Normal Harris Line Measurements: Cervical Radiography, CT, MRI
The Harris line (also called the Harris rule of 12s) is a geometric construct used on lateral cervical spine imaging to assess the relationship between the basion and the upper cervical spine, specifically the odontoid process and the posterior axial line. It serves as a practical screening tool for detecting occipitocervical and atlantoaxial dissociation, injuries that can be life-threatening yet subtle on initial imaging. Accurate measurement is essential in trauma evaluation to avoid missing unstable craniocervical injuries.
Normal Reference Values
| Orientation | Location | Measurement |
|---|---|---|
| Lateral | Basion – Odontoid Distance | <12 mm |
| Lateral | Basion – Posterior Axial Line Distance | <12 mm |
Clinical Significance
The Harris rule of 12s establishes two critical distances on the lateral projection: the basion-odontoid distance (BOD) and the basion-posterior axial line distance (BPAL). Both measurements should be less than 12 mm in normal adults. Exceeding either threshold strongly suggests craniocervical dissociation (CCD), an injury with high mortality that may be present even when plain films appear deceptively normal.
Elevated values may result from purely ligamentous disruption with no associated fracture, making CCD easy to overlook without systematic application of these measurements. The BPAL is particularly useful when odontoid fracture or hypoplasia obscures the BOD measurement. Notably, these thresholds apply primarily to adults; pediatric patients may have physiologically greater mobility and pseudo-subluxation that can complicate interpretation.
- Craniocervical dissociation (traumatic)
- Atlantoaxial instability (e.g., rheumatoid arthritis, Down syndrome)
- Odontoid fracture with displacement
- Occipital condyle fracture with ligamentous disruption
- Congenital os odontoideum or odontoid hypoplasia
Reference: Frymoyer JW, Wiesel SW. The Adult and Pediatric Spine. (2004).
Imaging Notes
On lateral radiography, the basion is identified as the anterior margin of the foramen magnum. The BOD is measured from the basion to the tip of the odontoid; the BPAL is measured from the basion perpendicular to a line drawn along the posterior cortex of the C2 vertebral body (the posterior axial line). A true lateral projection without rotation is mandatory, as obliquity artificially distorts both measurements.
On CT, sagittal multiplanar reconstructions in the midline provide the most accurate and reproducible measurements; CT is the preferred modality in acute trauma given superior osseous detail. MRI allows direct visualization of ligamentous integrity (tectorial membrane, alar ligaments) and is complementary when CCD is suspected clinically but CT measurements are borderline, or when spinal cord or soft-tissue injury assessment is required.