1167. Parallel Line Sign: A New MRI Finding Associated with Chronic Sacroiliitis - Prevalence, Characteristics, and Associations
Authors * Denotes Presenting Author
  1. Shubreet Randhawa *; Western University
  2. Timothy Miao; Western University
  3. Michael Roth; Western University
  4. Joanne Howey; Western University
  5. Greg Garvin; Western University
We have observed that some cases of chronic sacroiliitis have vertically oriented linear low signal in the subchondral bone parallel to the subchondral plate of the sacroiliac joints (SIJs) on MRI examinations. We propose that this line be called the parallel line sign (PLS). The objectives of this study are to validate the existence of the PLS and to determine its prevalence, characteristics, and associations.

Materials and Methods:
This institutional review board-approved retrospective study included 200 patients who underwent MRI of the SIJs at a single tertiary rheumatology referral centre between January 1, 2018 and December 30, 2019. MRI examinations were evaluated for the presence of the PLS, defined as a vertically oriented line of low signal on both T1-weighted and fluid-sensitive sequences, at least 1 cm in length, and parallel to and within 2 cm of the SIJ. The presence and extent of imaging features of sacroiliitis (edema, fatty infiltration, erosions, sclerosis, and ankylosis, with extent of involvement measured using an 8-point scale) were also evaluated. Chi-square and independent samples t tests were used for statistical analysis.

Prevalence of PLS in the study population was 11.5% (23/200), with 9 patients with PLS involving both SIJs simultaneously, resulting in 32 unique instances of PLS. Presence of PLS was not associated with HLA-B27 status or clinical diagnosis (ankylosing spondylitis, psoriatic arthritis, reactive arthritis, or other). Every PLS involved the synovial (rather than ligamentous) portion of the SIJ, and almost all (31/32, 96.9%) involved the iliac (rather than sacral) side of the SIJ. Every PLS occurred with at least one other imaging feature of sacroiliitis. The presence of PLS, compared to its absence, was associated with a higher prevalence of erosions (78.3% vs. 36.7%, p < 0.001), greater extent of SIJ involvement by erosions (3.6 ± 1.2 vs. 2.3 ± 1.1 quadrants of the SIJ involved, p < 0.001), and a higher density of erosions per centimeter (88.9% vs. 46.2% with >2 erosions/cm, p = 0.001). Although there was a higher prevalence of edema, fatty infiltration, and sclerosis in those with PLS compared to those without PLS (p = 0.001, p < 0.001, and p = 0.006, respectively), extent of SIJ involvement by any of these features was not different between the two groups (p = 0.174, 0.063, and 0.228, respectively).

The PLS is associated with imaging features of chronic sacroiliitis, especially erosions. Its MR appearance can mimic an insufficiency fracture, differing mainly in location (iliac instead of sacral side of the SIJ), associated MR findings, and clinical context.