Craniocervical Junction Measurements (ADI / PADI / Powers / BDI / Basilar Invagination) — free guideline decision tool

ADI, PADI, Powers ratio, basion-dens interval and dens position above McGregor's line for atlantoaxial instability, atlanto-occipital dislocation and basilar invagination. Includes how to measure and what each value means. Instant, browser-side.

ADI (adult) ≤ 3 mm: normal ADI (adult) 3–5 mm: transverse ligament injury / atlantoaxial instability likely ADI (adult) > 5 mm: transverse and alar ligament injury, marked instability (RA > 9 mm: consider surgery) ADI (child) ≤ 5 mm: normal in children ADI (child) > 5 mm: abnormal, atlantoaxial instability PADI < 14 mm: high cord-compression risk (surgical threshold in RA) PADI ≥ 14 mm: adequate space for the cord Powers ratio < 1: normal Powers ratio ≥ 1: anterior atlanto-occipital dislocation BDI (radiograph) ≤ 12 mm: normal (Harris rule of 12) BDI (radiograph) > 12 mm: possible atlanto-occipital dislocation BDI (CT) ≤ 8.5 mm: normal on CT BDI (CT) > 8.5 mm: abnormal, possible atlanto-occipital dislocation Dens tip above McGregor's line ≤ 4.5 mm: normal Dens tip above McGregor's line > 4.5 mm: basilar invagination

Craniocervical junction imaging: lateral cervical radiographs and sagittal CT — trauma, rheumatoid and congenital craniocervical assessment.

Lateral cervical radiograph or sagittal CT. Each value is graded against published reference ranges. Measurement technique and positioning change the numbers — compare like with like on follow-up. Part of the YuanDoctor MSK measurement topic.

For licensed clinicians and clinical researchers. Interpret results with history, investigations and local protocols; not a diagnosis or prescription, and not a substitute for multidisciplinary decision-making or local guidelines.

Calculation runs locally in the browser; inputs and results are not uploaded. For licensed clinicians only; interpret with history, investigations and local protocols — not a diagnosis or prescription.

Inputs: Population Adult, BDI modality Radiograph, Powers: basion → posterior C1 arch 30 mm, Powers: opisthion → anterior C1 arch 38 mm, ADI (adult) 2.5 mm, PADI 16 mm, BDI (radiograph) 8 mm, Dens tip above McGregor's line 3 mm → Result: ADI (adult) 2.5 mm（ADI (adult): 2.5 mm → ≤ 3 mm: normal, PADI: 16 mm → ≥ 14 mm: adequate space for the cord, Powers ratio: 0.79 → < 1: normal） Inputs: Population Child (≤ ~8 y), BDI modality CT, Powers: basion → posterior C1 arch 30 mm, Powers: opisthion → anterior C1 arch 38 mm, ADI (child) 4 mm, PADI 16 mm, BDI (CT) 6 mm, Dens tip above McGregor's line 3 mm → Result: ADI (child) 4 mm（ADI (child): 4 mm → ≤ 5 mm: normal in children, PADI: 16 mm → ≥ 14 mm: adequate space for the cord, Powers ratio: 0.79 → < 1: normal）