High Yield Overview

SPINOPELVIC DISSOCIATION - LUMBOPELVIC FIXATION

Posterior midline and paraspinal approach for unstable sacral fractures | High complexity

Critical Danger Structures - Spinopelvic Region

Cauda Equina & Sacral Nerve Roots

Location: Within sacral canal and exiting through dorsal foramina Protection: Careful dissection over sacral ala, avoid violation of sacral canal unless decompression indicated, identify fracture fragments compressing neural elements

L5 Nerve Root

Location: Crosses sacral ala at S1 pedicle level, 15-20mm from midline Protection: Identify transverse process and ala junction, stay subperiosteal, avoid aggressive lateral retraction during L5 pedicle screw placement

Superior Gluteal Artery & Vein

Location: Exits greater sciatic notch at piriformis superior border, crosses iliac wing medially Protection: Stay posterior and superior to sciatic notch during iliac dissection, control bleeding with bone wax or bipolar, avoid penetrating anterior cortex with iliac screws

Sacral Venous Plexus

Location: Anterior to sacrum between periosteum and presacral fascia Protection: Maintain subperiosteal dissection, avoid anterior cortex breach during S2AI screw placement, have hemostatic agents available, consider preoperative embolization if displaced fracture

Skin & Soft Tissue Envelope

Location: Often compromised by high-energy trauma, edema, degloving Protection: Delay surgery 5-7 days if significant swelling, use modified incisions for open wounds, avoid excessive undermining, consider staged approach with external fixation first

Mnemonic

SACRALSACRAL - Spinopelvic Dissociation Classification

Memory Hook:Remember: spinopelvic dissociation requires BOTH vertical sacral fractures AND anterior pelvic ring disruption for complete instability

Mnemonic

ILIACILIAC - S2-Alar-Iliac Screw Trajectory

Memory Hook:S2AI screws have 30% greater pullout strength than traditional iliac screws and avoid prominent hardware at PSIS

Indications for Lumbopelvic Fixation

Absolute Indications

U-type sacral fractures (bilateral vertical + horizontal component)
H-type sacral fractures (bilateral vertical fractures through sacral foramina)
Spinopelvic dissociation with vertical shear instability
Denis Zone III bilateral fractures with neurological deficit
Roy-Camille Type III transverse fractures at L5-S1 level

Relative Indications

Unilateral Denis Zone III fracture with neurological deficit requiring decompression
Lumbosacral kyphotic deformity greater than 20° post-injury
Bilateral Denis Zone II fractures with vertical displacement greater than 1cm
Failed nonoperative management of U-type or H-type fractures
Polytrauma patients requiring early mobilization with unstable sacral fractures

Contraindications

Absolute

Active infection at surgical site
Medical instability precluding prolonged surgery
Severe osteoporosis preventing adequate screw purchase

Relative

Extensive soft tissue injury requiring delay
Neurologically intact patient with minimally displaced fracture
Advanced age with low functional demand

Preoperative Planning

Imaging Requirements

AP pelvis radiograph - assess pelvic ring disruption and vertical displacement
Inlet view - evaluate posterior displacement and sacral kyphosis
Outlet view - assess cranial-caudal displacement and transverse fracture
CT scan with 3D reconstruction - define fracture pattern, canal compromise
MRI sacrum - evaluate neural compression, disc injury, soft tissue injury

Classification Systems

Denis Classification (Sacral Fracture Zones)

Zone I: Lateral to foramina (sacral ala) - 6% neurological injury
Zone II: Through foramina - 28% neurological injury
Zone III: Central canal involvement - 57% neurological injury

Roy-Camille Classification (Transverse Sacral Fractures)

Type I: Flexion injury, distal fragment flexed
Type II: Extension injury, distal fragment extended
Type III: Comminuted, associated with spinopelvic dissociation

AO/OTA Classification

61-C3: Complete pelvic ring disruption with sacral involvement
12-C: Posterior arch injury with spinopelvic dissociation

Templating

Measure sacral alar width and iliac corridor on axial CT
Plan pedicle screw trajectory at L4/L5 based on pedicle dimensions
Template S2AI screw trajectory: insertion point, angulation, length
Identify safe zones avoiding L5 and S1 nerve roots
Plan reduction maneuvers for kyphotic deformity

Patient Positioning

Exam Pearl

Positioning Strategy: Prone on radiolucent Jackson table or OSI frame. Free abdomen to reduce epidural venous pressure and blood loss. Padding at chest and iliac crests. Arms tucked or on arm boards. Ensure genitalia free in males. All pressure points padded.

