vertebral bodies are distinct from the nerve-carrying anterior sacral foramina; injury to these venous channels can cause severe presacral bleeding.[7] See presacral anatomy for sacral nerves, venous bleeding and fixation hazards.
The left common iliac vein courses from left toward the right to join the right common iliac vein, usually behind the right common iliac artery. Median sacral vessels descend anterior to the sacrum, with variable position relative to the midline. Their relationship to the intended fixation site is identified at surgery; an average clearance is not relied on.[2]
The coccyx contributes attachment to levator ani, coccygeus and anococcygeal tissues. Coccygeal injury, including at childbirth, can cause coccydynia. Coccygectomy is reserved for refractory coccydynia; it releases the anococcygeal attachments, and a sacral cutout cushion may be used to offload the postoperative wound.[8] Wound infection and dehiscence are the most frequent reported complications.[9] Pelvic-floor physical therapy was studied for persistent postcoccygectomy pain in a retrospective subgroup of 17 patients.[10]
Pelvic inlet, midpelvis and outlet
The pelvic brim separates the greater pelvis above from the lesser pelvis below. It follows the sacral promontory and alae, arcuate lines, pectineal lines and pubic crests toward the upper symphysis.
The ischial spines are landmarks of the midpelvis and the reference for fetal station zero. At the outlet, the pubic arch lies anteriorly, the ischial tuberosities laterally, and the coccyx and sacrotuberous ligaments posteriorly. A line between the tuberosities divides the perineum into urogenital and anal triangles.
Textbook female diameters are approximate averages.
| Plane | Anteroposterior | Transverse |
|---|---|---|
| Inlet | About 11 cm (obstetric conjugate); diagonal conjugate about 12.5 cm | About 13 cm |
| Midpelvis | About 12 cm | About 10 cm (interspinous, the narrowest transverse diameter) |
| Outlet | About 9.5–11.5 cm; increases with coccygeal mobility | About 11 cm (intertuberous) |
Typical sex differences are summarized below. They vary within and between populations.
| Feature | Female | Male |
|---|---|---|
| Subpubic angle | Greater than about 80°, rounded | Less than about 70°, V-shaped |
| Inlet | Round or oval | Heart-shaped |
| Sacrum | Shorter, wider, less curved | Longer, narrower, more curved |
| Ischial spines | Less prominent | More prominent |
| Ischial tuberosities | Everted, wider apart | Inverted, closer together |
| Greater sciatic notch | Wide | Narrow |
| Obturator foramen | Oval | Round |
A narrow subpubic angle, closely set tuberosities and a deep sacral curve make deep pelvic exposure by the vaginal or perineal route more difficult.
Procedure-specific landmarks and limits
| Procedure | Useful landmark | Limitation |
|---|---|---|
| Sacrocolpopexy / sacrohysteropexy | Promontory and anterior longitudinal ligament | Identify the left common iliac vein, median sacral vessels, S1 foramen and disc level; no fixed bony rectangle is safe in every patient |
| Sacrospinous ligament fixation | Ischial spine and coccygeus–sacrospinous ligament complex | The suture passes through the ligament, so needle depth and the S3–S4 branches and pudendal nerve near the spine are the limits |
| Burch colposuspension | Cooper's ligament | Retropubic vessels, bladder and paravaginal tissues are identified directly; sutures are placed to avoid the obturator neurovascular bundle |
| Transobturator sling | Ischiopubic ramus and obturator region | Protect the obturator canal and nearby vessels; the device technique defines the trajectory |
| Pudendal nerve block | Ischial spine | The needle is directed medial to the spine through the sacrospinous ligament |
| Urethroplasty / artificial urinary sphincter | Ischiopubic rami and perineal membrane | Protect the urethra, crura and adjacent neurovascular tissue during crural dissection |
