Motor Evoked Potentials (MEPs): Orthopaedics & Neurology

What Are MEPs?

MEPs in Orthopaedic Surgery

Role in Intraoperative Neurophysiological Monitoring (IONM)

According to Bailey & Love's Short Practice of Surgery (28th Ed., p. 541): During surgery for scoliosis, continuous spinal cord monitoring uses SSEPs, MEPs, and free-run/stimulated EMG activity to minimise the risk of neurological damage. The risk of neurological injury is 0.4% (1 in 250).

Surgeries Requiring MEP Monitoring

MEP vs SSEP: Complementary Roles

Key point: SSEPs alone miss anterior cord syndrome. MEPs are essential to detect motor pathway injury independently of SSEP changes.

Warning Criteria (Intraoperative Alerts)

• Amplitude decrease ≥50% from baseline — the most widely used criterion
• Complete loss of MEP — most serious; requires immediate response
• Increased stimulation threshold (voltage needed to elicit response rises significantly)

Response to MEP Alerts

1. Inform surgeon immediately
2. Check anaesthetic depth (volatile agents suppress MEPs significantly)
3. Raise mean arterial pressure (MAP) to ≥80 mmHg
4. Reverse any recent instrumentation or distraction
5. If MEPs do not recover → consider wake-up test (Stagnara test)

Anaesthetic Considerations

• Volatile agents (sevoflurane, isoflurane) — markedly suppress MEPs; avoid or use <0.5 MAC
• Propofol + remifentanil TIVA — preferred regimen for MEP monitoring
• Neuromuscular blockers — abolish muscle MEPs; avoid or use only partial block
• Nitrous oxide — additive suppression; generally avoided

MEPs in Neurology

Diagnostic Applications

Key Measurements

MEPs in Specific Neurological Conditions

1. Multiple Sclerosis (MS)

• Most established neurological use
• Prolonged CMCT reflects demyelination of corticospinal tracts
• MEPs can be abnormal when clinical exam and MRI are equivocal
• Used to objectively quantify disability and monitor progression

2. Amyotrophic Lateral Sclerosis (ALS)

• Upper motor neuron (UMN) signs can be subtle clinically
• Prolonged CMCT or absent MEP supports UMN involvement
• Helps distinguish ALS from pure lower motor neuron (LMN) syndromes
• Part of updated Awaji criteria evidence base

3. Stroke / Cerebrovascular Disease

• MEP presence or absence after stroke is a strong predictor of motor recovery
• Absent MEP in acute stroke → poor hand/arm recovery prognosis
• Serial MEPs track cortical reorganisation during rehabilitation
• TMS mapping identifies eloquent cortex before neurosurgery

4. Cervical Myelopathy

• CMCT prolongation occurs before severe clinical symptoms
• Helps differentiate cervical myelopathy from peripheral neuropathy
• Supports surgical decision-making in borderline cases

5. Parkinson's Disease

• Short cortical silent period; reduced MEP inhibition
• Motor cortex hyperexcitability reflected in lowered threshold
• Used in research; less routine diagnostically

6. Hereditary Spastic Paraplegia (HSP)

• Markedly prolonged CMCT or absent lower limb MEPs
• Useful in mild/subclinical cases

7. Friedreich's Ataxia

• MEPs often absent in lower limbs
• Helps differentiate from other ataxias

Therapeutic TMS (rTMS)

• Depression: FDA-approved; 10 Hz rTMS over left dorsolateral prefrontal cortex at 120% motor threshold (Bailey & Love; rTMS guideline data, p. 14)
• Stroke rehabilitation: Low-frequency rTMS to contralesional hemisphere suppresses interhemispheric inhibition, facilitating ipsilesional motor recovery
• Chronic pain / neuropathic pain: Motor cortex stimulation protocols
• OCD: FDA-approved rTMS protocol

MEP Recording Technique

Stimulation (Intraoperative TES)

• Corkscrew scalp electrodes at C3/C4 or Cz positions
• Short train stimuli (5–7 pulses, interstimulus interval 2–4 ms)
• Avoids single-pulse cardiac/movement artefacts

Recording

• Needle electrodes in target muscles bilaterally
• Upper limb: abductor pollicis brevis (APB), abductor digiti minimi (ADM)
• Lower limb: tibialis anterior, abductor hallucis
• Recorded as compound muscle action potential (CMAP)

Summary: MEPs at a Glance

Motor Evoked Potentials (MEPs) in Orthopaedic Surgery

Why MEPs Are Used

According to Bailey & Love's Short Practice of Surgery (28th Ed., p. 541): Continuous spinal cord monitoring using SSEPs, MEPs, and free-run/stimulated EMG is standard practice during corrective spinal surgery. The risk of neurological injury is 0.4% (1 in 250) — a risk MEP monitoring aims to prevent or detect early.

