Normal Boogaard’s Angle Size on X-ray, CT & MRI
Boogaard’s angle is a craniometric measurement drawn on lateral projections of the skull base, defined by the angle between the plane of the clivus and the plane of the anterior cranial fossa floor (specifically the line from the nasion to the tuberculum sellae and the line along the clivus to the basion). It serves as an important indicator of the normal curvature and inclination of the clivus relative to the skull base. Accurate measurement is clinically relevant because abnormal values may indicate structural deformities at the craniocervical junction, including basilar invagination and platybasia.
Normal Reference Values
| Orientation | Measurement |
|---|---|
| Lateral | <135¡ |
Clinical Significance
A normal Boogaard’s angle is less than 135° on lateral imaging. When this angle equals or exceeds 135°, the clivus is considered abnormally flattened — a condition termed platybasia. Platybasia may occur in isolation or in conjunction with basilar invagination, where upward migration of the odontoid process into the foramen magnum can compress the brainstem and upper cervical cord.
It is important to distinguish Boogaard’s angle from McGregor’s line and Chamberlain’s line, which assess vertical migration of the odontoid. Boogaard’s angle specifically evaluates clival inclination; a flattened clivus may not always produce symptoms unless accompanied by neural compression. Clinical correlation is therefore essential, as isolated platybasia can be an incidental finding.
- Basilar invagination — often coexists with platybasia; may cause myelopathy or lower cranial nerve palsies
- Chiari malformation Type I — frequently associated with craniocervical junction abnormalities
- Paget’s disease — softening of skull base bone leading to flattening of the clivus
- Osteogenesis imperfecta — bone fragility predisposing to skull base deformity
- Achondroplasia — developmental flattening of the skull base
Reference: Yochum TR, Rowe LJ. Essentials of Skeletal Radiology. (2005).
Imaging Notes
On lateral skull radiography, Boogaard’s angle is measured by drawing one line from the nasion to the dorsum sellae and a second line from the dorsum sellae to the basion along the clivus; the angle between them should be less than 135°. Adequate lateral positioning without rotation is critical, as head tilt will falsely alter the apparent angle.
On CT (sagittal multiplanar reconstructions) and MRI (T1-weighted sagittal sequences), the bony landmarks — nasion, dorsum sellae, and basion — are identified in the midline sagittal plane. CT offers superior cortical bone detail for landmark identification, while MRI provides simultaneous assessment of neural structures at the craniocervical junction, making it the preferred modality when symptomatic compression is suspected.