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Chapter Summary
• Context: Surgical approaches to the lumbar and lumbosacral spine must reconcile adequate operative exposure, preservation of critical neurovascular structures, and restoration of segmental stability and sagittal alignment. The selection among anterior (ALIF), anterolateral/oblique (OLIF/ATP), lateral transpsoas (LLIF/XLIF), and posterior/transforaminal (PLIF/TLIF) corridors depends on the underlying pathology, target vertebral level, and therapeutic goals—including interbody fusion, canal decompression, deformity correction, and management of spinal tumors or infections. The L4–L5 and L5–S1 levels demand particular anatomical vigilance due to their high prevalence of degenerative disc disease, high shear forces, and major contribution to total lumbar lordosis. In anterior and lateral corridors, dissection traverses the abdominal wall and retroperitoneal space, placing the ureter, lumbar plexus, sympathetic trunk, superior hypogastric plexus, great vessels (aorta and vena cava), and iliac bifurcation at direct risk. Minimally invasive techniques decrease muscle and abdominal wall trauma but restrict visualization to narrow corridors. Consequently, precise cross-sectional imaging, fascial plane dissection, and individualized trajectory selection are paramount.
• Chapter Objective: To present the surgical anatomy of the abdominal wall, retroperitoneum, and neurovascular structures relevant to lumbar and lumbosacral approaches. The reader should be able to understand the distinct technical features of anterior, posterior, transpsoas, and prepsoas corridors; recognize anatomical boundaries and limitations at each lumbar level; plan safe operative corridors using multiplanar MRI and CT; and apply strategies to prevent major vascular, neural, autonomic, ureteral, and abdominal wall complications.
• Abdominal Wall and Retroperitoneal CorridorsThe anterolateral abdominal wall comprises skin, subcutaneous tissue (Camper and Scarpa fasciae), external oblique, internal oblique, and transversus abdominis muscles, lined internally by the transversalis fascia and peritoneum. In lateral and retroperitoneal exposures, muscle-splitting dissection along fiber orientation minimizes abdominal wall denervation and pseudohernia formation. The retroperitoneal space is accessed by mobilizing the peritoneal envelope anteromedially. The ureter adheres to the posterior leaf of the peritoneum and is swept forward with it during blunt retroperitoneal development, protected from direct injury if mobilization remains in the correct fascial plane.
• Critical Neurovascular StructuresThe abdominal aorta descends to the left of the midline and bifurcates at L4 into common iliac arteries; the inferior vena cava lies to the right and bifurcates at L5. At L4–L5 and L5–S1, vascular anatomy is highly variable. The ascending lumbar vein, iliolumbar vein, and middle sacral vessels present significant surgical hazards during lateral and anterior mobilization. The lumbar plexus (femoral nerve, obturator nerve, genitofemoral nerve) is embedded within the posterior substance of the psoas major muscle, migrating progressively anterior from L1 to L5. The lumbar sympathetic trunk courses along the anteromedial border of the psoas, whereas the superior hypogastric plexus overlies the anterior aspect of the L5–S1 disc space in the bifurcation triangle.
• Corridor Selection: ALIF, OLIF, LLIF, and PosteriorApproaches are tailored to level and pathology: ALIF (retroperitoneal or transperitoneal) is ideal for L5–S1, providing direct access to the vascular bifurcation and allowing large hyperlordotic cages; LLIF/XLIF (transpsoas) provides excellent access from L1 to L4 but carries substantial risk of lumbar plexus and femoral nerve injury at L4–L5; OLIF/ATP (prepsoas) accesses L1–L5 via the corridor between the anterior psoas margin and aorta/iliac vessels, avoiding psoas splitting and lumbar plexus navigation; and posterior approaches (TLIF/PLIF) allow direct canal decompression, facetectomy, and interbody grafting from behind.
• Technical Steps and Complication AvoidanceIn anterior retroperitoneal surgery, a left-sided approach is generally preferred for L2–L5 due to the thicker, more resilient aortic wall compared to the thin, tear-prone vena cava. At L5–S1, midline exposure requires meticulous mobilization of the left common iliac vein and cautery-free dissection of the superior hypogastric plexus to prevent retrograde ejaculation in males. In lateral transpsoas surgery, intraoperative directional electromyographic neuromonitoring (EMG) is mandatory, and retractor time should be limited (<20–30 minutes) to avoid ischemic traction injury to the femoral nerve. In prepsoas OLIF, gentle mobilization of the psoas and left iliac vessels exposes the disc space without violating neural parenchyma.
• Clinical Application: Preoperative planning must evaluate multiplanar MRI and CT: assess the position of the iliac bifurcation and ascending lumbar vein at L4–L5 and L5–S1 (high bifurcation vs. low bifurcation); evaluate psoas morphology (high-riding psoas or "Mickey Mouse" sign) for lateral corridors; and measure the vascular corridor window in prepsoas OLIF. In anterior L5–S1 surgery, using blunt dissection without monopolar electrocautery over the anterior annulus protects the superior hypogastric plexus, keeping the incidence of retrograde ejaculation below 2%. In lateral transpsoas LLIF, placing the retractor within the anterior half of the disc space and using real-time triggered EMG avoids the femoral nerve and lumbosacral trunk. In posterior TLIF/PLIF, adequate neural retraction and nerve root identification prevent postoperative radiculopathy. Adherence to these anatomical principles maximizes fusion rates, restores lordosis, and prevents devastating neurovascular complications.
