Normal Cervical Basal Angle Size on X-ray, CT & MRI
The basal angle (also known as the Martin angle) is formed at the junction of a line drawn from the nasion to the center of the sella turcica and a second line from the sella to the anterior margin of the foramen magnum (basion). It serves as a key indicator of the geometric relationship between the skull base and the craniovertebral junction. Accurate measurement is essential in evaluating conditions such as platybasia and basilar invagination, which can compress the cervicomedullary junction and produce neurological deficits.
Normal Reference Values
| Orientation | Measurement |
|---|---|
| Lateral | 125-143¡ |
Clinical Significance
The normal basal angle measures between 125° and 143° on lateral projection. An angle exceeding 143° is consistent with platybasia, indicating a flattened skull base. While platybasia alone may be an incidental finding, it frequently coexists with basilar invagination, Chiari malformation, and other craniovertebral junction anomalies that carry significant neurological risk.
A reduced basal angle (<125°) suggests an abnormally acute skull base angulation, which is less common but may be seen in certain skeletal dysplasias. Clinicians should correlate basal angle measurements with other craniovertebral parameters (e.g., Chamberlain’s line, McGregor’s line) to avoid isolated over-interpretation. Measurement can be affected by patient positioning and image angulation, particularly on plain radiographs.
- Platybasia (basal angle >143°)
- Basilar invagination
- Chiari malformation type I
- Achondroplasia and other skeletal dysplasias
- Paget’s disease of the skull base
Reference: Menkes JH, Sarnat HB, Maria BL. Child Neurology, 7e. Lippincott Williams & Wilkins. (2006).
Imaging Notes
On lateral skull radiographs, the basal angle is measured by drawing a line from the nasion to the sella turcica and a second line from the sella to the basion; the angle between these two lines is recorded. True lateral positioning is critical — even minor rotation introduces significant measurement error. On CT, a sagittal midline reformation provides optimal anatomical landmarks, allowing precise identification of the nasion, sella floor, and basion. MRI sagittal T1-weighted sequences offer excellent soft-tissue contrast and clearly delineate the sella and clivus without ionizing radiation, making it the preferred modality in pediatric patients when repeated assessment is needed.