Direct Vertebral Rotation (DVR) - MS Orthopaedics (20 Marks)

Introduction

• Coronal plane - lateral curvature (measured by Cobb angle)
• Sagittal plane - abnormal kyphosis/lordosis
• Axial plane - vertebral rotation (the most difficult to correct)

Definition

Background and Rationale

Limitations of Prior Techniques

Rod Derotation vs DVR

• Pre-contoured concave rod is connected to each fixation site
• Rod is rotated ~90 degrees into the sagittal plane
• This "converts scoliosis to kyphosis" - an en bloc derotation maneuver
• Results in lateral translation of apical vertebrae (indirect derotation)
• Two forces are generated: (1) posteriorly/medially directed vector; (2) rotation of the rod on its own axis
• Limited ability to directly correct intervertebral rotation

• Acts directly on the vertebrae via screw derotators
• Corrects intervertebral rotation segment by segment
• Can be done en bloc (multiple levels simultaneously) or segmentally (individual levels)
• Opposite in direction to the rotational deformity
• Provides superior axial correction

Prerequisites for DVR

1. Bilateral pedicle screws at every level to be fused in the thoracic spine (monoaxial or uniplanar screws)
2. Knowledge of the direction of vertebral rotation at each level
3. Screw derotators (special instruments)
4. Intraoperative neuromonitoring (MEP - motor evoked potentials)
5. Adequate mean arterial pressure (MAP ≥ 70 mmHg) to maintain spinal cord perfusion during correction maneuvers

Understanding the Direction of DVR

In a Right Thoracic Curve (Lenke Type 1 - most common):

• Apical and juxta-apical thoracic vertebrae are rotated clockwise in the transverse plane (right rotation)
• DVR must be in the opposite direction - counterclockwise
• The rotation correction for the uppermost 1-2 vertebrae must also be opposite to the thoracic DVR
• For the lowermost 1-2 screws, DVR direction depends on the rotation of the compensatory lumbar curve

In a Double Major Curve:

• The lumbar DVR direction is opposite to the thoracic DVR direction
• The transitional vertebrae require careful assessment

Key Rule:

"The direction of DVR must be opposite to that of the vertebral rotation in the deformed curve."

Step-by-Step Surgical Technique (TECHNIQUE 40.14 - Campbell's)

Figure: DVR - screw derotators applied to bilateral pedicle screws at concave (concave side, pull-up force) and convex (push-down force) sides.

Step 1 - Pedicle Screw Insertion

• Insert bilateral pedicle screws at every level to be fused in the thoracic spine
• Identify the neutral (least rotated) vertebra distally - begin derotation proximal to this level

Step 2 - Neuromonitoring Setup

• Because of the corrective forces placed on the spine, a MAP of at least 70 mmHg is essential to maintain spinal cord perfusion
• Obtain a baseline motor-evoked potential (MEP) measurement before any reduction maneuver for comparison once maneuvers are complete

Step 3 - Apply Screw Derotators

• Insert screw derotators onto the pedicle screws of the juxta-apical vertebrae on both the concave and convex sides

Step 4 - Derotation Maneuver

• Derotate the vertebrae as much as possible in the direction opposite to the deformity
• This can be done in:
  • En bloc fashion: multiple levels rotated simultaneously
  • Segmental fashion: each individual level rotated one at a time
• Simultaneously:
  • Push down on convex screws
  • Pull up on concave screws
  • An assistant should apply downward pressure on the convex apical ribs to aid derotation

Step 5 - Locking

• After completion of derotation, lock the rod into position by tightening set screws fully
• This process can be repeated multiple times until the desired correction is obtained

Step 6 - Completion

• If the curve is rigid, DVR may be limited - consider additional techniques like rod bending or apical posterior release
• Apply distraction/compression as needed for balance
• Perform decortication and bone grafting
• Close wound

Mechanisms of Correction

1. Axial plane: Direct rotation of vertebra toward neutral - reduces apical vertebral rotation (AVR)
2. Coronal plane: Derotation of rotated vertebrae assists in reducing the Cobb angle (coronal correction enhanced by DVR)
3. Sagittal plane: En bloc DVR does not reduce thoracic kyphosis - it may even slightly increase kyphosis, which is beneficial in AIS patients who commonly have thoracic hypokyphosis

Advantages of DVR over Simple Rod Derotation

• DVR showed better rotational and coronal corrections than SRD
• Both groups had similar preoperative curve patterns

