ABCDS Search Pattern
Apply systematically on every craniocervical CT:
| Letter | Evaluate |
|---|---|
| A | Alignment — lordotic curve, anterior and posterior vertebral lines, spinolaminar line |
| B | Bones — each vertebral body, posterior elements, odontoid, occipital condyles |
| C | Cartilage/disc — disc space height, facet joints |
| D | Disc spaces — intervertebral widening or narrowing |
| S | Soft tissues — prevertebral soft tissue swelling |
Key Measurements
| Measurement | Normal Threshold | Clinical Significance |
|---|---|---|
| Basion-dens interval (BDI) | ≤12 mm | Distance from basion to tip of dens; >12 mm = occipitocervical dissociation |
| Basion-axial interval (BAI) | ≤12 mm | Distance from basion to posterior axial line; >12 mm = anterior occipitocervical displacement |
| Atlantodental interval (ADI) | ≤3 mm (adult); ≤5 mm (child) | Gap between anterior arch of C1 and dens; widening = transverse atlantal ligament injury |
| Powers ratio | <1.0 | BC/OA ratio; ≥1.0 = anterior occipital-C1 dissociation |
| Prevertebral soft tissue at C2 | <7 mm | Swelling suggests anterior ligamentous or osseous injury at upper cervical levels |
| Prevertebral soft tissue at C6 | <22 mm | Swelling at subaxial levels; may indicate disc, ligament, or vertebral injury |
Powers ratio ≥1.0 indicates anterior atlantooccipital dissociation. BDI and BAI are complementary — use both, as each detects different displacement directions. Prevertebral soft tissue thresholds are adjunctive and not diagnostic in isolation.
Reporting Checklist — Craniocervical Junction
- Occipitocervical alignment: normal / abnormal
- BDI: ___ mm (normal ≤12 mm)
- BAI: ___ mm (normal ≤12 mm)
- ADI: ___ mm (normal ≤3 mm adult, ≤5 mm child)
- Powers ratio: ___ (normal <1.0)
- Prevertebral soft tissue at C2: ___ mm (normal <7 mm)
- Prevertebral soft tissue at C6: ___ mm (normal <22 mm)
- Occipital condyles: intact / fractured (side; type)
- C1 arch: intact / fractured (anterior / posterior / bilateral)
- Odontoid: intact / fractured (Anderson-D'Alonzo type)
- C2 body / Hangman fracture: present / absent
- Facet joints: normal / widened / locked / perched
- Spinous process alignment: normal / fanned (widened interspinous distance)
- MRI recommended: yes / no — indication: ___
Reference
ABCDS search pattern — standard radiologic teaching; measurements sourced from established craniocervical trauma literature.