Background Backfill (BF) is a specific sign seen on magnetic resonance imaging (MRI) in patients with ankylosing spondylitis (AS). BF is considered to be a key intermediary in the development of ankylosis in patients with AS. But the research on BF is rare.
Objectives To summarize the characteristics of BF in axial Spondyloarthritis (SpA) patients and patients with non-specific back pain (NSBP) and healthy controls, and to assess the value of BF in diagnose axial SpA.
Methods Three readers blinded recorded BF seen on T1SE MRI scans from 647 subjects: 297 patients with AS, 126 patients with non-radiographic axial SpA (nr-axSpA), 147 patients with NSBP, and 77 healthy controls. The SPARCC SIJ Structural Score (SSS) method was used to assess BF. The changes of BF were evaluated by the follow-up MRI scans from 157 patients. We summarized the characteristics of BF and calculated its sensitivity and specificity for diagnosing axial SpA.
Results BF was recorded in 78.8% AS patients, 11.1%nr-axSpA patients, 1.8% patients with NSBP, and no healthy control. BF affected more frequently at ilium bone, lower half of sacroiliac joints in axial SpA (both p<0.05), but not in patients with NSBP. The SSS score of BF was much higher in axial SpA than in patients with NSBP (both p<0.01) and it did not correlate with demographics and BASDAI, BASFI, and CRP (all p>0.05). BF score only positively correlated with symptom duration in AS (r=0.251, p<0.01) and in nr-axSpA (r=0.743, p<0.01) patients. Only 8.9% patients had the change of BF in an average follow-up time of 1.09 years. BF had high specificity (0.98) and moderate sensitivity (0.59) for diagnosing axial SpA.
Conclusions We summarized the characteristics of BF and found that BF is a specific sign of axial SpA seen on MRI.
Chiowchanwisawakit P, Lambert RGW, Conner-Spady B, et al. Focal fat lesions at vertebral corners on magnetic resonance imaging predict the development of new syndesmophytes in ankylosing spondylitis. Arthritis Rheum 2011;63:2215-25.
Maksymowych WP, Wichuk S, Chiowchanwisawakit P, et al. Fat Metaplasia and Backfill are Key Intermediaries in the Development of Sacroiliac Joint Ankylosis in Patients with Ankylosing Spondylitis. Arthritis Rheumatol 2014;66(11):2958-67.
Disclosure of Interest None declared