Clinical Results

• DVR significantly reduces apical rotation of the spine
• Enhances coronal correction ability
• More efficient in the thoracic spine than lumbar spine (better reduction in rib prominence/rib hump)
• En bloc DVR does not reduce thoracic kyphosis; may increase it (preferable to lordotic effect)
• DVR with apical posterior release and use of highly rigid rods enhances radiological results, particularly in stiff curves and adult patients
• No clear correlation found between the amount of axial derotation and degree of coronal/sagittal correction achieved

Complications and Pitfalls

1. Neurological injury - most serious; hence mandatory MEP monitoring and MAP maintenance ≥ 70 mmHg
2. Screw pullout/plow - occurs if excessive force applied at a single level; distribute forces across multiple levels
3. Inadequate correction in rigid curves - DVR may be limited; consider supplemental posterior release, Ponte osteotomies, or rib release
4. Wrong direction of DVR - can worsen deformity; meticulous preoperative planning essential
5. Instrument failure - breakage of screw derotators
6. Decompensation - if fusion levels are not selected correctly (use Lenke classification for guidance)
7. Loss of sagittal balance - under-contouring of rods can create flatback

Comparison with Other 3D Correction Methods

Vertebral Body Derotation (VBD) / Direct Vertebral Body Derotation:

• Uses a clamp attached to the vertebral body anteriorly via VATS/MISS approach
• More aggressive derotation possible but requires anterior approach or combined surgery

Differential Rod Contouring (DRC):

• Includes some axial correction via differential contouring of concave vs convex rods
• Less powerful than DVR for axial correction
• Avoids additional surgical effort
• Recent studies (MDPI 2023) suggest DVR may not improve clinical outcomes (SRS-22 scores) vs DRC alone, though radiographic axial correction is better with DVR

Thoracoplasty:

• Rib resection to reduce rib hump
• Acts on the cosmetic deformity but not directly on vertebral rotation
• Can be combined with DVR

Indications for DVR

1. Adolescent Idiopathic Scoliosis (AIS) - primary indication, especially Lenke Type 1, 2, 3, 6
2. Significant apical vertebral rotation (Nash-Moe Grade II or higher)
3. Rigid curves - DVR provides added benefit over rod derotation alone
4. Adult idiopathic scoliosis with significant rotational deformity
5. Any structural thoracic or lumbar curve requiring pedicle screw-based posterior correction

Summary Points for Exam

Conclusion

3D Printing in Orthopaedics - MS Orthopaedics (20 Marks)

Introduction

Principles and Terminology

3D Printing Technologies Used in Orthopaedics

1. Stereolithography (SLA)

• Oldest method - uses a UV laser to cure liquid photopolymer resin layer by layer
• Produces highly detailed and accurate models
• Used for anatomical models, preoperative planning, custom implant prototypes
• Limitation: photopolymer resins are not biocompatible for implantation

2. Selective Laser Sintering (SLS)

• A high-power laser sinters powdered materials (polymers, ceramics) layer by layer
• Can handle a variety of biocompatible and bioresorbable materials
• Produces robust, complex implants and prosthetics
• No support structures needed; high design freedom

3. Fused Deposition Modeling (FDM) / Fused Filament Fabrication (FFF)

• Thermoplastic filament (PLA, ABS, PEEK) is extruded through a heated nozzle layer by layer
• Most affordable and accessible technology
• Used for anatomical teaching models, surgical planning tools, prototypes
• Limited for direct implantation due to material properties

4. Selective Laser Melting (SLM) / Direct Metal Laser Sintering (DMLS)

• Most important for orthopaedic implants
• High-power laser fuses metal powder (titanium alloy Ti-6Al-4V, cobalt-chromium, stainless steel) completely layer by layer
• Produces dense, load-bearing metallic implants with complex internal architectures
• Enables creation of highly porous titanium surfaces with controlled pore size (100-400 μm) ideal for bone ingrowth - superior to traditional sintered bead coatings
• Used for: custom joint prostheses, spinal cages, acetabular cups, tumor reconstruction implants

5. Electron Beam Melting (EBM)

• Uses electron beam instead of laser to melt titanium powder
• Operates in vacuum; reduces residual stresses
• Produces highly porous structures for cementless fixation
• Used for trabecular metal-like structures and spinal implants

6. Binder Jetting

• Liquid bonding agent selectively deposits onto a powder bed
• Multi-material capability; faster than laser-based methods
• Used for anatomical models and surgical planning aids

