Normal Atlanto-Temporomandibular Index Size on Radiography, CT, MRI
The atlanto-temporomandibular index (ATMI) is a craniocervical measurement used to evaluate the spatial relationship at the craniovertebral junction, particularly around the atlas (C1) and surrounding structures. Accurate measurement is clinically important because abnormalities at this level can indicate ligamentous laxity, congenital anomalies, or instability syndromes that carry significant neurological risk. This index is applied across multiple imaging modalities, including radiography, CT, and MRI, to guide both diagnosis and surgical planning.
Normal Reference Values
| Orientation | Measurement |
|---|---|
| Lateral | >22 mm |
Clinical Significance
A normal atlanto-temporomandibular index measures greater than 22 mm on lateral projection. Values falling below this threshold may indicate craniocervical instability or pathological narrowing at the craniovertebral junction, raising concern for compromise of adjacent neural structures including the cervicomedullary junction and vertebral arteries.
Reduced ATMI values are associated with several conditions affecting craniovertebral alignment. Clinicians should be aware that this measurement alone is not diagnostic and must be interpreted in the context of clinical presentation, other craniometric indices, and dynamic imaging findings.
- Atlantoaxial instability — trauma, rheumatoid arthritis, or Down syndrome
- Basilar invagination / cranial settling — congenital or inflammatory
- Chiari malformation with associated craniocervical anomalies
- Odontoid fractures or os odontoideum altering C1-C2 geometry
- Ligamentous laxity syndromes — Ehlers-Danlos or Marfan syndrome
Reference: Freyschmidt J, Khler A, Brossmann J et al. Freyschmidt’s “Koehler.” George Thieme Verlag. (2003).
Imaging Notes
On lateral radiography, the ATMI is measured in the true lateral projection with the patient in neutral position and the mouth closed; strict patient positioning minimizes rotational artifact that can falsely reduce measured values. CT offers multiplanar reformatting, allowing precise identification of bony landmarks at the craniovertebral junction even in the setting of trauma or complex anatomy; thin-slice axial acquisition with sagittal reconstruction is preferred. MRI is particularly valuable for assessing soft-tissue and ligamentous contributions to craniocervical stability, and sagittal T1-weighted sequences provide optimal landmark delineation for ATMI calculation. Dynamic or flexion-extension imaging may be warranted when static measurements are equivocal.