Surface and Applied Pelvic Anatomy

Iliac crests: palpable along their length, the highest points used clinically as the intercristal (supracristal) line, corresponding approximately to the L4 vertebral level, a key landmark for safe lumbar puncture.

• ASIS: palpable anterior termination of the iliac crest; reference point for numerous measurements (McBurney's point, Nélaton's line, pelvic inlet diameters) and for palpating pelvic bony landmarks during examination.
• Pubic symphysis: palpable in the midline, the anterior union of the two pubic bones; a key obstetric and surgical landmark, and the anterior boundary of the perineum/pelvic outlet.
• Pubic tubercles: palpable lateral to the symphysis, inguinal ligament attachment, useful landmark for distinguishing femoral from inguinal hernias (femoral hernia lies below and lateral to the tubercle; inguinal hernia lies above and medial).
• Ischial tuberosities: palpable in the seated position (weight-bearing), the lateral boundaries of the pelvic outlet/perineum, hamstring origin.
• Sacrum: palpable in the midline posteriorly, the sacral promontory an important internal (not externally palpable, but assessable per vaginam/rectum) obstetric landmark.
• Coccyx: palpable at the inferior termination of the sacrum, posterior perineal boundary; site of coccydynia after direct trauma.
• Perineal boundaries: the diamond formed by the pubic symphysis, ischiopubic rami/ischial tuberosities, sacrotuberous ligaments, and coccyx; the interischial (bituberous) line dividing it into anal and urogenital triangles is a key surface reference for both clinical examination and procedures (e.g., episiotomy planning, pudendal block).

Pelvic Fractures

• The pelvis forms a rigid bony ring (hip bones + sacrum, joined at the sacroiliac joints and pubic symphysis); disruption at one point of the ring is mechanically likely to be accompanied by disruption at a second point (either another fracture or a joint diastasis), given the ring's structural continuity.
• High-energy pelvic fractures carry a significant risk of major vessel injury (pelvic venous plexuses particularly, given their diffuse, plexiform, difficult-to-control nature) and pelvic organ injury (bladder, urethra, rectum), making pelvic fracture a potentially life-threatening injury even without directly visible external bleeding.

Bladder Rupture

• Extraperitoneal rupture (more common, ~60-80% of cases), typically associated with pelvic fracture, where bony fragments directly injure the extraperitoneal bladder wall/base; urine leaks into the extraperitoneal pelvic space.
• Intraperitoneal rupture (less common), typically from a direct blow to a full/distended bladder, causing the dome (the only peritoneum-covered part) to burst into the peritoneal cavity, given that a full bladder rises above the pubic symphysis into direct abdominal contact and loses the bony pelvis's protective coverage.

Urethral Injury

• Male: The membranous urethra (fixed, within the deep perineal pouch) is particularly vulnerable to shearing injury in pelvic fracture (given its fixed position relative to the more mobile prostatic and bulbar segments); the bulbar urethra is vulnerable to direct straddle injury (e.g., falling astride an object).
• Female: Urethral injury is less common overall, given the urethra's short length and relative mobility, but can occur with severe pelvic fracture or obstetric trauma, given its close relationship to the anterior vaginal wall.

Pudendal Nerve Block

• Performed at the ischial spine, anaesthetising the perineum via the pudendal nerve's course at this point.

Pelvic Floor Injury

• Results from obstetric trauma (the leading cause: direct muscle stretch/tear and pudendal nerve stretch injury during vaginal delivery), chronic raised intra-abdominal pressure, and age-related connective tissue changes.

Pelvic Organ Prolapse

• Descent of pelvic viscera through the urogenital hiatus, the structural weak point in the pelvic diaphragm, classified by compartment and organ involved.

Rectocele

• Posterior compartment prolapse: herniation of the anterior rectal wall into the posterior vaginal wall, through a defect in the rectovaginal septum/fascia, typically related to obstetric or chronic straining-related pelvic floor weakness.
• Presents with a bulge in the posterior vaginal wall, and can cause defecatory dysfunction (incomplete emptying, need for digital assistance/splinting).