7. Bioprinting (Emerging)

• Uses "bioinks" containing living cells, hydrogels, and growth factors
• Can print tissue-engineered scaffolds seeded with cells
• Current research: cartilage repair, bone regeneration scaffolds, osteochondral constructs

Materials Used in 3D Printing for Orthopaedics

Clinical Applications in Orthopaedics

1. Preoperative Planning and Anatomical Models

• Complex fractures: acetabular fractures (both-column), tibial plateau, pilon, calcaneum, pelvis
• Model enables: understanding fracture anatomy, simulating reduction ("practice surgery"), pre-contouring plates, determining implant sizes, screw lengths and trajectories
• Liu et al.: complex T-type acetabular fracture treated 25 days after injury using 3D model - reduction and fixation sequence planned on model, plates pre-bent and then sterilized for use
• You et al.: proximal humeral fractures - 3D-printed model group had reduced surgical time, less blood loss, and less fluoroscopy time vs. standard CT planning group
• Meta-analysis (Liu 2025, PMID 40420305): 3D printing-assisted ORIF for acetabular fractures - significantly reduced operating time, blood loss, fluoroscopy exposure vs conventional surgery
• Reduces interobserver variability in fracture classification

• Bone tumors: complex resections near neurovascular structures (periacetabular, sacral, vertebral)
• Enables planning of resection margins and reconstruction geometry

• Models used to plan corrective osteotomies in malunions and deformities
• Quantitative 3D assessment (Q3DCT) of fracture displacement supplements existing classification systems

2. Patient-Specific Surgical Guides (PSG / PSI)

• Total Knee Arthroplasty (TKA): custom cutting blocks that snap onto bone surfaces to guide tibial and femoral cuts without intramedullary rods - reduces blood loss, fat embolism risk
• Total Hip Arthroplasty (THA): acetabular cup orientation guides
• Total Ankle Arthroplasty (TAA): CT-based patient-specific cutting jigs provide accuracy and reproducibility; can decrease operative time and costs - particularly useful in complex deformities
• Shoulder arthroplasty: patient-specific instrumentation reduces deviations between planned and post-osteotomy humeral retrotorsion and height
• Pedicle screw placement: guides for accurate screw trajectory in scoliosis and spinal fusion
• Tibial plateau fractures: 3D-printed drilling guides for medial tibial plateau fractures - guide drill bit along predetermined screw trajectories (Assink et al., 2024)
• Corrective osteotomies: malunion of proximal humerus, distal radius, high tibial osteotomy - patient-specific guides ensure accurate osteotomy planes

3. Patient-Specific Implants (PSI) / Custom Prostheses

• Bone tumors requiring wide resection: hemipelvis, proximal femur, distal femur, proximal tibia, scapula, vertebral body
• Custom 3D-printed titanium implants designed to fill exact resection gap
• Integrated porous surfaces for bone ingrowth
• Meta-analysis (Mounsef 2025, PMID 40923871): 3D printing in orthopedic oncology shows enhanced functional and surgical outcomes vs conventional techniques
• Avoids the need for allograft or custom implant with long lead time from manufacturer

• Large acetabular or tibial bone defects after failed THA/TKA
• Custom augments and cage designs to bridge bone loss
• Highly porous titanium with trabecular architecture promotes bone ongrowth/ingrowth

• Patient-specific titanium implants for tibial plateau fractures (Assink 2024): designed to follow curvature of tibial shaft and medial plateau - restores coronal and sagittal alignment
• Patient-specific plates for both-column acetabular fractures - designed from virtual fracture reduction (contralateral mirrored template)
• Periarticular malunions: custom implants for complex deformities with limited fixation options

• Custom implant sizing guided by weight-bearing CT-based planning

• 3D-printed titanium interbody fusion cages: patient-specific geometry, lordosis, porous architecture for fusion
• Custom vertebral body replacement implants for tumor/trauma

• Additive manufacturing used to produce highly porous titanium surfaces on femoral components, ideal for bone ingrowth (Campbell's, 15th Ed)
• Pore size 100-400 μm optimized for bone ingrowth
• Mimics trabecular bone architecture; superior osseointegration vs traditional sintered bead coatings

