Distinct characteristics and progression patterns of facet joint structural lesions in radiographic axial spondyloarthritis.

Background: Both vertebral bodies and posterior elements of the vertebrae (facet joints, FJ) can engage in bone formation in radiographic axial spondyloarthritis (r-axSpA). However, little is known about the specific structural lesions and progression patterns of FJs in r-axSpA.

Objectives: To identify specific lesions related to r-axSpA and to investigate the distinct progression patterns by comparing the FJ changes of r-axSpA with that of diffuse idiopathic skeletal hyperostosis (DISH), osteoarthritis (OA), and control group (CG).

Design: Single-center, retrospective study. Longitudinal imaging data were retrieved and collected.

Methods: Age- and sex-matched patients with complete thoracic and lumbar spine computed tomography (CT) data were included and their bilateral FJs were assessed. FJ changes were divided into erosions, ankylosis, joint-space narrowing, osteophytes, subchondral sclerosis, subchondral cysts, and vacuum phenomena. Average progressed year was defined as "number of changed vertebrae × interval years"/number of changed vertebrae.

Results: In all, 50 patients in each group were included. Subchondral cysts and vacuum phenomena were not observed. Bilateral FJ ankylosis (FJA)/erosions in the thoracic and lumbar spine, and unilateral ankylosis/erosions in T1-4, T9-12 were significantly more common in r-axSpA. Joint-space narrowing/osteophytes/subchondral sclerosis were significantly more common in DISH and OA. FJ lesions progressed in 56.34% of vertebrae of r-axSpA. The most common pattern was "FJ normal advanced to ankylosis" (17.54%) which required 2.63 years. It was followed by "erosions advanced to ankylosis" (12.3%) which took 2.05 years, and by "normal FJ advanced to erosions" (11.04%) which took 2.29 years, respectively. Degenerative changes could also progress to FJ erosions/ankylosis (24.83%). The majority pattern in DISH/OA was "FJ changes advanced to subchondral sclerosis/osteophytes/joint-space narrowing."

Conclusion: Bilateral FJA/erosions are r-axSpA-specific lesions. The specific progression pattern for r-axSpA was "FJ changes advanced to ankylosis/erosions." Repeated CT examination in intervals of at least 2 years will be more appropriate for monitoring FJ progression.