Backfill is a specific sign of axial spondyloarthritis seen on MRI

To summarize the characteristics of backfill in patients with axial spondyloarthritis (SpA) and patients with non-specific back pain (NSBP) and healthy controls, and to assess the value of backfill in diagnosing axial SpA.

Three readers blinded recorded backfill seen on T1SE MRI scans from 647 subjects: 297 patients with ankylosing spondylitis (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 backfill. The changes of backfill were evaluated by the follow-up MRI scans from 157 patients. We summarized the characteristics of backfill and calculated its sensitivity and specificity for diagnosing axial SpA.

Backfill was recorded in 78.8% AS patients, 11.1% nr-axSpA patients, 1.8% patients with NSBP, and no healthy control. Backfill affected more frequently at ilium bone, lower half of sacroiliac joints in axial SpA (both P<0.05). The SSS score of backfill 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). The score of backfill 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 backfill in an average follow-up time of 1.09 years. Backfill had high specificity (0.98) and moderate sensitivity (0.59) for diagnosing axial SpA.

We summarized the characteristics of backfill and found that backfill is a specific sign of axial SpA seen on T1SE MRI.