Safety Measures in Scoliosis Surgery

Introduction

I. PREOPERATIVE SAFETY MEASURES

1. Patient Evaluation

• Thorough clinical evaluation including pulmonary function tests (PFTs), cardiac assessment, and nutritional status.
• Identification of high-risk patients: neuromuscular scoliosis patients have 3x greater blood loss than AIS patients.
• Baseline neurological examination documented.
• MRI spine to detect unrecognized cord pathology - tethered cord, Arnold-Chiari malformation, diastematomyelia - which if missed, dramatically increase risk of neurological injury during correction.
• ASA classification and co-morbidity optimization.

2. Preoperative Neurological Baseline

• Any pre-existing neurological deficit must be documented and investigated before surgery. Correcting a scoliosis with an undiagnosed intrinsic cord lesion is a major cause of intraoperative neurological catastrophe.

3. Blood Conservation Planning

• Preoperative autologous donation - patient donates their own blood 2-4 weeks before surgery for re-transfusion intraoperatively.
• Optimization of hemoglobin - target Hb >12 g/dL preoperatively; iron supplementation and erythropoietin if indicated.
• Planning for intraoperative cell salvage (cell saver).
• Informed consent discussing risk of neurological injury, infection, implant failure, and need for blood transfusion.

4. Antibiotic Prophylaxis

• The most important factor in infection prevention is timely dosing of prophylactic antibiotics within 1 hour of skin incision.
• First-generation cephalosporin (cefazolin) is standard. Vancomycin is added for MRSA-high-risk patients.

II. INTRAOPERATIVE SAFETY MEASURES

A. Anesthesia-Related Safety

1. Patient Positioning

• Prone position is standard for posterior scoliosis surgery. This must be performed carefully to:
  • Avoid abdominal compression (increases epidural venous pressure and blood loss)
  • Prevent pressure injuries to eyes, face, genitalia, and anterior superior iliac spine
  • Maintain normovolemia - abdominal compression reduces venous return causing cardiovascular collapse
  • Use specialized frames (Jackson frame, Wilson frame, or Montreal mattress) that free the abdomen
• Padded chest bolsters to allow diaphragmatic excursion
• Arms are tucked or abducted no more than 90 degrees to prevent brachial plexus injury

2. Controlled Hypotension

• Mean arterial pressure (MAP) is maintained at 50-65 mmHg intraoperatively to reduce blood loss from epidural vessels.
• Caution: Excessive hypotension is a major contributory factor to spinal cord ischemia, especially when combined with distraction forces. MAP should not be allowed to drop below 50 mmHg.
• After any neuromonitoring alert, blood pressure is immediately raised to maximize spinal cord perfusion.

3. Anesthetic Technique for Neuromonitoring

• When both SSEP and MEP monitoring are used, the anesthetic regimen must be compatible:
  • Avoid volatile anesthetics in high doses - they suppress SSEPs and MEPs in a dose-dependent manner
  • TIVA (Total Intravenous Anesthesia) with propofol + remifentanil is preferred - least interference with evoked potentials
  • Avoid neuromuscular blocking agents (muscle relaxants) during MEP monitoring - they abolish myogenic MEPs
  • Low-dose volatile agents (< 0.5 MAC) may be acceptable for SSEP alone
  • Nitrous oxide and opioids can be used as adjuncts

B. Intraoperative Neurological Monitoring (IONM) - The Cornerstone of Safety

1. Wake-Up Test (Stagnara Wake-Up Test)

• Principle: The patient is lightened from anesthesia after spinal instrumentation and asked to move hands and feet voluntarily, confirming intact motor function.
• Method: Anesthesia is maintained with volatile agent, N2O, and opioids. The agent is lightened until the patient responds to command. The patient is asked to move hands (tests thoracic cord) and feet (confirms absence of paraplegia).
• If hands move but feet do not: The rod correction is released one notch and the test is repeated.
• Advantages: Simple, inexpensive, provides definitive motor function assessment, no specialized equipment needed.
• Disadvantages:
  • Only a single time-point assessment; does not provide continuous monitoring
  • Risk of patient dislodging endotracheal tube, moving unexpectedly, or falling from table
  • Patient recall (0-20%), though rarely distressing
  • Cannot detect delayed neurological injury
  • Not feasible in very young children or cognitively impaired patients
• Role today: Reserved for cases where SSEP/MEP monitoring is not available, equivocal, or has failed. Should be discussed with patient preoperatively.

2. Somatosensory Evoked Potentials (SSEP)

• Principle: Peripheral nerve (typically posterior tibial or common peroneal) is stimulated; the ascending signal through the dorsal columns (posterior spinal artery territory) is recorded from scalp electrodes.
• Monitoring criterion: A >50% decrease in amplitude or >10% increase in latency constitutes a significant alert.
• Advantages: Continuous, non-invasive, correlates with dorsal column and posterior spinal artery integrity.
• Limitations:
  • Monitors only sensory (dorsal column) pathways; motor tracts are NOT directly monitored
  • The anterior spinal artery (ASA) supplies motor tracts - so pure motor deficits can occur with intact SSEPs (false negatives)
  • False positives are common (technical artifacts, anesthetic depth changes, hypothermia, hypotension, hypercarbia)
  • Overall false-negative rate (intact SSEPs with postoperative deficit) is <1%

