Background: Acute calcium pyrophosphate (CPP) crystal arthritis is a crystal-induced arthropathy that typically presents as acute monoarthritis of peripheral joints such as the knee, wrist, or ankle, and is characterized by fever and severe joint pain. In the spine, CPP crystal arthritis is most commonly recognized as crowned dens syndrome, whereas involvement of the lumbar facet joint is rare. Lumbar facet joint CPP crystal arthritis has been reported sporadically and is associated with acute low back pain and fever. We report an extremely rare case in which acute CPP crystal arthritis initially developed in a unilateral lumbar facet joint and subsequently progressed to bilateral involvement.

Case Presentation: A 65-year-old woman presented with acute right-sided low back and buttock pain that developed one day prior to admission. Her body temperature at presentation was 36.9°C. Computed tomography revealed calcification at the L4/5 level. Magnetic resonance imaging demonstrated a small joint effusion in the right L4/5 facet joint with signal changes in the adjacent paraspinal muscles. Acute CPP crystal arthritis of the lumbar facet joint was suspected, and oral NSAIDs were initiated. On the night of admission, the patient developed a fever of 38.3°C. On hospital day 3, the low back pain progressed to bilateral involvement, with elevated inflammatory markers. Repeat magnetic resonance imaging revealed newly developed joint effusion in the left L4/5 facet joint with corresponding paraspinal muscle signal changes. Aspiration of the right L4/5 facet joint identified CPP crystals. Treatment with NSAIDs alone was continued. The patient became afebrile by hospital day 4, and her low back pain improved. Inflammatory markers decreased, and she was discharged on hospital day 5. Laboratory findings normalized by day 13 after admission.

Conclusion: Acute CPP crystal arthritis can involve the lumbar facet joint and may progress dynamically during the clinical course.