Normal McGregor’s Line Measurement: Radiography, CT, MRI
McGregor’s line is a craniometric reference drawn from the posterosuperior margin of the hard palate to the most caudal point of the occipital curve on a lateral projection. It serves as a practical landmark for evaluating the position of the odontoid process (dens) relative to the skull base. Accurate measurement is clinically essential for detecting basilar invagination, a potentially serious craniovertebral junction abnormality.
Normal Reference Values
| Orientation | Measurement |
|---|---|
| Lateral | <4.5 mm |
Clinical Significance
In a normal adult, the tip of the odontoid process should project no more than 4.5 mm above McGregor’s line on a lateral view. Protrusion beyond this threshold raises concern for basilar invagination, in which the upper cervical spine effectively telescopes into the skull base, potentially compressing the brainstem or cervical cord.
Basilar invagination may be primary (developmental) or secondary to a range of pathological processes. Clinical consequences range from asymptomatic incidental findings to life-threatening myelopathy. Key pitfalls include poor patient positioning causing chin elevation (which artificially elevates the apparent odontoid height) and misidentification of the hard palate endpoint on degraded images.
- Rheumatoid arthritis — erosive atlantoaxial disease with secondary basilar invagination
- Paget’s disease — bone softening leading to upward migration of the dens
- Osteogenesis imperfecta — congenital bone fragility with craniovertebral settling
- Chiari malformation — often coexists with or mimics craniovertebral junction anomalies
- Occipital condyle fractures or os odontoideum — traumatic or developmental variants altering alignment
Reference: M.D. TJ, M.D. BS, M.D. AW. Surgical Management of Spinal Deformities [With Access Code]. Saunders. (2009).
Imaging Notes
On lateral radiography, McGregor’s line is drawn from the posterosuperior hard palate to the most caudal occipital point; the perpendicular distance from the odontoid tip to this line is then measured. True lateral positioning is mandatory — even slight rotation introduces measurement error. On CT, sagittal multiplanar reconstructions allow precise bony landmark identification, making it the preferred modality when plain film quality is suboptimal or surgical planning is required. MRI sagittal sequences (T1 or T2) enable simultaneous assessment of neural structures at risk, though bony landmark delineation may be less sharp than CT; bone-windowed sequences or combined CT-MRI correlation is recommended for definitive measurement.