4. Prosthetics and Orthotics

• Upper limb prostheses: custom 3D-printed myoelectric hands and body-powered prostheses; significantly cheaper than conventional devices
• Lower limb prostheses: custom sockets, ankle-foot orthoses (AFOs) with biomechanically optimized geometry
• Osseointegrated limb prostheses: titanium implants for direct skeletal attachment - porous 3D-printed surfaces for bone-implant integration
• Spinal orthotics: scoliosis braces (TLSO) - custom 3D-printed braces that fit precisely, improve comfort, and may improve compliance vs standard thermoplastic braces
• Pediatric applications: growing children - custom prosthetics can be reprinted quickly and cheaply as the child grows

5. Medical Education and Training

• Anatomical models for medical students and residents
• Surgical simulation before complex cases
• Improved spatial understanding of complex 3D anatomy (acetabulum, wrist, foot)
• Models for patient education - visual representation of fracture and planned surgery

6. Tissue Engineering and Bioprinting (Future)

• 3D-printed scaffolds (HA, TCP, biopolymers) seeded with mesenchymal stem cells for bone regeneration in large defects
• Osteochondral scaffolds for cartilage repair
• Bioprinting: cell-laden "bioinks" print living tissue constructs
• Systematic review (Lu 2026, PMID 41722744): mesenchymal stem cells with bone-implant scaffolds for bone regeneration

Advantages of 3D Printing in Orthopaedics

1. Patient-specific fit - implants and guides conform exactly to individual anatomy
2. Reduced operative time - less intraoperative decision-making
3. Reduced blood loss and fluoroscopy - verified in multiple RCTs
4. Better osseointegration - complex porous architectures impossible with conventional manufacturing
5. Improved 3D anatomical understanding - reduces interobserver variability in fracture assessment
6. Oncological reconstruction - fills defects of any geometry
7. Reduced revision surgery - better fit leads to better long-term outcomes
8. Rapid prototyping - patient-specific titanium implants can be produced in as little as 5 days (Assink 2024)
9. Cost savings - reduced inventory, less waste, fewer revision surgeries
10. Education and training - inexpensive simulation models

Limitations and Challenges

1. Cost - high upfront cost of printers; metal printing especially expensive
2. Time - printing and quality control adds lead time; not practical for acute emergency cases daily
3. Regulatory hurdles - custom implants require FDA/CE regulatory approval; varies by country
4. No soft tissue modeling - 3D models represent only hard tissues; ligaments, tendons, neurovascular structures absent
5. Material fatigue - SLM/EBM parts may have different fatigue properties than forged implants
6. Sterilization - printed models and guides require validated sterilization; risk of contamination
7. Lack of long-term data - for custom implants, especially in benign conditions
8. Skill requirements - 3D segmentation and virtual surgical planning require trained personnel
9. PSI in TKA - no proven benefit in functional outcomes (Miller's Review)

Recent Evidence (2024-2026)

Summary Table: Applications at a Glance

Conclusion

3D Printing in Orthopaedics - MS Orthopaedics (20 Marks)

Introduction

CT/MRI scan → DICOM data → 3D segmentation software → STL file → 3D printing → Sterile model/guide/implant

Principles and Terminology

3D Printing Technologies Used in Orthopaedics

1. Stereolithography (SLA)

• Oldest method - uses a UV laser to cure liquid photopolymer resin layer by layer
• Produces highly detailed, accurate models
• Used for anatomical models, preoperative planning, implant prototypes
• Limitation: resins are not biocompatible for implantation

2. Selective Laser Sintering (SLS)

• High-power laser sinters powdered materials (polymers, ceramics) layer by layer
• Can handle a variety of biocompatible and bioresorbable materials
• Produces robust, complex implants and prosthetics
• No support structures needed; high design freedom

3. Fused Deposition Modeling (FDM)

• Thermoplastic filament (PLA, ABS, PEEK) extruded through heated nozzle layer by layer
• Most affordable and accessible technology
• Used for anatomical teaching models, planning tools, prototypes
• Limited for direct implantation due to material properties

4. Selective Laser Melting (SLM) / Direct Metal Laser Sintering (DMLS)

• Most important for orthopaedic implants
• High-power laser completely fuses metal powder (Ti-6Al-4V, cobalt-chromium, stainless steel) layer by layer
• Produces dense, load-bearing metallic implants with complex internal architectures
• Enables highly porous titanium surfaces with pore size 100-400 μm ideal for bone ingrowth - superior to traditional sintered bead coatings
• Used for: custom joint prostheses, spinal cages, acetabular cups, tumor reconstruction implants