For sacrospinous fixation, a study using eight fresh female cadavers plus 17 additional embalmed dissections found the fewest nerves and arteries at the middle segment of the ligament. With a suture-capture device, sutures lay a median 20.5 and 24.8 mm medial to the ischial spine, and no structure was damaged.[3] The nerves to coccygeus and levator ani lay closest to the sutures.[3] A separate 14-cadaver study found the S3 nerve a median 3 mm from the superior border of the ligament midpoint and S3 or S4 branches piercing coccygeus in 94% of specimens.[4] It concluded that limiting needle depth and keeping the needle entry and exit below the upper edge of the ligament may reduce nerve entrapment and gluteal pain.[4]
Pelvic trauma and pubic diastasis
Pelvic-ring displacement, including symphyseal diastasis and displaced inferomedial pubic fractures, raise suspicion for pelvic-fracture urethral injury (PFUI). In a retrospective nested case-control study of 119 men, 25 had urethral injury. Symphysis pubis diastasis (OR 11.8, 95% CI 4.0–34.5) and displaced inferomedial pubic fractures (OR 6.4, 95% CI 1.6–24.9) were independently associated with injury, and each millimeter of diastasis or displacement was associated with an approximately 10% relative increase in risk.[5][6] Mechanism and clinical signs, including meatal blood, perineal hematoma, inability to void, and difficult catheter passage, guide urethral imaging.[6]
Straddle injury is the most common mechanism of anterior bulbar urethral trauma, caused by compression of the urethra against the pubic region, and differs from posterior PFUI. When male urethral injury is suspected, retrograde urethrography is the preferred initial urethral imaging test. For suspected female urethral injury, which is rare and occurs with pelvic fracture, the ACS guideline states that cystourethroscopy and vaginoscopy are warranted, along with a complete speculum examination to identify associated vaginal or rectal injury.[6]
The symphysis widens by a few millimeters in late pregnancy, and symphyseal pain can accompany wider separation. Pregnancy-related widening and traumatic pelvic-ring disruption are distinct.
References
1. 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.
2. 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.
3. Katrikh AZ, Ettarh R, Kahn MA. Cadaveric nerve and artery proximity to sacrospinous ligament fixation sutures placed by a suture-capturing device. Obstet Gynecol. 2017;130(5):1033–1038. doi:10.1097/AOG.0000000000002324.
4. Florian-Rodriguez ME, Hare A, Chin K, et al. Inferior gluteal and other nerves associated with sacrospinous ligament: a cadaver study. Am J Obstet Gynecol. 2016;215(5):646.e1–646.e6. doi:10.1016/j.ajog.2016.06.025.
5. Basta AM, Blackmore CC, Wessells H. Predicting urethral injury from pelvic fracture patterns in male patients with blunt trauma. J Urol. 2007;177(2):571–575. doi:10.1016/j.juro.2006.09.040.
6. American College of Surgeons. Best Practices Guidelines: Management of Genitourinary Injuries. 2025; urethral injury chapter. Full guideline.
7. Casal Nuñez JE, García Martínez MT, Ruano Poblador A, et al. "Presacral haemorrhage during rectal cancer resection: morphological and hydrodynamic considerations." Cir Esp. 2012;90(4):243–247. doi:10.1016/j.ciresp.2011.11.015
8. Kwon HD, Schrot RJ, Kerr EE, Kim KD. "Coccygodynia and Coccygectomy." Korean J Spine. 2012;9(4):326–333. doi:10.14245/kjs.2012.9.4.326
9. Sagoo NS, Haider AS, Palmisciano P, et al. "Coccygectomy for refractory coccygodynia: a systematic review and meta-analysis." Eur Spine J. 2022;31(1):176–189. doi:10.1007/s00586-021-07041-6
10. Scott KM, Fisher LW, Bernstein IH, Bradley MH. "The Treatment of Chronic Coccydynia and Postcoccygectomy Pain With Pelvic Floor Physical Therapy." PM R. 2017;9(4):367–376. doi:10.1016/j.pmrj.2016.08.007