The spine is a common site of various pathologic changes. On bone scans, differential diagnosis between metastasis and benign changes is not always easy. We describe a characteristic bone scan appearance of facet osteoarthritis most often seen at the fifth lumbar level. The lesion typically appears oval/elongated in shape, located along the lateral margin of the spine; its superior end is usually outside (lateral to) or just at the margin of the spine, and the inferior end is just at or inside (medial to) the margin, yielding a pattern of slightly oblique activity.
Bone scans performed in 448 patients with known malignancy and interpreted as having some sort of spinal abnormalities were reviewed. Of these, the scans in 36 patients were judged to show the facet osteoarthritis sign (FOS). Follow-up was available in 28 of the 36 patients. The presence or absence of metastasis at the site of the FOS was determined by a minimum of 6-month follow-up by chart review and correlation with plain radiographs, computed tomography (CT), magnetic resonance imaging, and/or serial bone scans.
None of the 28 patients had metastasis at the site of the FOS. CT and/or plain radiographs performed in 13 patients invariably showed facet disease.
When planar bone scan images show the FOS, further evaluation of this region with SPECT imaging or other imaging, ie, plain radiography, CT, or magnetic resonance imaging, would be unnecessary because this sign has an extremely high negative predictive value for metastasis (100% in this series).