Normal Digastric Line Cervical Measurements on X-ray, CT, MRI
The digastric line is a craniometric reference line drawn between the bilateral digastric grooves on the mastoid processes, used to assess craniovertebral junction alignment. It serves as a key landmark for detecting superior migration of the odontoid process and atlanto-occipital instability. Accurate measurement is essential in evaluating trauma, rheumatoid arthritis, and congenital craniovertebral anomalies.
Normal Reference Values
| Orientation | Location | Measurement |
|---|---|---|
| Odontoid | Distance With Atlanto-Occipital Articulation | >4 mm |
| Odontoid | Distance With Odontoid Process | >1 mm |
Clinical Significance
The digastric line provides two critical measurements at the craniovertebral junction. A distance of >4 mm between the digastric line and the atlanto-occipital articulation indicates superior displacement and raises concern for atlanto-occipital dissociation or basilar invagination. A distance of >1 mm between the digastric line and the odontoid process tip similarly suggests cranial settling or basilar invagination, commonly encountered in rheumatoid arthritis with pannus formation eroding the transverse ligament and dens.
These thresholds must be interpreted alongside complementary craniovertebral measurements such as the McGregor line, McRae line, and Chamberlain line. Pitfalls include patient positioning artifacts, congenital variants of the mastoid anatomy, and poor landmark identification on low-quality radiographs.
- Basilar invagination (primary or secondary)
- Atlanto-occipital dissociation following trauma
- Cranial settling in rheumatoid arthritis
- Platybasia and achondroplasia
- Congenital odontoid hypoplasia or os odontoideum
Reference: Yochum TR, Rowe LJ. Essentials of Skeletal Radiology. (2005).
Imaging Notes
On radiography, the digastric line is drawn on an anteroposterior (AP) open-mouth or Towne projection by connecting the two digastric notches. Perpendicular distances to the atlanto-occipital joint and odontoid tip are then measured. Optimal positioning is critical, as rotation or tilt significantly alters measured distances. CT with multiplanar reformats offers superior anatomic delineation of the digastric notches and odontoid, allowing precise coronal plane measurement even in trauma patients. MRI is preferred when soft-tissue pathology such as pannus, ligamentous injury, or cord compression is suspected, and coronal sequences enable equivalent landmark identification to CT.