MRI Findings of Facet Joint Septic Arthritis

ABSTRACT Background Facet Joint Septic Arthritis (FJSA) is an infection of the spinal facet joints, for which previously described imaging findings are non‐specific. We present MRI findings of 20 patients who underwent facet biopsy for possible FJSA. Materials and Methods An internal database was reviewed for cases who underwent image‐guided spinal facet biopsy for possible infection and had pre‐biopsy contrast‐enhanced MRI. Cases were categorized as biopsy FJSA positive or negative and MRI was reviewed for synovitis, bone marrow enhancement, epidural phlegmon/abscess, and paraspinal muscle enhancement. The MRI findings were considered extensive if the involvement of the posterior paraspinal muscles spanned at least two vertebral bodies in the craniocaudal direction and at least half the length of the muscles in the anterior‐posterior direction. Results Twenty patients were included. Nine patients were FJSA positive, all of whom had synovitis, bone marrow enhancement of the facet joints, and epidural phlegmon. No patients had epidural abscess, but five patients had a paraspinal abscess. All nine patients demonstrated extensive paraspinal muscular enhancement. Eleven biopsy cases that were negative for FSJA were included. Their MRI findings demonstrated all with synovitis, six with bone marrow enhancement of the facet joints, six with epidural phlegmon, none with epidural abscess, and none with paraspinal abscess. Only one FJSA negative case had extensive paraspinal muscle enhancement. Conclusion The MRI appearance of FJSA has rarely been described in the literature; however, the diagnosis has a significant clinical impact. Facet enhancement with florid posterior paraspinal muscle enhancement should raise suspicion for FSJA.