How is cervical instability measured on flexion and extension X-rays (X-radiographs)?

From the Guidelines

Cervical instability on flexion and extension X-rays is not recommended as a primary assessment tool due to its limited clinical usefulness and potential to cause neurologic injury. The measurement of cervical instability is crucial for identifying patients who may require surgical intervention to prevent neurological compromise. However, according to the American College of Radiology 1, 2, flexion-extension views have not been shown to be of clinical usefulness in assessing cervical instability.

Key Points to Consider

• The literature has been uniformly negative in assessing the utility of static flexion-extension radiographs or dynamic fluoroscopy for detection of cervical spine ligamentous injuries 2.
• Studies have reported anywhere from 28% to 97% of flexion-extension studies are inadequate for evaluating ligament injury 2.
• Even when flexion-extension radiographs are technically adequate, they rarely demonstrate evidence of ligament instability 2.
• The low rate of technically adequate studies along with the low sensitivity and specificity of flexion-extension radiographs makes this study undesirable for assessment of cervical spine ligament injuries 2.

Alternative Assessment Tools

• MRI has a high sensitivity for cervical ligament injury and identifies many ligament injuries that are clinically insignificant 2.
• In the very limited circumstance where MRI findings are equivocal for ligamentous injury, flexion-extension radiographs may be useful to determine whether the MRI findings correlate with pathologic motion 2.

Important Considerations

• The primary goal is to prioritize patient safety and prevent neurologic injury.
• Care should be taken to ensure adequate flexion and extension when performing flexion-extension radiographs 2.
• The assessment should be performed carefully, ensuring true lateral views and maximum but safe flexion and extension to avoid false negative results.

From the Research

Measuring Cervical Instability on Flexion and Extension X-rays

• Cervical instability can be measured on flexion and extension X-rays by assessing various radiographic variables, including subluxation, interspinous distance, and vertebral angulation 3.
• A study found that subluxation greater than 2 mm in men 18 to 40 years of age may be a useful variable for further study as an indicator of ligamentous injury 3.
• Another study suggested that the index of horizontal displacement ≥0.3 is suggestive of cervical instability 4.
• Flexion and extension radiographs can be useful in identifying cases of degenerative cervical spondylolisthesis that would be missed by obtaining solely neutral upright radiographs 5.
• The reliability of magnetic resonance imaging (MRI) in diagnosing degenerative cervical spondylolisthesis is limited, with 38% of cases identified by radiographs being missed by MRI 5.

Radiographic Criteria for Flexion-Extension Studies

• A study proposed the following radiographic criteria for flexion-extension studies:
  • Subluxation: greater than 2 mm
  • Interspinous distance: variable, depending on the level of cervical spine studied
  • Vertebral angulation: variable, depending on the level of cervical spine studied 3.
• Another study used established criteria to define instability as ≥2 mm of listhesis on neutral imaging, and ≥1 mm of motion between flexion-extension radiographs 5.

Limitations of Flexion and Extension Radiographs

• Flexion and extension radiographs may not be clinically useful in assessing acute instability in patients hospitalized with blunt trauma with negative computed tomography scans 6.
• Patients with acute injuries and severe pain and muscle spasms may not be able to move their necks effectively, severely compromising the diagnostic yield of the radiographs 7.
• Approximately 30-40% of flexion and extension radiographs may be deemed inadequate due to pain and muscle spasms experienced by patients 7, 6.