Positioning Hazards

Abdominal compression increases venous bleeding
Excessive hip flexion limits sacral reduction
Inadequate padding causes pressure ulcers
Eye/facial injuries from improper headrest

Operative Steps

Step 1: Skin Incision and Exposure

Technique: Midline incision from L4 spinous process to sacral hiatus (approximately 18-22cm). Deepen through skin and subcutaneous tissue with electrocautery. Incise thoracolumbar fascia in midline. Use Cobb elevator to elevate paraspinal muscles subperiosteally from spinous processes bilaterally.

Exam Pearl

Dissection Plane: Stay strictly subperiosteal to avoid muscle ischemia and denervation. Identify L4, L5 spinous processes by palpation - L4 intersects iliac crest line. Cauterize muscular branches of lumbar arteries close to bone.

Dangers at this step

Superficial wound infection risk 8-12% in trauma cases
Excessive undermining causes skin necrosis
Injury to cutaneous nerves causes painful neuromas at incision

Step 2: Exposure of L4/L5 Pedicles

Technique: Expose lamina, facets, and transverse processes of L4 and L5. Use Penfield elevator to define pedicle boundaries. L4 pedicle: entry 2-3mm lateral to midpoint of facet line. L5 pedicle: entry at junction of transverse process, superior articular process, and lateral pars.

Exam Pearl

Pedicle Landmarks: L5 transverse process is short and stubby. Identify by tracing facet line caudally. Mamillary process marks superior pedicle boundary. Avoid excessive lateral dissection to protect L5 nerve root crossing sacral ala.

Dangers at this step

L5 nerve root injury from lateral dissection or retractor placement
Facet joint violation compromises future fusion options
Dural tear if dissection extends too far medially

Step 3: Exposure of Sacral Ala and Iliac Crest

Technique: Extend subperiosteal dissection laterally over sacral ala to SI joint. Elevate multifidus and erector spinae from posterior sacrum. Expose PSIS bilaterally. If using S2AI screws, identify S1 dorsal foramen (palpable dimple 15-20mm from midline).

Exam Pearl

Sacral Ala Dissection: Stay posterior and superior. The superior gluteal artery exits the greater sciatic notch inferior to PIIS. Maintain continuous subperiosteal elevation. Use sponge sticks to control venous ooze from fracture surfaces.

Dangers at this step

Superior gluteal artery injury causes massive venous bleeding
Anterior cortex penetration risks presacral vessels
Inadequate exposure leads to malpositioned screws

Step 4: Fracture Pattern Assessment

Technique: Palpate sacral fracture lines. Assess displacement and instability with gentle manual pressure. Identify comminution. Use intraoperative fluoroscopy: AP pelvis, inlet, outlet views. Evaluate canal compromise requiring decompression.

Exam Pearl

Fracture Reduction Assessment: Measure lumbosacral kyphosis on lateral fluoroscopy (normal 30-45°). Assess vertical displacement on AP and outlet views. Plan reduction maneuvers before instrumentation. May need temporary Schanz pins for manipulation.

Dangers at this step

Missed injury patterns lead to inadequate fixation
Excessive manipulation causes neurological deterioration
Undisplaced fracture propagation from handling

Step 5: L4 and/or L5 Pedicle Screw Placement

Technique: Use anatomic landmarks or image guidance. Create starting point with burr or awl. Palpate pedicle with curved probe (gear shift technique). Confirm 4-wall integrity with ball-tip probe. Tap, measure depth. Insert polyaxial pedicle screws (6.5-7.5mm diameter, 40-50mm length). Confirm position with fluoroscopy and neurophysiological monitoring.

Exam Pearl

L5 Pedicle Technique: Convergent medial trajectory 25-35°. Caudal angulation 10-15° to follow endplate. Deeper insertion (45-50mm) provides better purchase. Bicortical purchase not necessary and risks anterior vessel injury.

