Normal Klaus Index Size on X-ray, CT & MRI
The Klaus Index is a linear measurement used to assess the vertical position of the odontoid process (dens) relative to the posterior fossa on lateral cervical imaging. It is drawn from the tip of the dens perpendicular to the tuberculum-opisthion line (also called the Klaus line). Accurate measurement is clinically important for identifying basilar invagination, a potentially life-threatening craniovertebral junction anomaly that can compress the brainstem and upper cervical cord.
Normal Reference Values
| Orientation | Measurement |
|---|---|
| Lateral | >30 mm |
Clinical Significance
A Klaus Index of >30 mm is considered normal. Values at or below 30 mm suggest superior migration of the dens into the posterior fossa, consistent with basilar invagination. This condition may be congenital or acquired and can cause progressive myelopathy, lower cranial nerve deficits, or sudden neurological deterioration with minor trauma.
Common pitfalls include suboptimal lateral positioning, which can distort the reference line, and failure to account for developmental variants such as os odontoideum or atlantoaxial instability, which may independently alter dens position. The Klaus Index should always be interpreted alongside other craniovertebral measurements (e.g., McGregor’s line, Chamberlain’s line) for comprehensive assessment.
- Basilar invagination (primary or secondary)
- Platybasia associated with osteogenesis imperfecta or Paget disease
- Rheumatoid arthritis with cranial settling
- Achondroplasia with foramen magnum narrowing
- Down syndrome (trisomy 21) with atlantoaxial instability
Reference: Yochum TR, Rowe LJ. Essentials of Skeletal Radiology. (2005).
Imaging Notes
On lateral radiography, the Klaus Index is measured from the tip of the dens to the tuberculum-opisthion line in the true lateral projection. Patient positioning is critical — rotation or flexion/extension can artifactually alter the measurement. A dedicated neutral lateral cervical or skull radiograph with precise centering at the craniovertebral junction is preferred.
On CT and MRI, mid-sagittal reformats provide the most accurate and reproducible assessment of the Klaus Index. CT offers superior bony landmark delineation, making it the modality of choice when plain films are equivocal. MRI adds the ability to directly visualize neural compression at the craniocervical junction, making it essential when myelopathy is suspected alongside an abnormal Klaus Index.