Anatomically the pelvis structures can be separated into the true pelvis, located below the iliopectineal line (pelvic brim) and the false pelvis, located above the iliopectineal line. Numerous anatomical structures, including vascular supply for the buttocks and lower extremities, pass between the false and true pelvis. The true pelvis contains the floor of the pelvis along with the urethra, rectum, prostate, and vagina. The false pelvis surrounds the lower intraabdominal contents along with the iliacus muscle.

It is important to understand the location of major blood vessels which lie on the inner wall of the pelvis, since injury to these vessels is commonly associated with severe hemorrhage (Fig. 10.2). An understanding of pelvic anatomy will help the orthopedic surgeon recognize which fracture patterns are more likely to cause direct damage to major vessels and result in significant bleeding. The common iliac artery divides into the external and internal branches. The external iliac artery exits the pelvis anteriorly over the pelvic brim to become the femoral artery. The internal iliac artery lies over the pelvic brim and courses anterior and in close proximity to the sacroiliac joint. The posterior branches of the internal iliac artery include the iliolumbar, superior gluteal, and lateral sacral arteries. The superior gluteal artery, which is the largest branch of the internal iliac artery, courses across the sacroiliac joint in the true pelvis and exits through the greater sciatic notch to supply the gluteus medius, gluteus minimus, and tensor fascia lata muscles. It is the most commonly injured vessel in pelvic fractures with posterior ring disruptions. Anterior branches of the internal iliac artery include the obturator, umbilical, vesical, pudendal, inferior gluteal, rectal, and hemorrhoidal arteries. The inferior gluteal artery exits the pelvis through greater sciatic notch inferior to the piriformis and supplies the gluteus maximus. The pudendal and obturator arteries are adjacent to the pubic rami. In addition to the arteries, there is an associated large venous plexus which drains into the internal iliac vein. Injury to this venous plexus is the major source of hemorrhage in most pelvic fractures.

The neural structures that traverse the pelvis can also be injured in displaced pelvic fractures, leading to long-term morbidity. The sciatic nerve is formed by roots from the lumbosacral plexus (L4, L5, S1, S2, S3) and exits the pelvis deep to the piriformis muscle. The lumbosacral trunk is formed from the anterior rami of L4 and L5, and it crosses anterior to the sacral ala and SI joint. Fractures of sacral ala or dislocations of SI joint are most likely to injure the lumbosacral trunk. Typical displacement patterns in posterior pelvic fractures include cranial and posterior displacement of the hemipelvis. This may actually decrease the tension on the nerve roots exiting the pelvis posteriorly. More concerning are pelvic injuries with anterior (and caudal) displacement of the hemipelvis, as these displacement patterns potentially put the nerve roots on continued and significant stretch. The L5 nerve root exits below the L5 transverse process and crosses the sacral ala approximately 2 cm medial to the sacroiliac joint. It may be injured in SI joint disruptions and during anterior surgical approaches to the SI joint.

Significant anterior ring disruption can also damage the urethra and/or bladder. The female urethra is short and not rigidly fixed to the pubis or pelvic floor. Since it is more mobile it is less susceptible to injury from shear forces associated with pelvic fractures. The male urethra is less mobile and is making it more susceptible to injury in pelvic fractures. Stricture is the most common long-term complication observed in male patients who have sustained a urethral injury, but impotence may also occur in 25–47 % of patients with urethral rupture and is likely due to associated injury of the parasympathetic nerves (S2–S4). In males the bladder neck is attached to the pubis by puboprostatic ligaments and is contiguous with prostate, whereas in females the bladder lies on pubococcygeal portion of levator ani muscles. The superior and upper posterior portion of the bladder is covered by peritoneum, while the remainder of the bladder is extraperitoneal and covered with loose areolar tissue. Bladder injuries may be caused by a variety of mechanisms including bony spicules from the pubic rami fractures, blunt force injuries causing rupture, or shearing injuries. Intraperitoneal bladder ruptures require operative repair. Extraperitoneal bladder ruptures can usually be managed nonoperatively unless there is a bony spicule invading the bladder. Nonoperative management consists of catheter drainage and broad spectrum antibiotics. Most bladder injuries heal by 3–6 weeks, and a cystogram is obtained prior to catheter removal to confirm bladder healing.