Dangers at this step

Nerve root injury 1-3% risk with medial or inferior breach
Dural tear with medial pedicle wall violation
Screw loosening if inadequate purchase in osteoporotic bone
Anterior cortex penetration risks iliac vessels

Step 6: Iliac Screw Insertion (Traditional Technique)

Technique: Entry point at PSIS, 1-2mm lateral to lateral edge. Trajectory: 40-45° lateral in axial plane, 20-30° caudal in sagittal plane toward AIIS. Use fluoroscopy to confirm corridor position between inner and outer tables. Drill with 3.2mm bit followed by 4.0mm bit. Measure depth (usually 70-90mm). Tap and insert 7.5-8.5mm diameter screw.

Exam Pearl

Iliac Corridor Navigation: Inlet view shows medial-lateral position. Outlet view shows superior-inferior position. Oblique view visualizes full screw trajectory. Feel resistance changes: hard cortex at entry, softer cancellous bone, hard anterior cortex. Do not penetrate anterior cortex.

Dangers at this step

Superior gluteal artery injury if trajectory too inferior/medial
Iliac crest fracture with excessive force during tapping
Prominent hardware at PSIS causes pain, requires later removal 20-30%
Screw cutout through outer table with incorrect trajectory

Step 7: S2-Alar-Iliac (S2AI) Screw Placement (Preferred Technique)

Technique: Entry point 1-2mm lateral to S1 dorsal foramen on sacral slope. Starting trajectory: 40-45° lateral, 20-30° caudal. Palpate S1 and S2 pedicle boundaries. Use guidewire under fluoroscopy. Cannulated drill over wire. Measure depth (90-120mm for bicortical purchase at AIIS). Insert cannulated 7.5-8.5mm screw.

Exam Pearl

S2AI Advantages Over Traditional Iliac Screws: (1) Starting point closer to midline allows easier rod contouring, (2) 30-40% greater pullout strength, (3) Lower prominence and pain, (4) Reduced removal rate (5% vs 25%), (5) Crosses SI joint for additional stability, (6) Can be placed percutaneously.

Dangers at this step

L5 nerve root risk if trajectory too superior/medial
S1 nerve root risk if starting point too medial
Anterior cortex penetration risks presacral vessels
Inadequate length provides suboptimal fixation

Step 8: Rod Contouring and Placement

Technique: Measure distance between screw heads. Contour 5.5mm or 6.0mm titanium or cobalt-chromium rods to restore lumbar lordosis and correct sacral kyphosis. Insert rods into screw heads, usually starting caudal and working cephalad. Use rod holders and persuaders. Achieve provisional reduction.

Exam Pearl

Rod Contouring Strategy: Pre-bend rods to restore normal lumbosacral alignment (lumbar lordosis 40-60°, sacral slope 30-45°). Create smooth curve without sharp bends (stress risers). S2AI screws require less rod offset than traditional iliac screws, easier to connect.

Dangers at this step

Excessive force during rod insertion can displace screws
Sharp rod bends create fatigue failure points
Incomplete seating prevents solid connection

Step 9: Fracture Reduction

Technique: Use rods as reduction tools. Compress to close vertical fracture gaps. Apply extension force to correct kyphotic deformity. May use temporary Schanz pins in sacrum for manipulation. Confirm reduction with fluoroscopy: AP pelvis (vertical displacement), inlet (posterior displacement), outlet (cranial-caudal displacement), lateral (kyphosis).

Exam Pearl

Reduction Maneuvers: For vertical displacement: compression along rod. For kyphosis: extend lumbosacral junction (may need prone positioning adjustment). For rotation: de-rotation around bilateral fixation points. Accept up to 5-10mm residual displacement if neurologically intact.

Dangers at this step

Neurological deterioration from manipulation
Screw pullout with excessive force
Propagation of undisplaced fracture lines
Overcorrection into hyperlordosis

Step 10: Set Screw Tightening

Technique: Provisionally tighten all set screws. Recheck reduction and alignment. Definitively tighten set screws sequentially (not alternating) to approximately 8-10 Nm torque. Confirm no screw toggle or rod slippage.

Exam Pearl

Tightening Sequence: Start at apex of construct (L5 or S1). Work toward ends (L4 and iliac/S2AI). This prevents locking in malalignment. Recheck reduction after each screw tightened. Final torque 8-10 Nm using torque-limiting driver.