5. Electron Beam Melting (EBM)

• Uses an electron beam (in vacuum) instead of laser to melt titanium powder
• Reduces residual stresses; produces trabecular metal-like porous structures
• Used for cementless fixation surfaces and spinal implants

6. Bioprinting (Emerging)

• Uses "bioinks" containing living cells, hydrogels, and growth factors
• Prints tissue-engineered scaffolds seeded with cells
• Current research: cartilage repair, bone regeneration, osteochondral constructs

Materials Used in 3D Printing for Orthopaedics

Clinical Applications in Orthopaedics

1. Preoperative Planning and Anatomical Models

• Complex injuries: acetabular (both-column), tibial plateau, pilon, calcaneum, pelvis
• Enables: understanding fracture anatomy, simulating reduction ("practice surgery"), pre-contouring plates, determining implant sizes, screw lengths and trajectories
• Liu et al.: complex T-type acetabular fracture - reduction and fixation sequence planned on 3D model; plates pre-bent on model and then sterilized for actual use
• You et al.: proximal humeral fractures - 3D-printed model group had reduced surgical time, less blood loss, and less fluoroscopy time vs standard CT planning
• Reduces interobserver variability in fracture classification and displacement measurement
• 3D gap area (Q3DCT) method: quantifies articular displacement more reliably than 2D CT measurements

• Bone tumors near neurovascular structures (periacetabulum, sacrum, vertebral body)
• Plan resection margins and reconstruction geometry before surgery

• Plan corrective osteotomies in malunions and deformities pre-operatively

2. Patient-Specific Surgical Guides (PSG / PSI)

• Total Knee Arthroplasty (TKA): custom cutting blocks snap onto bone surfaces to guide tibial and femoral cuts - no intramedullary rods, reduced blood loss
• Total Hip Arthroplasty (THA): acetabular cup orientation guides
• Total Ankle Arthroplasty (TAA): CT-based cutting jigs provide accuracy and reproducibility; decrease operative time; especially useful in complex deformities (Campbell's, 15th Ed)
• Shoulder arthroplasty: reduces deviations between planned and post-osteotomy humeral retrotorsion and height
• Pedicle screw placement: guides for accurate screw trajectory in scoliosis and spinal fusion
• Tibial plateau fractures: 3D-printed drilling guides with drill sleeves guiding bit along predetermined screw trajectories (Assink et al., 2024, Rockwood & Green's 10th Ed)
• Both-column acetabular fractures: custom drilling guides combined with patient-specific plates
• Corrective osteotomies: malunion of proximal humerus, distal radius, high tibial osteotomy

3. Patient-Specific Implants (PSI) / Custom Prostheses

• Wide resection of bone tumors: hemipelvis, proximal/distal femur, proximal tibia, scapula, vertebral body
• Custom 3D-printed titanium implants designed to fill exact resection gap
• Integrated porous surfaces promote bone ingrowth
• Meta-analysis (Mounsef 2025, PMID 40923871): 3D printing in orthopedic oncology - enhanced functional and surgical outcomes vs conventional techniques

• Failed THA/TKA with large acetabular or tibial defects
• Custom augments and cage designs to bridge complex bone loss
• Porous trabecular-like architecture on titanium promotes osseointegration

• Patient-specific titanium implants for tibial plateau fractures (Assink 2024): designed to follow exact curvature of tibial shaft and medial plateau - restores coronal and sagittal alignment; entire process from design to surgery within 5 days
• Custom plates for both-column acetabular fractures - designed from virtual fracture reduction using contralateral mirrored hemipelvis template (Rockwood & Green's 10th Ed)
• Periarticular malunions: custom implants where conventional fixation is anatomically limited

• 3D-printed titanium interbody fusion cages: patient-specific geometry, lordosis, and porosity for fusion
• Custom vertebral body replacement implants for tumor/trauma

• Additive manufacturing used to produce highly porous titanium surfaces on femoral components
• Pore size 100-400 μm optimized for bone ingrowth - better than traditional sintered bead or fiber mesh coatings (Campbell's 15th Ed)
• More closely mimics trabecular bone architecture

