Patterns of spinal new bone formation in patients with axial spondyloarthritis, psoriatic arthritis, rheumatoid arthritis and healthy controls: a low-dose CT study

OBJECTIVE: This study (the SpACT study) aimed to investigate the frequency and localisations of different types of spinal new bone formation (NBF) in patients with axial spondyloarthritis (axSpA), psoriatic arthritis (PsA), rheumatoid arthritis (RA) and healthy controls (HCs) using low-dose CT (ldCT).

METHODS: Patients with axSpA, PsA or RA, and HC without a history of chronic back pain were included and underwent ldCT of the entire spine. Three readers blinded to all clinical information, including diagnosis, assessed sagittal and coronal images for NBF: (1) marginal syndesmophytes, (2) non-marginal syndesmophytes and (3) osteophytes.

RESULTS: 69 participants (33 females, mean age 51.4 years) were included: AxSpA: 30; PsA: 19; RA: 10; HC: 10. Across all groups, the thoracic spine consistently showed the highest number of NBFs, especially for marginal syndesmophytes and osteophytes. Furthermore, on sagittal images, NBF, regardless of type, occurred predominantly at the anterior vertebral corners. Coronal images showed right-sided dominance of NBFs, particularly osteophytes and non-marginal syndesmophytes, whereas marginal syndesmophytes had an almost equal overall distribution. Both sagittal and coronal reconstructions demonstrated high inter-reader reliability (intraclass correlation coefficient >0.9) for almost all groups for detecting any type of NBF.

CONCLUSION: AxSpA exhibited a distinct NBF pattern characterised by frequent marginal syndesmophytes, particularly in the thoracic spine. In contrast, the most prevalent findings in the other groups were osteophytes, and no consistent NBF pattern was observed to distinguish the groups from each other. Further studies, especially longitudinal assessments using ldCT or equally bone-sensitive imaging methods, are needed to further increase our understanding of NBF patterns.