SLICS vs. AO Spine Subaxial Cervical Spine Injury Classification
The Subaxial Injury Classification System (SLICS) is a treatment-oriented scoring system that guides surgical decision-making through a severity score, while the AO Spine classification is primarily a morphological descriptive system based on CT imaging that categorizes injury patterns but provides less direct treatment guidance.
Core Structural Differences
SLICS System Components
SLICS uses three weighted major categories that are summed to create a severity score:
- Injury Morphology (compression, burst, distraction, rotation/translation) 1, 2, 3
- Discoligamentous Complex (DLC) Integrity (intact, indeterminate, or disrupted - assigned 2 points if disrupted) 2, 3
- Neurological Status (intact, nerve root injury, incomplete cord injury [3 points], complete cord injury [2 points]) 2, 3, 4
The total SLICS score directly determines treatment: ≥5 points indicates surgical intervention, <4 points suggests conservative management 1, 2, 5
AO Spine Classification Structure
AO Spine uses a hierarchical morphological system based on CT imaging with ascending severity:
- Type A injuries: Compression injuries with anterior element injury and preserved posterior ligamentous complex 6
- Type B injuries: Tension band failure (B1 = transosseous posterior failure; B2 = posterior failure with vertebral body fracture; B3 = hyperextension injuries) 6
- Type C injuries: Complete disruption of all elements with displacement/dislocation 6
The AO system adds a neurological grading component (N0-N4) and case-specific modifiers but does not provide a treatment algorithm 6
Clinical Application and Reliability
SLICS Advantages
SLICS demonstrates superior clinical utility for treatment decisions:
- Excellent reliability with intraclass correlation coefficients ranging 0.49-0.90 for different components 2, 4
- 93.3% agreement with treatment recommendations in validation studies 4
- Prospective validation showed 100% concordance for surgical cases (score >4) and 76.9% for conservative cases (score <4) 5
- Better therapeutic correlation (76.9-100% concordance) compared to AO Spine (28.6-66.7% concordance) for treatment decisions 7
AO Spine Advantages
AO Spine provides superior morphological description:
- Better anatomical correlation for describing fracture patterns on CT imaging 7
- Good to excellent inter- and intraobserver reliability for the main injury types (A, B, C) 6
- Comprehensive yet simple scheme with greater reproducibility than older classification systems 6
Key Practical Distinctions
SLICS is treatment-prescriptive while AO Spine is descriptive:
- SLICS incorporates neurological status as a weighted component affecting the severity score, while AO Spine adds it as a separate grading system (N0-N4) without integration into treatment algorithms 2, 3, 4
- SLICS explicitly addresses discoligamentous integrity as a critical decision point (2 points if disrupted), requiring MRI assessment when suspected 1, 3
- AO Spine focuses on CT-based morphological patterns and does not provide specific surgical approach recommendations (anterior vs. posterior) 2, 7
Clinical Pitfalls and Caveats
Common misapplications to avoid:
- Do not use AO Spine alone for treatment decisions—it lacks validated treatment thresholds unlike SLICS 2, 7
- SLICS requires MRI evaluation when DLC injury is suspected, as this significantly impacts the severity score 1, 3
- A patient with SLICS score of 4 represents a gray zone requiring individualized assessment (may include incomplete spinal cord injury, compression injuries with incomplete deficits, or burst fractures with complete deficits) 8
- AO Spine's morphological detail is superior for research and communication but does not translate directly to surgical indications 7
Both systems have Level I evidence for reliability in grading cervical spine injuries, but SLICS has superior validated treatment guidance 1, 2