The Rule of Spence is a radiographic method used to assess the integrity of the transverse ligament during open-mouth (odontoid) radiography. In essence, it helps determine if there is excessive atlanto-axial translation, indicating a possible transverse ligament rupture or insufficiency.
Understanding the Rule of Spence
The rule specifically measures the atlantodental interval (ADI) on an open-mouth odontoid radiograph. The ADI is the space between the anterior aspect of the odontoid process (dens) of the axis (C2) and the posterior aspect of the anterior arch of the atlas (C1).
- Normal ADI: In adults, a normal ADI is typically less than 3 mm. In children, it is normally less than 5 mm.
- Abnormal ADI: An ADI greater than 3 mm in adults or greater than 5 mm in children suggests disruption of the transverse ligament. This disruption allows the atlas (C1) to subluxate (move) anteriorly on the axis (C2).
How it Works
The transverse ligament is crucial for maintaining the stability of the atlanto-axial joint. It prevents excessive anterior translation of the atlas on the axis. When the transverse ligament is damaged (ruptured or insufficient), the ADI increases. The Rule of Spence quantifies this increase to aid in diagnosis.
Clinical Significance
An increased ADI, as assessed by the Rule of Spence, warrants further investigation. MRI or CT imaging may be needed to confirm the diagnosis of transverse ligament injury and to evaluate for other associated injuries, such as fracture of the odontoid process or other ligamentous injuries. Early diagnosis and treatment are critical to prevent neurological compromise resulting from cervical spine instability.
Important Considerations
- The Rule of Spence is just one component of a thorough clinical and radiographic evaluation.
- Clinical correlation is essential. The patient's symptoms and neurological examination findings should be considered in conjunction with the radiographic findings.
- The quality of the radiograph is important. Proper positioning is essential for accurate measurement of the ADI.
- False negatives and false positives can occur.
