The sciatic nerve arises from L4, L5, S1, S2, and S3. The nerve emerges from the pelvis through the greater sciatic notch inferior to the piriformis muscle and enters the thigh lateral to the ischial tuberosity. In 10% of patients, the sciatic nerve penetrates the substance of the piriformis muscle. The sciatic nerve is accompanied by the inferior gluteal artery.

It is essential to protect the sciatic nerve early in most procedures. Inside the pelvis, the nerve should be identified distally at the greater sciatic notch. Proximally, it should be picked up below the psoas muscle. The sciatic nerve is formed at the junction of the lumbar sacral plexus where these two trunks come together.

Great care must be taken, as the nerve exits the pelvis at the level of the greater sciatic notch, not to injure the accompanying inferior and superior gluteal nerves and arteries because these supply the abductors as well as the gluteus maximus muscle. The gluteus maximus muscle is essential for closure of most pelvic resections.

The femoral nerve arises from posterior divisions of the ventral rami of L2 and L3 and passes inferolaterally between the psoas and iliacus muscles. It passes over the superficial iliacus muscle to enter the proximal thigh underneath the inguinal ligament, just lateral to the superficial femoral artery.

This nerve is almost always preserved during pelvic resections. It should be identified early during most procedures. The femoral nerve is identified in the space between the iliacus and psoas muscles as they exit the pelvis. The femoral nerve lies just below the fascia, bridging the interval between the two muscles, lateral to the femoral artery and vein.

The obturator nerve, formed from the anterior branches of L2, L3, and L4, is the largest nerve formed from anterior divisions of the lumbar plexus. The nerve descends thru the iliopsoas muscle and courses distally over the sacral ala into the lesser pelvis, lying lateral to the ureter and under the internal iliac vessels. It then traverses the obturator foramen into the medial thigh, under the superior pubic ramus, dividing into anterior and posterior branches.

This nerve is routinely transected during pelvic floor resections (type III) due to its intimate proximity to the tumor.

The iliohypogastric (L1), ilioinguinal (L1), genitofemoral (L1, L2) and lateral femoral cutaneous nerves, which arises from L2 and L3, travel downward laterally along the iliopsoas muscle, pass underneath the lateral aspect of the inguinal ligament, and pass just distal and medial to the anterior superior iliac crest to innervate the anterolateral thigh.

This nerve is sacrificed during most pelvic surgical procedures.

Descending the abdomen to the left of the vena cava, the aorta bifurcates at the level of L4 into common iliac vessels at the level of L4-L5. The common iliac bifurcates into internal and external iliacus vessels at the level of S1, the ala sacralis. The level of these bifurcations may vary, especially if the vessels are pushed by a large adjacent tumor mass (FIG 1A).

It is essential to identify two levels of bifurcations before any ligation: the aortic bifurcation and the common iliac bifurcation. Even the best surgeons have ligated the wrong vessels due to distorted anatomy. Such a misstep is especially possible with tumors that cross the midline. Preoperative evaluation with angiography is required for a comprehensive evaluation.

The common iliac artery must be identified early to correctly identify the aorta as well as the internal iliac (hypogastric)

artery (FIG 1B). To the surgeon, the major anatomic features of the common iliac artery are as follows:

The external iliac artery contributes to the inferior epigastric artery and extends distally, as the superficial femoral artery, into the femoral triangle, where it is a useful landmark in identifying neighboring structures.

The internal iliac (hypogastric) artery descends from the lumbosacral articulation to the greater sciatic notch and branches into several arteries. The internal iliac artery and vein often are difficult to identify or ligate. The internal iliac artery lies on top of its vein, which often is large and is easily injured. The hypogastric vessels are routinely ligated in performing modified hemipelvectomies as well as many pelvic resections.

Anterior branches

Posterior branches

The ureter originates from the renal pelvis at the level of L1 and courses in the retroperitoneum to the medial surface of the psoas major muscle, crossed by spermatic or ovarian vessels.

The ureter crosses from lateral to medial on the surface of the peritoneum at the level of the common iliac bifurcation.

The ureter then courses medially at the level of the sciatic notch to insert into the trigone of the bladder.

The corona mortis is an anastomosis of the external iliac, inferior epigastric, and obturator vessels located in the retropubic region approximately 3 cm from the symphysis pubis. Laceration during an ilioinguinal approach can lead to extensive bleeding.

The retroperitoneal space between pubis and bladder is called the space of Retzius.

The anatomic confines of the inguinal canal are described as 4 cm from the deep inguinal ring to the subcutaneous ring. This “deep ring” is the “direct” inguinal space originating lateral to the epigastric vessels. Hesselbach triangle is the “indirect” hernia space originating medial to the epigastric vessels.

The inguinal contents vary by gender:

Most patients who undergo extended hemipelvectomy, which necessitates transection of the sacrum through the ipsilateral neural foramina, regain function of the gastrointestinal and genitourinary tracts. Adding a contralateral compromise of the sacral nerve root will create a severe dysfunction. Tumors that penetrate the sacrum and cross the midline are considered unresectable because of the involvement of bilateral sacral nerve roots (FIG 5). The tumor can be resected but the morbidity of bilateral sacral nerve root loss usually outweighs the questionable oncologic benefit from surgery.

The common iliac vessels are just anterior to the sacral ala, and any cortical breakthrough by a tumor in that site may be expected to extend directly to the blood vessels. The sacroiliac (SI) joint is a key anatomic landmark. The major nerves and blood vessels are medial to it; therefore, any tumor or pelvic resection lateral to the SI joint may be expected not to violate the major neurovascular bundle. Tumor transgression through the SI joint should be documented prior to surgery by using the combination of CT, MRI, and bone scan.

The common iliac artery bifurcates along the sacral ala, and the ureter crosses the bifurcation on its ventral side. Large tumors around the sacral ala commonly displace and occasionally invade these structures. The mere presence of major blood vessel or pelvic viscus involvement by tumor is not an indicator of unresectability and demands preoperative planning. If necessary and curative resection is planned, both structures can be excised en bloc with the tumor and then can be repaired with a graft. However, when a complex resection (bony pelvis and viscus resection) is anticipated, the patient must be informed, and surgical assistance and necessary equipment must be prepared in advance.

Current MRI imaging techniques cannot accurately identify nerve involvement by tumor.

Clinical evidence of femoral, sacral, or sciatic nerve dysfunction usually suggests direct tumor involvement. In most cases, the presence and extent of nerve involvement is established only at the time of surgery.

Sacral plexus invasion by tumor has the same significance in terms of resectability as tumor invasion of the sacrum; bilateral involvement is an indicator of probable unresectability.

The sciatic notch is the site of pelvic osteotomy in resections of the ilium or periacetabular region and in modified hemipelvectomy. CT establishes tumor extension to the sciatic notch, a tight space through which the sciatic nerve and superior gluteal vessels and nerve pass (FIG 6).

The piriformis muscle, which divides the sciatic notch, is a key structure because the sciatic nerve exits the pelvis underneath it and the superior gluteal artery exits the pelvis above it. The patency of the superior and inferior gluteal arteries, which supply the gluteal vasculature, is established by angiography.

Adequate blood supply of the gluteal region is a major preoperative consideration in flap design, and the artery must be
preserved in any pelvic resection, if oncologically feasible. The artery is located only a few millimeters from the periosteum of the sciatic notch roof, and it should be dissected carefully.