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Posterior Instrumentation of the Spine

For Orthopaedic Postgraduate Exam - Complete Guide

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1. INTRODUCTION / BASIC CONCEPT

• Posterior instrumentation = placing implants (screws, hooks, wires, rods) through a posterior (back) midline approach to stabilize, correct, or fuse the spine
• Goal: restore alignment, provide stability, allow fusion, decompress neural elements
• Three-column control (Denis): anterior, middle, and posterior columns - posterior instrumentation ideally controls all 3 via pedicle screws

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2. HISTORICAL EVOLUTION (Important for MCQs)

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3. TYPES OF POSTERIOR IMPLANTS

A. Pedicle Screws (Most Important)

• Pass through pedicle into vertebral body
• Three-column fixation - strongest construct
• Control in all 3 planes (sagittal, coronal, axial)
• Allow derotation during correction
• Extraspinal - no implant inside canal (safer than hooks/wires)
• Biomechanically superior in osteoporotic bone

• Type A - Large cancellous channel, probe inserts smoothly
• Type B - Small cancellous channel, snug fit
• Type C - Cortical channel, needs mallet
• Type D - Absent channel, requires juxtapedicular placement

B. Hooks

• Placed over lamina (laminar hook), under lamina, or on transverse process
• Claw hook construct = two hooks facing each other = very strong fixation
• Hooks occupy spinal canal = risk of cord compression if dislodged
• Used when pedicle too small (upper thoracic)

C. Sublaminar Wires (Luque technique)

• Passed under lamina, around spinal cord
• Segmental fixation but risk of neurological injury during passage
• Still used in some pediatric and neuromuscular scoliosis cases

D. Rods

• Connect screws/hooks together
• Usually cobalt-chrome (stiffer, better correction) or titanium (MRI compatible, less stiff)
• Contoured to restore normal sagittal alignment (lordosis/kyphosis)

E. Connectors / Cross-links

• Connect two parallel rods
• Increase rotational stiffness of the construct

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4. PEDICLE SCREW ANATOMY (High-Yield)

• Widest pedicle: L5 (horizontal plane)
• Narrowest pedicle: T5 (horizontal plane) - hardest to place screw
• Largest angle: L5 (most lateral angulation needed)
• Sagittal plane: widest at T11, narrowest at T1

• Pedicle = 62-79% cancellous bone
• Lateral wall thinnest - vertebral artery at risk (cervical spine)
• Medial wall thickest - ~2x thickness of lateral wall (protects spinal cord)
• Insertion point: junction of transverse process midpoint + facet joint line (most common landmark)

• Recommended: ~1.6 screws/level (intermediate density)
• Higher density at ends of construct and at apex of curve
• No proven difference between high vs low density for curve correction

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5. PEDICLE SCREW INSERTION TECHNIQUE

1. Identify entry point (TP mid-height + lateral facet line)
2. Open pedicle cortex with awl/drill
3. Probe the pedicle channel
4. Check all 4 cortices with ball-tipped probe (floor/roof/medial/lateral walls)
5. Tap the pedicle if needed
6. Insert screw

• Intraoperative fluoroscopy
• CT-based navigation
• Robotic-assisted placement
• Patient-specific jigs
• 3D-printed models

• True lateral (0°) misleadingly shows tip far from cortex
• Optimal angle: 30° oblique - best shows true anterior penetration

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6. INDICATIONS FOR POSTERIOR INSTRUMENTATION

• Flexible curve <70°
• Hypokyphosis (less than normal kyphosis)
• Flexible compensatory lumbar curve
• King type V or Lenke 2/4 structural upper thoracic curve

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7. CORRECTION MANEUVERS (Posterior Approach)

• Rod rotation / derotation: Rod pre-contoured in sagittal plane, then rotated axially to correct the coronal curve
• In-situ bending: Rod bent while connected
• Compression: Applied on convex side to correct coronal curve
• Distraction: On concave side (caution - kyphogenic if in lumbar spine beyond L2!)
• Cantilever correction: One end fixed, other end pulled to rod
• Vertebral column resection (VCR): For rigid severe deformities

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8. SACROPELVIC FIXATION

• S1 pedicle screws alone insufficient
• Add: iliac screws (Galveston technique) or S2-alar-iliac (S2AI) screws
• S2AI screws are lower-profile, less prominent, don't need connectors

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9. FUSION LEVELS (Selection Principles)

• Fuse from stable vertebra to stable vertebra
• Stable vertebra = bisected by the center sacral vertical line (CSVL)
• With pedicle screw constructs: can stop 1-3 levels short of the stable vertebra vs hook constructs
• End result: balanced spine - head centered over pelvis in both planes

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10. COMPLICATIONS OF POSTERIOR INSTRUMENTATION

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11. ADVANTAGES OF ALL-PEDICLE SCREW CONSTRUCT (vs Hooks)

1. Segmental fixation at each level
2. Three-column control via vertebral body purchase
3. Derotation possible in axial plane
4. Extraspinal implants - no canal compromise (unlike hooks/wires)
5. Superior biomechanics - cortical purchase stronger, especially in osteoporotic bone
6. Can save 1-3 fusion levels distally

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12. MONITORING DURING SURGERY

• Wake-up test (Stagnara): Patient woken intraoperatively, asked to move feet/ankles. Gold standard historically
• Somatosensory evoked potentials (SSEP): Monitors dorsal columns (sensory)
• Motor evoked potentials (MEP): Monitors corticospinal tract (motor)
• EMG: Pedicle screw placement monitoring
• Combo of SSEP + MEP = most sensitive for cord injury detection

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13. KEY EXAM POINTS / ONE-LINERS

• Harrington rod → flatback deformity (loss of lumbar lordosis)
• Luque rod wires → risk of neurological injury during passage
• Narrowest pedicle = T5 (hardest screw to place)
• Widest pedicle = L5
• Medial pedicle wall = thicker (protects cord); lateral = thinner (vertebral artery at risk)
• Pedicle screw density: 1.6 screws/level is optimal
• All-pedicle screw = 3-column control = best correction
• Optimal view for anterior penetration = 30° oblique "near-approach"
• S2AI screws = lower profile, no connector needed vs iliac screws
• PJK = proximal junctional kyphosis = complication at upper end of long fusions

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