Dangers at this step

Screw head fracture with excessive torque
Construct failure if undertightened
Rod slippage if not properly seated before tightening

Step 11: Cross-Connector Application

Technique: Measure distance between bilateral rods. Apply cross-connectors at two levels (typically proximal and distal). Tighten cross-connector screws. Confirm construct rigidity by gentle manual stress.

Exam Pearl

Cross-Connector Biomechanics: Increases torsional rigidity by 45%. Essential for bilateral unstable injuries. Place one connector near top of construct (L4-L5) and one near sacrum. Offset connectors axially for maximum rigidity. Consider multiple connectors for long constructs.

Dangers at this step

Prominent connectors cause discomfort
Inadequate tightening allows micromotion
Excessive length prevents skin closure

Step 12: Supplemental SI Screw Fixation

Technique: If anterior pelvic ring unstable or inadequate lumbopelvic fixation, place percutaneous SI screws. Entry point: lateral sacrum below iliac crest. Trajectory: perpendicular to fracture plane, parallel to S1 superior endplate. Confirm position in inlet, outlet, and lateral views. Insert 7.3mm or 8.0mm cannulated screw across SI joint into S1 body.

Exam Pearl

SI Screw Indications: (1) Unilateral fixation requiring contralateral SI stabilization, (2) Comminuted sacral ala fracture, (3) Osteoporotic bone requiring augmentation, (4) Vertical shear pattern requiring compression across SI joint. Start SI screws before lumbopelvic fixation for easier trajectory.

Dangers at this step

L5 nerve root injury if trajectory too superior (must stay parallel to S1 endplate)
Anterior cortex penetration risks iliac vessels
Screw too short provides inadequate purchase
Dysmorphic sacrum (25% prevalence) requires careful templating

Step 13: Neural Decompression (If Indicated)

Technique: If neurological deficit present, perform laminectomy of involved sacral segments. Use Kerrison rongeurs and high-speed burr. Remove bone fragments compressing nerve roots. Decompress central canal and lateral recesses. Preserve posterior tension band if possible. Confirm decompression by visualizing dura and nerve roots.

Exam Pearl

Decompression Timing: Acute neurological deficit (less than 8 hours): emergent decompression. Subacute (8-72 hours): urgent decompression improves outcomes. Chronic (greater than 2 weeks): limited recovery potential, decompress for pain. Incomplete cauda equina has better prognosis than complete.

Dangers at this step

Dural tear requires meticulous repair (high CSF leak risk in sacrum)
Nerve root injury from manipulation
Destabilization if excessive bone removed
Persistent deficit despite adequate decompression

Step 14: Fluoroscopic Confirmation

Technique: Obtain AP pelvis, inlet, outlet, and lateral radiographs. Confirm: (1) Screw positions within bone, (2) Adequate reduction, (3) No hardware impingement on neural structures, (4) Restored alignment. Perform final neurophysiological monitoring checks.

Exam Pearl

Fluoroscopic Assessment Checklist: AP: screw trajectories, vertical displacement reduced. Inlet: posterior displacement corrected, iliac/S2AI screws in corridor. Outlet: cranial-caudal displacement reduced, transverse sacral screws below L5-S1. Lateral: lumbosacral kyphosis corrected, no anterior screw penetration.

Dangers at this step

Missed screw malposition causes postoperative neurological deficit
Incomplete reduction leads to progressive deformity
Inadequate documentation for medicolegal purposes

Step 15: Wound Closure

Technique: Copious irrigation (3-6 liters with bacitracin or chlorhexidine). Place drains if extensive dead space (controversial). Close fascia with #1 absorbable suture in interrupted or running fashion. Close subcutaneous layer with 2-0 absorbable suture. Skin closure with staples or subcuticular suture. Apply sterile dressing.

Exam Pearl

Closure Considerations: Fascia closure essential for strength. Avoid excessive tension (use relaxing incisions if needed). Drains may reduce hematoma but increase infection risk - author preference varies. Subcuticular closure improves cosmesis. Negative pressure wound therapy for high-risk wounds.

