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Chapter Summary
• Context: Traumatic fractures of the sacrum represent complex, high-energy injuries that are frequently underdiagnosed or delayed in diagnosis in the polytrauma patient. The sacrum serves as the foundational anatomical and biomechanical keystone linking the axial spine to the pelvic ring, transmitting upper body gravity loads to the lower extremities and housing the sacral spinal canal and sacral nerve roots (S1–S5) responsible for lower extremity motor/sensory function, urinary bladder control, anal sphincter continence, and sexual function. Fractures occur in high-energy trauma (motor vehicle collisions, falls from height, crush injuries) typically accompanied by pelvic ring disruptions (Tile, Young-Burgess classifications), but can also occur as low-energy insufficiency fractures in osteoporotic or radiated bone. Classical anatomical classifications include the Denis classification (Zone I alar, Zone II foraminal, Zone III central canal) and the modern AO Spine Sacral and Spinopelvic Classification, which distinguishes isolated pelvic ring fractures from true spinopelvic dissociation (U-shaped, H-shaped, or bilateral vertical fractures separating the spine from the pelvis). High-energy sacral fractures carry high rates of hemodynamic instability from retroperitoneal hemorrhage (presacral venous plexus, internal iliac branches), severe soft tissue Morel-Lavallée shearing lesions, and neurological deficits (up to 50–60% in Zone III injuries). Management spans non-operative mobilization, percutaneous sacroiliac/transiliac-transsacral screw fixation, and robust triangular spinopelvic instrumentation (lumbopelvic fixation).
• Chapter Objective: To present the anatomical, diagnostic, classification, and management principles of sacral fractures and spinopelvic dissociation. The reader should be able to apply the Denis and AO Spine Sacral classifications; distinguish pelvic ring instability from spinopelvic dissociation; select and interpret pelvic/sacral multi-detector CT and MRI; identify sacral nerve root deficits and cauda equina injury; and formulate treatment strategies ranging from percutaneous transsacral screw fixation to open decompression and lumbopelvic triangular fixation.
• Sacral Anatomy and the Spinopelvic KeystoneThe sacrum functions as the mechanical keystone of the pelvic ring. Axial load transfers from L5 to S1 across the sacral promontory, dispersing laterally across the sacral alae and sacroiliac joints to the iliac bones and femoral heads. Anterior and posterior sacroiliac, sacrotuberous, and sacrospinous ligaments provide exceptional multiplanar stability. The sacral canal contains descending S1–S5 nerve roots forming the sacral plexus and pelvic splanchnic autonomic nerves (S2–S4).
• Mechanisms, Denis Classification, and AO Spine ClassificationDenis classification divides sacral fractures into three sagittal zones: 1) Zone I (Alar region lateral to sacral foramina, 50% of cases, ~6% neurological deficit); 2) Zone II (Foraminal region traversing sacral neural foramina, 34% of cases, ~28% neurological deficit, often unilateral sciatica); 3) Zone III (Central canal region medial to foramina, 16% of cases, ~57% neurological deficit with severe bowel, bladder, and sexual dysfunction). The AO Spine Sacral classification categorizes: Type A (lower sacrococcygeal fractures below S2, stable); Type B (vertical sacral fractures compromising pelvic ring stability but maintaining spinopelvic continuity); Type C (spinopelvic dissociation: bilateral vertical fractures connected by a transverse fracture, U-shaped, H-shaped, or Y-shaped variants, resulting in complete mechanical dissociation of the axial spine from the pelvis).
• Clinical Assessment and Pelvic Hemodynamic EmergenciesIn polytrauma, primary evaluation addresses hemorrhagic shock. Sacral fractures frequently lacerate the presacral Batson venous plexus and internal iliac arterial branches (superior gluteal, lateral sacral arteries), leading to life-threatening retroperitoneal hemorrhage. Emergency pelvic circumferential compression (pelvic binder) stabilizes bony disruption and promotes tamponade. Pelvic angiographic embolization is indicated for ongoing arterial bleeding. Examine for closed internal degloving Morel-Lavallée lesions over the lumbosacral region and perform digital rectal examination to test perianal sensation, voluntary anal contraction, and rule out open pelvic fractures communicating with the rectum or vagina.
• Diagnostic Imaging: Multi-Detector CT and 3D ReconstructionsPlain pelvic radiographs (AP, Inlet, Outlet views) provide initial screening but miss up to 30–50% of sacral fractures. Multi-detector thin-cut CT with coronal, sagittal, and 3D reconstructions is the gold standard, identifying vertical fracture lines, foraminal impaction, transverse fracture components in S1/S2 ("jumped" or spondylolisthesis of the upper sacrum on lower sacrum in spinopelvic dissociation), and sacral dysmorphism (present in up to 40% of populations, featuring upper sacral alar slope and collinearity with the iliac crest that narrows safe transsacral osseous corridors). MRI assesses nerve root compression, hematomas, and insufficiency edema.
• Surgical Strategies: Percutaneous Fixation vs. Lumbopelvic FixationManagement depends on stability and neurology: 1) Non-operative: stable Denis Zone I or Type A fractures without displacement or neurological deficit (early progressive mobilization); 2) Percutaneous Sacroiliac / Transiliac-Transsacral Screws (7.0–8.0 mm cannulated screws placed into S1 and S2 corridors under fluoroscopic or 3D navigation): ideal for nondisplaced or reducible Type B pelvic ring injuries; 3) Triangular Spinopelvic Fixation (Lumbopelvic Fixation connecting L4/L5 pedicle screws to iliac or S2-alar-iliac [S2AI] screws linked to transsacral screws): the gold standard for Type C spinopelvic dissociation and highly comminuted vertical shear fractures, neutralizing axial, shear, and torsional loads and allowing immediate patient mobilization; 4) Sacral laminectomy/decompression: indicated for progressive neurological deficit from displaced bone fragments in the central canal (Zone III).
• Clinical Application: In the polytrauma emergency bay, a hemodynamically unstable patient with an open-book or vertical shear pelvic fracture must be immediately stabilized with a pelvic binder placed at the level of the greater trochanters (not over the iliac crests). Obtain emergent pelvic CT: if a transverse fracture across S1/S2 is identified connecting bilateral transforaminal vertical fractures (U-shaped spinopelvic dissociation / AO Type C), recognize that the patient cannot bear weight and is at high risk for progressive cauda equina compression. Perform an urgent neurological exam evaluating S2–S5 dermatomes, perianal sensation, and anal sphincter tone. In spinopelvic dissociation, rigid triangular lumbopelvic fixation (bilateral L4–L5 pedicle screws, S2AI/iliac screws, and transiliac rods) provides immediate mechanical stability, eliminates axial shear, and allows the polytrauma patient to sit upright and mobilize early, preventing catastrophic pulmonary and thromboembolic complications.
