C2-3 facet joint "pseudo-fusion": anatomic basis of a normal variant.
OBJECTIVE: To identify the anatomic basis for apparent C2-3 facet joint fusion (pseudo-fusion) on lateral cervical spine radiographs. DESIGN AND PATIENTS: The studies of 81 consecutive blunt trauma patients who had both plain radiographs and a CT scan of the upper cervical spine were reviewed. The C2-3 facet joints were evaluated on lateral cervical spine radiographs and graded "normal" (category 1), "indistinct" (category 2), or "fused" (category 3), relative to the C3-4 level. The accompanying CT scans were reviewed for the presence of fusion and the angle of orientation of the facet joints relative to the axial and coronal planes. RESULTS: In category 1 ("normal"), the C2-3 facet joints were oriented nearly parallel to the true coronal and axial plane. In category 2 ("indistinct"), both the C2-3 facet joints were oriented obliquely to the true coronal and axial planes. In category 3 ("fused"), the C2-3 facet joints were also oriented obliquely, but at a steeper angle than in category 2. Head tilt/rotation caused a change in category rating in 5 of 81 cases (6.2%). CONCLUSION: The appearance of C2-3 facet joint fusion (pseudo-fusion) on lateral cervical spine radiographs may be a normal anatomic variant. This "pseudo-fusion" is due to the oblique orientation of these facet joints relative to the X-ray beam and is usually unaffected by patient position.