Dangers at this step

Wound dehiscence if fascia not properly closed
Deep infection risk 2-8% in trauma setting
Seroma/hematoma if inadequate hemostasis
Skin necrosis from excessive tension

Exam Viva Scenarios

Practice these scenarios to excel in your viva examination

VIVA SCENARIOStandard

EXAMINER

"A 28-year-old motorcyclist has sustained a U-type sacral fracture in a high-speed collision. Walk me through your surgical plan for lumbopelvic fixation. What are the critical steps and potential pitfalls?"

EXCEPTIONAL ANSWER

I would approach this as a high-energy spinopelvic dissociation requiring urgent stabilization. After confirming neurological status and obtaining comprehensive imaging including CT with 3D reconstruction, I would plan lumbopelvic fixation. The critical elements are: (1) Positioning prone with free abdomen to reduce venous bleeding. (2) Midline approach from L4 to sacrum with subperiosteal dissection. (3) Placement of bilateral pedicle screws at L5 (possibly L4 if osteoporotic or comminuted). (4) Bilateral S2-alar-iliac screws which I prefer over traditional iliac screws due to superior biomechanics and lower hardware prominence. (5) Rod contouring to restore lumbosacral alignment, correcting any kyphotic deformity. (6) Application of cross-connectors for rotational stability. (7) Supplemental SI screws if anterior ring unstable. (8) Neural decompression if neurological deficit present. The key pitfalls include: L5 nerve root injury during lateral dissection or S2AI screw placement, superior gluteal artery injury during sacral ala exposure, inadequate reduction of kyphotic deformity leading to sagittal imbalance, and screw malposition causing neurological injury.

VIVA SCENARIOStandard

EXAMINER

"You are placing an S2-alar-iliac screw and encounter brisk bleeding that is difficult to control. What is your differential diagnosis and immediate management?"

EXCEPTIONAL ANSWER

Brisk bleeding during S2AI screw placement suggests vascular injury, most likely the superior gluteal artery or presacral venous plexus. My immediate management would be: (1) Apply direct pressure with sponge on a stick or cottonoid. (2) Ensure adequate exposure and lighting to identify bleeding source. (3) If superior gluteal artery (pulsatile bright red blood exiting greater sciatic notch), attempt bipolar coagulation, bone wax at exit point, or packing with thrombin-soaked gelfoam. (4) If presacral venous plexus (darker oozing from anterior sacrum), this suggests anterior cortex penetration - remove the screw immediately, pack the tract with bone wax or hemostatic agents like Gelfoam or Floseal. (5) If uncontrolled, consider placing screw to tamponade (controversy - may need vascular surgery consultation). (6) Have anesthesia prepare for potential massive transfusion protocol. (7) Consider interventional radiology for emergent embolization if bleeding continues despite packing. Prevention strategies include staying within the safe corridor on fluoroscopy, using guidewire technique, and avoiding anterior cortex penetration by measuring screw length carefully on preoperative CT.

VIVA SCENARIOStandard

EXAMINER

"Six months postoperatively, your patient with lumbopelvic fixation for spinopelvic dissociation complains of persistent buttock pain and painful hardware prominence at the PSIS. What is your assessment and management plan?"

EXCEPTIONAL ANSWER

This presentation is consistent with symptomatic hardware prominence, occurring in 20-30% of traditional iliac screws but only 5% of S2AI screws. My assessment would include: (1) History: Characterize pain - worse with sitting or lying supine, direct pressure over PSIS, no radicular symptoms. (2) Exam: Palpable prominent hardware, no signs of infection (erythema, warmth, drainage), assess for SI joint tenderness versus isolated hardware pain. (3) Imaging: AP pelvis and lateral lumbosacral radiographs to assess fracture healing and hardware position. Consider CT if nonunion suspected. (4) Labs: ESR/CRP if any concern for infection. Management depends on fracture healing status: If fracture NOT healed (less than 12 months, persistent fracture lines, no bridging bone): Conservative management - activity modification, cushioning, NSAIDs, reassurance that removal possible after union. If fracture solidly healed (confirmed radiographic/CT union): Offer hardware removal - remove iliac screws bilaterally, can retain pedicle screws and rods if asymptomatic, warn about less than 5% risk of recurrent instability. If nonunion present: Revision fixation with bone grafting, not simple removal. Prevention for future cases: Use S2AI screws instead of traditional iliac screws as they have much lower prominence rates and equivalent/superior biomechanics.