Cystocele

• Anterior compartment prolapse: herniation of the bladder base into the anterior vaginal wall, through a defect in the pubocervical fascia, related to the same pelvic floor weakness spectrum.
• Presents with a bulge in the anterior vaginal wall, and can cause urinary symptoms (incomplete emptying, stress incontinence, or occasionally obstructive voiding if severe).

Uterine Prolapse

• Apical compartment prolapse: descent of the uterus/cervix, related to weakness of the transverse cervical/uterosacral ligaments and levator ani.

Ectopic Pregnancy

• Implantation outside the uterine cavity, most commonly in the ampulla of the uterine tube; rupture causes intraperitoneal haemorrhage, classically pooling in the rectouterine pouch (Douglas's pouch).

Ureteric Injury During Pelvic Surgery

• Two classic risk points in female pelvic surgery: the uterine artery crossing ("water under the bridge," near the lateral cervix, hysterectomy risk) and the infundibulopelvic ligament (oophorectomy risk).
• In male pelvic surgery, the ureter's relationship to the ductus deferens (crossing anterior/superior near the bladder base) is the analogous consideration.
• In colorectal surgery, the ureter's course along the pelvic side wall (crossing the common iliac vessel bifurcation, then running close to the rectal lateral ligaments) is a further recognised injury risk site.

BPH Anatomy

• Arises from the transition zone of the prostate, compressing the preprostatic urethra.

Prostate Cancer Spread

• Predominantly arises from the peripheral zone; local spread toward the neurovascular bundles, rectum (limited by Denonvilliers' fascia), and seminal vesicles/bladder base; lymphatic spread to internal iliac/obturator nodes; haematogenous spread via Batson's plexus to the vertebral column.

Haemorrhoids

• Internal (above the pectinate line, portal venous drainage, visceral/painless innervation, classically at 3/7/11 o'clock) vs external (below the line, systemic venous drainage, somatic/painful innervation).

Anal Fissure

• A tear in the anoderm below the pectinate line, classically in the posterior midline, related to a local watershed in blood supply and perpetuated by reflex internal sphincter spasm.

Ischioanal Abscess

• Infection within the fat-filled ischioanal fossa, often originating from an infected anal gland/sinus at the pectinate line, tracking laterally into the fossa.
• Given the posterior communication between the two ischioanal fossae (via the deep postanal space), infection can track to the contralateral side, producing a "horseshoe" abscess: a key anatomical explanation for this recognised clinical pattern, and a reason surgical drainage must account for this potential bilateral extension.

Episiotomy Anatomy

• A surgical incision of the perineum performed during childbirth to enlarge the vaginal outlet and (theoretically) reduce the risk of uncontrolled perineal tearing.
• Mediolateral episiotomy: angled away from the midline (typically at 45-60° from the midline, directed away from the anal sphincter complex), deliberately avoids the perineal body and anal sphincter, reducing the risk of extension into the anal sphincter/rectum compared to a midline incision, at the cost of somewhat more difficult repair and healing.
• Midline episiotomy: directed straight posteriorly through the perineal body, technically easier to repair and generally less painful during healing, but carries a higher risk of extension into the anal sphincter complex/rectum (third/fourth-degree tear) given its direct alignment with these structures.

Perineal Tears

• Classified by degree based on the depth of tissue involvement: first degree (vaginal/perineal skin only), second degree (perineal muscles, sparing the anal sphincter, the most common degree, and anatomically equivalent in extent to a mediolateral episiotomy), third degree (extends into the anal sphincter complex, further subclassified by the percentage of external sphincter thickness torn, and whether the internal sphincter is also involved), fourth degree (extends through the anal sphincter complex into the rectal mucosa).
• The perineal body's central role as the convergence point for multiple muscles explains why it is the focal point of tearing during childbirth, and why third/fourth-degree tears specifically threaten faecal continence (given the anal sphincter complex's direct involvement), the most functionally significant complication of severe perineal trauma.