3. Motor Evoked Potentials (MEP) / Transcranial Motor Evoked Potentials (tcMEP)

• Principle: High-voltage transcranial electrical stimulation activates corticospinal tracts; myogenic compound muscle action potentials (CMAPs) are recorded from limb muscles.
• Monitors the anterior spinal artery territory - directly assesses the motor tracts.
• Significant alert: >50% amplitude decrease or complete loss of MEP signals.
• Advantage: Detects anterior cord ischemia which SSEP would miss.
• Limitation: Requires absence of neuromuscular blockade; more technically demanding.
• Gold standard: Combined SSEP + MEP monitoring - the American Society of Neurophysiologic Monitoring consensus states this is well established for preventing injury to sensory and motor tracts. This combination reduces false negatives significantly.

4. Electromyography (EMG) - Pedicle Screw Safety Testing

• Triggered EMG (t-EMG): The pedicle screw pilot hole or shank is stimulated with a ball-tip probe with increasing current.
• If current threshold to elicit a myotomal CMAP is low (<10 mA), it suggests a pedicle wall breach with the screw too close to a nerve root, especially in lumbar spine.
• Helps detect malpositioned pedicle screws before neurological damage occurs.
• Limitation: Thresholds vary between cervical, thoracic, and lumbar levels, and between healthy vs diseased roots. In thoracic spine, a medially malplaced screw contacts the corticospinal tract, not a nerve root.

5. Response to Neuromonitoring Alerts

1. Inform surgeon immediately
2. Release corrective forces / reduce distraction on the rod
3. Raise mean arterial pressure (increase vasopressor/fluid) to improve spinal cord perfusion
4. Check anesthetic factors: ensure no sudden deepening of volatile agent, no hypotension, normothermia, normocapnia
5. Avoid hypothermia and anemia
6. Perform wake-up test if signals do not recover
7. If neurological deficit confirmed: implant removal should be considered

C. Blood Conservation and Hemostasis

1. Intraoperative Cell Salvage (Cell Saver): Blood lost is collected, washed, and re-infused. Highly effective in multilevel fusions.
2. Controlled Hypotension: As described above.
3. Tranexamic Acid (TXA): An antifibrinolytic agent that reduces surgical bleeding and transfusion requirements in scoliosis surgery without increased incidence of PE, DVT, or myocardial infarction. Given as an IV infusion perioperatively.
4. Proper Prone Positioning: Abdomen-free position reduces intra-abdominal pressure, thereby reducing epidural venous engorgement and operative blood loss.
5. Maintaining Normovolemia: Sudden volume depletion combined with venous air embolism (a risk in prone surgery) can cause cardiovascular collapse.
6. Coagulation Monitoring: Significant deviation from baseline prothrombin time or aPTT predicts bleeding. Coagulation factors and/or platelets should be replaced accordingly.

D. Implant-Related Safety (Pedicle Screw Safety)

• Pedicle screws are the most common component of spinal instrumentation. Malpositioned screws are among the most common causes of reoperation.
• Fluoroscopic guidance / O-arm imaging intraoperatively confirms screw position.
• Triggered EMG testing of each screw as described above.
• Bilateral symmetry of correction is checked before rod locking to ensure no sudden asymmetrical distraction.
• Ponte osteotomies have been associated with increased rates of neuromonitoring alerts - extra vigilance required.

E. Infection Prevention

1. Timely antibiotic prophylaxis (within 1 hour of incision).
2. Vancomycin powder applied to the wound before closure - reduces incidence of infection, especially in neuromuscular patients.
3. Dilute betadine (povidone-iodine) irrigation of wound at end of procedure.
4. Obesity is the single most important predictor of infection in AIS patients (sevenfold increased risk).
5. Strict aseptic technique throughout.

III. POSTOPERATIVE SAFETY MEASURES

1. Close neurological monitoring in recovery room and ward - any new deficit should trigger immediate imaging and return to OT.
2. Maintenance of MAP >70 mmHg postoperatively to prevent spinal cord ischemia.
3. Early mobilization to reduce ileus, atelectasis, and DVT risk.
4. Temperature elevation >102°F beyond postoperative day 4 should raise suspicion of deep wound infection; prompt wound aspiration and debridement indicated.
5. Ileus management: multimodal approach - reduce narcotics, early feeding, early mobilization.
6. Superior Mesenteric Artery (SMA) syndrome - rare complication in thin habitus with significant spinal lengthening; suspect if vomiting persists postoperatively; managed conservatively with nasogastric decompression and positional maneuvers.
7. Pulmonary complications (atelectasis) are reduced with early patient rehabilitation and incentive spirometry.

IV. SPECIFIC SAFETY MEASURES BY COMPLICATION TYPE

Conclusion

• Campbell's Operative Orthopaedics 15th Ed 2026 - Complications of Posterior Scoliosis Surgery
• Miller's Anesthesia, 10th Ed - Scoliosis, Adult Correction, and Spinal Cord Surgery; Neurophysiologic Monitoring
• Barash, Cullen, and Stoelting's Clinical Anesthesia, 9th Ed - Spinal Cord Monitoring, Blood Conservation in Spine Surgery