Bony Pelvic Anatomy
The pelvic bones provide landmarks for reconstructive surgery, but the vessels, nerves and soft tissues around them vary. A palpable spine or an average distance from a cadaver study helps orientation; it does not establish a safe needle, suture or trocar trajectory.
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 surface projection onto the lumbar spine varies; do not select a neuraxial level from an assumed fixed L4 relationship.
- Anterior superior iliac spine (ASIS): attachment of the inguinal ligament and sartorius; a reference for abdominal-wall incisions and nerve-block planning.
- Anterior inferior iliac spine: attachment of the direct head of rectus femoris.
- Posterior superior iliac spine: surface landmark near the sacroiliac joint.
- Iliac fossa: internal concavity occupied by iliacus.
- Arcuate line: part of the pelvic brim.
- 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. Their proximity matters during urethral exposure and crural dissection; bone is a landmark, not evidence that an adjacent dissection plane is free of neurovascular structures.
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; the neurovascular canal is a structure to protect. Use the specific procedure and device technique rather than a generic instruction to aim at the canal or pass in a presumed safe quadrant. 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). This thin layer demands controlled suture depth and recognition of the adjacent L5–S1 disc; the measurement does not establish a universally safe bite depth or guarantee fixation strength.[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. Distinguish these neural openings from the smaller venous channels communicating with cancellous bone. 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 actual relationship to the intended fixation site matters more than an average clearance.[2]
The coccyx contributes attachment to levator ani, coccygeus and anococcygeal tissues. Coccygeal trauma can cause pain, including after childbirth, but dislocation should not be described as a common injury of all deliveries. Coccygectomy is a selected treatment for persistent coccydynia after evaluation; anatomy alone does not determine an indication.
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.
Pelvic dimensions, sacral curvature and subpubic angle vary within and between populations. Female pelves are broader on average, but sex-based shape categories and textbook diameter averages cannot determine an individual's pessary size, mesh length, port placement or operative feasibility. Plan from examination, relevant imaging and the proposed exposure.
Procedure-specific landmarks and limits
| Procedure | Useful landmark | Limitation |
|---|---|---|
| Sacrocolpopexy / sacrohysteropexy | Promontory and anterior longitudinal ligament | Identify vessels, nerves and disc level; there is no universal bony rectangle that guarantees safe fixation |
| Sacrospinous ligament fixation | Ischial spine and coccygeus–sacrospinous ligament complex | Fixation intentionally engages the ligament; depth and individual nerve relationships remain important |
| Burch colposuspension | Cooper's ligament | Retropubic vessels, bladder and paravaginal tissues require direct identification |
| Transobturator sling | Ischiopubic ramus and obturator region | Protect the obturator canal and nearby vessels; follow the operation's specific trajectory |
| Pudendal nerve block | Ischial spine | A landmark for a regional-anesthesia technique, not a complete needle-placement instruction |
| Urethroplasty / artificial urinary sphincter | Ischiopubic rami and perineal membrane | Protect the urethra, crura and adjacent neurovascular tissues |
For sacrospinous fixation, a study using eight fresh female cadavers plus 17 additional embalmed dissections found fewer nerves and arteries associated with the middle ligament segment. A separate 14-cadaver study found sacral nerve branches very close to its superior border. These findings support careful placement and limited needle penetration; “2 cm medial to the spine” does not guarantee nerve avoidance, and passing through the ligament itself is not an error.[3][4]
Pelvic trauma and pubic diastasis
Pelvic-ring displacement, including symphyseal diastasis and displaced inferomedial pubic fractures, should increase suspicion for pelvic-fracture urethral injury (PFUI). A retrospective nested case-control study of 119 men included 25 urethral injuries and found an association with increasing displacement. Its model is not an individual bedside risk calculator, and the association between displacement and injury must not be converted into an absolute percentage-point increase in an individual patient.[5][6]
Straddle injury is a separate common mechanism of anterior bulbar urethral trauma, caused by compression against the pubic region; it is not synonymous with posterior PFUI. When male urethral injury is suspected, retrograde urethrography is the preferred initial urethral imaging test. For suspected female urethral injury, cystourethroscopy and vaginal examination are important. The absence of a classic external sign does not exclude injury.[6]
Pregnancy-related symphyseal widening and traumatic pelvic-ring disruption should also be distinguished. Pain and instability require clinical assessment; a gap measurement alone does not establish the cause or severity of symptoms.
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.