Anatomical Basis

• Mechanical compression by instrumentation
• Anterior spinal artery ischaemia
• Distraction forces during deformity correction

Surgeries Where MEP Monitoring Is Used

MEP Technique in the Operating Theatre

Stimulation

• Transcranial electrical stimulation (TES) via corkscrew scalp electrodes
• Electrode positions: C1/C2 or C3/C4 (international 10-20 system) — contralateral placement targets opposite limbs
• Short train stimuli: 4–7 pulses, interstimulus interval 2–4 ms, high voltage (up to 200–400 V)
• Short trains allow the stimulus to summate and traverse injured/partially blocked cord segments

Recording

• Surface or intramuscular needle electrodes in target muscles
• Recorded as compound muscle action potential (CMAP)

MEP vs SSEP: Critical Comparison

Alert Criteria (Warning Signs)

The 50% amplitude drop rule is the universally accepted alert criterion. Complete loss mandates immediate surgical action.

Response Protocol to MEP Alerts

1. Notify the surgeon immediately — pause instrumentation
2. Check anaesthetic depth — volatile agents, bolus propofol, or recent neuromuscular blockade can cause false positives
3. Optimise haemodynamics — raise MAP to ≥80 mmHg (or >10 mmHg above baseline) to improve cord perfusion
4. Correct mechanical cause — remove distraction, back off implant, decompress if applicable
5. Irrigate wound with warm saline (cord temperature matters)
6. Reassess MEPs after each corrective step
7. If MEPs do not recover → perform wake-up test (Stagnara test) to clinically confirm motor function

Anaesthetic Management for MEP Monitoring

The Wake-Up Test (Stagnara Test)

• Anaesthesia is lightened until the patient can follow commands
• Patient asked to move hands, then feet
• Positive response (movement preserved) = reassuring
• Limitations: patient cooperation required, one-time snapshot, awareness risk, cannot be repeated frequently

Specific Orthopaedic Contexts

Scoliosis Surgery

• Highest-risk spinal operation for neurological injury
• Distraction forces across multiple levels can compromise anterior cord perfusion
• MEPs + SSEPs + EMG used together
• Alert during rod rotation or final compression/distraction manoeuvres

Cervical Spine Surgery

• Risk of cord injury from retraction, instability, or direct compression
• Upper limb MEPs (APB, deltoid) are particularly important
• Myelopathic patients may have reduced baseline MEP amplitudes — establishing a good baseline before incision is critical

Pedicle Screw Placement

• Stimulated EMG (not MEP per se) is used to detect medial wall breach
• A screw stimulation threshold <6–8 mA suggests medial breach into the canal
• Free-run EMG detects nerve root irritation in real time

Thoracic Surgery / Aortic Cross-Clamping

• Anterior spinal artery watershed zone (T4–T9) is most vulnerable
• MEP loss during aortic cross-clamping → reposition clamp, insert intercostal reimplantation, raise MAP

Limitations of MEP Monitoring

Summary

Navigation in Orthopaedic Surgery

Definition

Core Components of a Navigation System

How It Works: The Workflow

1. Image Acquisition

• Preoperative CT/MRI → imported into workstation; virtual 3D model constructed
• Intraoperative fluoroscopy (most common) → C-arm images taken after patient positioning, automatically calibrated
• Intraoperative 3D CT (O-arm, Vario 3D) → highest accuracy; spin CT acquired on table, registered automatically

According to Rockwood & Green's Fractures in Adults (10th Ed., p. 232): "Computer-assisted surgical navigation utilizing fluoroscopy is an emerging technology applied to percutaneous pelvic and acetabular screw placement. Fluoroscopy is commonly used for surgical navigation because of its flexibility, convenience, lower radiation exposure, and lower cost."

