1) Description of Measurement

Facet joint overlap (percent overlap method) is a quantitative CT-based assessment of cervical facet articulation used to identify facet fractures, subluxation, dislocation, and instability, particularly in the trauma setting.

The method expresses how much of the inferior articular facet of the superior vertebra remains overlapped with the superior articular facet of the inferior vertebra, normalized to the full anteroposterior length of the facet. Loss of overlap reflects posterior element disruption and correlates with mechanical instability and risk of neurologic injury.

2) Instructions to Measure

• Obtain thin-slice cervical CT with sagittal and/or oblique reconstructions optimized to visualize the facet joints.

• Select the facet level of interest, ideally the level with suspected injury or maximal malalignment.

• Identify the articulating facets:

• Inferior articular facet of the superior vertebra

• Superior articular facet of the inferior vertebra

•  Measure Facet Length (A):

• Measure the total anteroposterior length of the superior articular facet of the inferior vertebra.

•  Measure Overlap Length (B):

• Measure the portion of the inferior articular facet that remains overlapped with the superior articular facet in the anteroposterior direction.

•  Calculate Percent Facet Overlap:

Facet Overlap (%) = (BA)100

• Perform measurements bilaterally; record the lowest overlap percentage, as this represents the worst instability.

3) Normal vs. Pathologic Ranges

• Normal facet articulation: ≥ 50%

• Partial subluxation: 25-49%; potentially unstable

• Severe instability: <25%

• Facet dislocation: 0%; perched or jumped facet

Key points:

• < 50% overlap is generally considered abnormal

• Unilateral loss may indicate rotational injury

• Bilateral loss strongly suggests gross instability

4) Important References

Dvorak MF, Fisher CG, Fehlings MG, et al. The surgical approach to subaxial cervical spine injuries: an evidence-based algorithm based on the SLIC classification system. Spine (Phila Pa 1976). 2007 Nov 1;32(23):2620-9. doi: 10.1097/BRS.0b013e318158ce16.

Daffner RH, Sciulli RL, Rodriguez A, Protetch J. Imaging for evaluation of suspected cervical spine trauma: a 2-year analysis. Injury. 2006 Jul;37(7):652-8. doi: 10.1016/j.injury.2006.01.018.

Allen BL Jr, Ferguson RL, Lehmann TR, O'Brien RP. A mechanistic classification of closed, indirect fractures and dislocations of the lower cervical spine. Spine (Phila Pa 1976). 1982 Jan-Feb;7(1):1-27. doi: 10.1097/00007632-198200710-00001.

Harris JH Jr, Mirvis SE. The Radiology of Acute Cervical Spine Trauma. 3rd ed. Williams & Wilkins; 1996.

5) Other info....

Facet overlap is a direct structural measure of stability, unlike purely alignment-based metrics

Particularly useful for:

• Facet fractures

• Perched or jumped facets

• Rotational or distraction injuries

Should be interpreted alongside:

• Anterior/posterior translation

• Interspinous widening

• Disc space disruption

• Neurologic findings

CT is the gold standard for bony facet evaluation; MRI complements this by assessing ligamentous and disc injury

Even with preserved overlap, associated ligamentous disruption may still confer instability—clinical correlation is essential