4. Prosthetics and Orthotics

• Upper limb prostheses: custom 3D-printed hands and terminal devices; fraction of cost of conventional prostheses; ideal for children
• Lower limb prostheses: custom sockets, AFOs (ankle-foot orthoses) with biomechanically optimized geometry; improved fit and comfort
• Osseointegrated limb prostheses: porous 3D-printed titanium surfaces for direct skeletal attachment (bone-implant integration)
• Scoliosis braces (TLSO): custom 3D-printed braces - precise fit, improved comfort, better patient compliance vs standard thermoplastic braces
• Pediatric applications: growing children - prosthetics reprinted quickly and cheaply as anatomy changes

5. Medical Education and Training

• Anatomical models for medical students, residents, and fellows
• Pre-surgical simulation for complex cases
• Improved spatial understanding of 3D anatomy (acetabulum, wrist, foot, spine)
• Patient education: visual representation of fracture and planned surgery
• Interinstitutional collaboration: physical model shared between centers

6. Tissue Engineering and Bioprinting (Future Directions)

• 3D-printed scaffolds (HA, TCP, biopolymers) seeded with mesenchymal stem cells for bone regeneration in large segmental defects
• Osteochondral scaffolds for articular cartilage repair
• Bioprinting: cell-laden "bioinks" to fabricate living tissue constructs
• Systematic review (Lu 2026, PMID 41722744): MSCs with bone-implant scaffolds for bone regeneration shows promising preclinical results
• Goal: functional, vascularized bone and cartilage replacements

Advantages of 3D Printing in Orthopaedics

1. Patient-specific fit - implants and guides conform exactly to individual anatomy
2. Reduced operative time - less intraoperative decision-making and adjustment
3. Reduced blood loss and fluoroscopy - verified in multiple studies and meta-analyses
4. Better osseointegration - complex porous architectures impossible with conventional manufacturing
5. Improved 3D anatomical understanding - reduces interobserver variability in fracture assessment
6. Oncological reconstruction - fills defects of any geometry
7. Rapid production - patient-specific titanium implants within 5 days
8. Cost savings - reduced inventory, fewer revisions, streamlined supply chain
9. Education - inexpensive simulation and teaching models
10. Implant accuracy - 2024 cohort study showed higher implant placement accuracy, lower heterotopic ossification, and smaller pain scores vs traditional surgery

Limitations and Challenges

1. Cost - high upfront investment; metal printing especially expensive
2. Time - not practical for routine daily trauma cases
3. Regulatory hurdles - custom implants require FDA/CE approval; complex pathway
4. No soft tissue modeling - models represent only bone; ligaments, tendons, neurovascular structures absent
5. Material fatigue - SLM/EBM implants may have different fatigue characteristics vs forged implants
6. Sterilization - validated sterilization required; risk of contamination
7. Lack of long-term clinical data - especially for custom implants in non-oncological indications
8. Skilled personnel - 3D segmentation and virtual surgical planning require trained team
9. PSI in TKA - no proven benefit in functional outcomes

Recent Evidence Summary (2024-2026)

Summary Table: Applications at a Glance

Conclusion

Motor Evoked Potentials in Spine Surgery - MS Orthopaedics (20 Marks)

Introduction

• Corrective forces are applied to the spine
• Osteotomies are made
• The spinal canal is surgically invaded
• Instrumentation is placed (pedicle screws, rods)

Historical Background

• 1973: The Stagnara Wake-Up Test was first described - the original method of intraoperative spinal cord monitoring
• 1970s-80s: Somatosensory evoked potentials (SSEP) were introduced for intraoperative monitoring
• 1990s: Transcranial motor evoked potentials (tcMEP) introduced to monitor the motor pathways specifically
• 2000s onward: Multimodal IONM (MEP + SSEP + EMG) became the gold standard

The Wake-Up Test (Historical Basis)

• Decreasing/reversing anesthesia after correction of spinal deformity
• Bringing the patient to a conscious level
• Asking the patient to move both lower extremities
• Once voluntary movement is noted, anesthesia is restored and surgery completed

1. Assesses function only at the time performed - not continuous
2. Risk that uncooperative patient moves and dislodges endotracheal tube
3. Risk of falling from the table (patient prone, intubated)
4. Recall of event in 0-20% of patients (rarely viewed as unpleasant)
5. Provides false reassurance after instrumentation but prior to unexpected injury
6. Many patients with neuromuscular scoliosis cannot cooperate