2. Registration

• The process of matching the virtual image model to the actual patient anatomy in real space
• Methods:
  • Point-to-point registration: surgeon touches anatomical landmarks with a tracked probe
  • Surface matching: probe swept over bone surface; software matches contour to CT model
  • Image-based (automatic): O-arm/C-arm auto-registers after intraoperative scan — no manual step required

3. Tracking

• A reference frame (star-shaped array with retroreflective spheres) is rigidly fixed to the patient's bone (e.g., spinous process pin, iliac crest pin, or tibial pin)
• Instruments fitted with tracking arrays
• The camera continuously tracks both patient and instrument positions
• Real-time position displayed on workstation in 2D/3D reconstructions

4. Guided Surgery

• Surgeon views instrument trajectory on screen relative to anatomy in axial, sagittal, coronal, and 3D views
• Allows precise planning and execution of cuts, screw trajectories, implant placement

Navigation Technologies: Types

Clinical Applications

1. Total Knee Arthroplasty (TKA)

• Imageless CAS: sensors capture knee kinematics (range of motion, femoral/tibial axes) without imaging; software guides cutting blocks
• CT-based / robotic: preoperative plan mapped onto patient in theatre

Intraoperative CAS-TKA showing navigation trackers rigidly fixed to femur and tibia via bicortical pins (Stryker-Leibinger system)

Schwartz's Principles of Surgery (11th Ed., p. 1937): "Computer navigation in total joint arthroplasty has been shown to minimize outliers in alignment, but there has been no proven benefit in survival or function secondary to computer-navigated or robotic-assisted joint replacement."

Miller's Review of Orthopaedics (9th Ed.): "Evidence supports not using intraoperative navigation because there is no difference in outcomes or complications" (conventional TKA instrumentation).

Robotic targeting arm for computer-assisted total knee replacement — Schwartz's Principles of Surgery, 11th Ed.

2. Total Hip Arthroplasty (THA)

• Cup malposition is the leading cause of dislocation and impingement
• Imageless CAS or robot-assisted (e.g., MAKO, Stryker) guides acetabular reaming and cup insertion
• Leg length and offset can be measured intraoperatively with navigation

3. Pedicle Screw Placement (Spine)

Posterior cervicothoracic navigation setup: imaging reference frame fixed to spinous processes, replaced by optical tracker frame for real-time screw guidance (Surgivisio® system)

Navigated dilator and Jamshidi needle for percutaneous pedicle screw placement in MIS spine surgery — reflective tracking spheres visible on instrument arrays

Rockwood & Green's (10th Ed., p. 895): "Intraoperative CT systems such as the O-arm (Medtronic), Vario 3D by Ziehm, or Surgivisio (eCential) can be coupled with navigation systems and may improve reduction and fixation in trauma surgery. Intraoperative CT coupled to robotics is currently used in spine and arthroplasty surgery."

4. Fracture Fixation & Trauma Surgery

• Percutaneous pelvic and acetabular screw placement (iliosacral, supra-acetabular, column screws)
• Femoral nail antegrade/retrograde — guidewire placement
• Tibial plateau and pilon fractures
• Distal radius fixation

Medtronic StealthStation reference frame fixed to iliac crest via pin — used for navigated pelvic ring fracture fixation

Rockwood & Green's (10th Ed.): Computer-assisted fluoroscopy navigation has "been applied to percutaneous pelvic and acetabular screw placement with expected continued progress."

5. Complex Spinal Deformity

• Navigated osteotomies (e.g., posterior vertebral column resection) guided in real time
• Multiplanar CT reconstructions allow precise bone removal and implant trajectory planning

Navigation during posterior vertebral column resection (p-VCR) for severe kyphosis in achondroplasia — multiplanar CT views with real-time instrument tracking displayed on navigation monitor

Robotic-Assisted Navigation

• Passive: navigation display only, surgeon controls instrument freely
• Semi-active (haptic): robotic arm creates a virtual boundary; surgeon cannot cut outside planned zone
• Active: robot moves autonomously to planned position, surgeon supervises

Miller's Anesthesia (10th Ed.): "Many robotic spine surgery platforms have been introduced, but their acceptance has [been variable] — robots provide intraoperative image-guided navigation combined with robotic assistance for 3D reconstruction for screw placement."

Advantages of Navigation

Limitations and Disadvantages

Patient-Specific Instrumentation (PSI) — A Related Concept

Miller's Review of Orthopaedics: "Evidence supports not using patient-specific instrumentation compared to conventional instrumentation for TKA because there is no difference in pain or functional outcomes."

Summary Table: Navigation by Application