Anatomical Basis of Monitoring

Types of Motor Evoked Potentials

1. Transcranial Electrical Motor Evoked Potentials (tcMEP) - MOST USED

• Electrodes placed on the scalp over the motor cortex (C1-C2 or C3-C4 positions per international 10-20 EEG system)
• Short train stimulation: 4-9 pulses at 3-7 ms inter-stimulus intervals
• Stimulus intensity: 100-400 V (voltage controlled) or 100-400 mA (current controlled)
• Electrical pulses strong enough to discharge the axon hillock of motor cortex pyramidal cells

• Needle electrodes placed in limb muscles - typically:
  • Thenar muscles (hand - C8/T1)
  • Tibialis anterior (L4-L5)
  • Gastrocnemius (S1)
  • Abductor hallucis (S1-S2)
  • Quadriceps (L2-L4)
• Recordings at 25-45 ms after stimulation

• D (Direct) waves: Compound corticospinal action potentials from direct axonal activation; conduction velocity ~50 m/s. Obtained by single transcranial electrical stimulation (intensity 80-100 mA; duration 0.5-1 ms; frequency 0.5-2 Hz). Recorded from epidural/subdural space. Do not require averaging - near real-time feedback. Alert criterion: >50% amplitude decrease.
• I (Indirect) waves: Generated by indirect transsynaptic activation of corticospinal neurons; obtained by repetitive train stimulation; recorded from muscles (tcMEP)

2. Spinal (Epidural) MEP

• Epidural electrode placed in the surgical field rostral to the operative level
• Records direct D-waves from the corticospinal tract
• Very rapid acquisition; does not require averaging
• Cannot be recorded below T12 (insufficient corticospinal fibers)
• Cannot distinguish laterality
• Cannot be used in children under 4 years (incomplete motor pathway myelination)

3. Neurogenic MEP

• Stimulate spinal cord via epidural electrodes
• Record from peripheral nerves (posterior tibial nerve, internal popliteal sciatic nerve)
• Stimulation parameters: 20-50 mA; duration 1 ms; frequency 4.1 Hz
• Monitors overall spinal cord function
• Largely replaced by tcMEP in modern practice

Somatosensory Evoked Potentials (SSEP) - Complementary Monitoring

• Ulnar nerve at wrist (cervical surgery - better coverage of lower cervical cord)
• Posterior tibial nerve at ankle (thoracolumbar surgery)
• Peroneal nerve at knee (alternative in elderly, diabetics, peripheral neuropathy)
• Rate: several per second

• Multiple sites: cortical, subcortical, spinal, peripheral
• Averaged SEP peaks (50 ms following stimulation) at standardized scalp locations
• Repeated every few minutes

• >50% decrease in amplitude, OR
• >10% increase in latency

• Monitors only sensory (posterior column) pathways
• Does NOT detect anterior spinal cord injury (anterior spinal artery syndrome)
• False negatives: postoperative paraplegia has occurred despite preserved intraoperative SSEPs
• Affected by: neural injury, volatile anesthetics, hypercarbia, hypoxia, hypotension, hypothermia

Combined MEP + SSEP - The Gold Standard

• MEPs are more sensitive to mean arterial pressure and hypotensive anesthesia than SSEPs
• Changes in MEP occur more rapidly than SSEP changes after neurologic injury (Campbell's 15th Ed)
• This is because anterior cord (motor pathway) has a single blood supply (anterior spinal artery) with less collateral circulation
• In thoracolumbar surgery, upper extremity MEP and SSEP serve as control channels to differentiate systemic/anesthetic causes from surgical causes

Key: (A) Surgical change in scoliosis repair - MEP loss in lower extremities after distraction, with recovery after distraction released. (B) Anesthetic change - BOTH upper and lower extremity responses affected (key distinguishing feature). - Miller's Anesthesia, 10th Ed

Alert Criteria (Significant Change Thresholds)

• Surgical change: affects lower extremity signals only (unilateral or bilateral) while upper extremity signals are preserved
• Anesthetic change: affects ALL signals including upper extremity controls simultaneously (see figure above)

Response to Intraoperative MEP Alert

Step 1 - Pause Surgery

• Immediately alert surgeon and anesthesiologist
• Discontinue active surgical manipulation

Step 2 - Optimize Physiology

• Increase blood pressure to normal or 20% above normal (crucial - MEP highly sensitive to MAP)
• Maintain MAP ≥ 70 mmHg (especially important during DVR and correction maneuvers)
• Check oxygen saturation and ventilation (rule out hypoxia, hypercarbia)
• Reduce or discontinue volatile anesthetic agents
• Check arterial blood gases - rule out metabolic derangement

Step 3 - Surgical Intervention

• Release distraction on the cord
• Remove or reposition suspicious implants (rod, screw)
• Check screw positions (fluoroscopy)

Step 4 - Reassess

• Allow time for signal recovery
• If signal recovers - may resume surgery cautiously
• If signal does not return to normal - consider wake-up test

Step 5 - Wake-Up Test

• Perform definitive wake-up test if signal does not recover or if there is continued clinical concern

Triggered EMG - Pedicle Screw Monitoring

• >6 mA (Campbell's) / >8 mA (Miller's): screw properly placed within intact pedicle
• <6 mA (Campbell's): alert the surgeon to possible pedicle breach
• Pedicle screws in healthy bone typically require >8 mA to elicit muscle responses (Miller's)

1. Triggered EMG (active): stimulate screw/pedicle hole; record muscle CMAP
2. Free-running (passive) EMG: continuously records all muscle responses; "neurotonic discharges" from nerve irritation appear as:
   • Brief "popcorn" discharges: benign contact with nerve
   • Response trains: significant nerve irritation
   • Neurotonic discharges: significant nerve irritation/damage

Anesthesia Considerations for MEP Monitoring

• Total Intravenous Anesthesia (TIVA) with propofol + ultrashort-acting opioid (remifentanil/fentanyl) infusion
• With EEG or BIS monitoring to minimize risk of intraoperative awareness
• No neuromuscular blockade (except transient for intubation)
• Low-dose or no volatile agents

Indications for Intraoperative MEP Monitoring in Spine Surgery

1. Scoliosis correction (AIS, neuromuscular, congenital) - primary indication; correction/distraction forces endanger cord
2. Cervical spine surgery (decompression, fusion, corpectomy)
3. Thoracic spine surgery (discectomy, tumour, deformity)
4. Spinal cord tumours (intramedullary, intradural)
5. Spinal osteotomies (PSO, VCR, Smith-Petersen)
6. Complex revision spinal surgery
7. Thoracoabdominal aortic aneurysm repair (threat to artery of Adamkiewicz)
8. Tethered spinal cord release
9. Pedicle screw insertion in deformed/dysplastic anatomy

Contraindications and Precautions

Specific Situations in Orthopaedic Spine Surgery

Scoliosis Surgery

• Standard of care (100% sensitivity, 87% specificity per Campbell's)
• Establish baseline MEP before any corrective maneuver
• Mandatory before rod derotation and direct vertebral rotation (DVR) maneuvers
• MAP ≥ 70 mmHg required during correction
• In neuromuscular scoliosis: wake-up test often not feasible; MEP/SSEP invaluable; achievable in 80% of cases including cerebral palsy patients (Campbell's)
• Congenital scoliosis: highest risk procedure for paraplegia after instrumentation - MEP monitoring most critical here

Cervical Surgery

• Ulnar nerve preferred over median for better lower cervical cord coverage
• MEP monitors C5-T1 cord and tracks impending myelopathy

Scoliosis Case Example (PMC review):

Complications of MEP Monitoring

1. Bite injuries (tongue, lip) - prevented by bite block
2. Scalp burns at electrode sites (rare)
3. Cardiac arrhythmias - rare; use low current protocols
4. Seizures - rare with modern short-train stimulation protocols
5. Patient movement during stimulation - may cause surgical mishap; careful coordination

Summary Comparison: Monitoring Modalities

Key Points for Exam

1. MEP monitors the anterior corticospinal tracts (anterior spinal artery territory) - pathway missed by SSEP alone
2. Transcranial electrical stimulation → descends corticospinal tract → recorded as CMAP in muscles at 25-45 ms
3. Alert criterion: >50% amplitude decrease in MEP
4. MEP changes occur faster than SSEP after neurologic injury
5. MEP is more sensitive to hypotension than SSEP - MAP must be maintained ≥ 70 mmHg
6. TIVA (propofol + opioid) is optimal; volatile agents and NMB must be avoided
7. Distinguishing surgical vs anesthetic change: anesthetic affects both upper and lower extremity channels
8. Triggered EMG threshold: <6 mA = pedicle breach likely (Campbell's); >8 mA = safe placement (Miller's)
9. Combined SSEP + MEP: 100% sensitivity, 87% specificity for spinal cord injury (Campbell's)
10. Wake-up test: first described 1973 (Stagnara); still used when MEP/SSEP equivocal or unavailable

Conclusion