Identification of subgroups of inflammatory and degenerative MRI findings in the spine and sacroiliac joints: A latent class analysis

Background: The use of magnetic resonance imaging (MRI) has increased dramatically in recent decades in an attempt to optimise the diagnostic process in persistent low back pain (LBP) and spondyloarthritis. Nevertheless, many uncertainties remain about the association between MRI findings and the clinical presentation of back pain. The objectives of this explorative study were: 1) Investigate subgroups of MRI findings of the spine and sacroiliac joints (SIJs) using Latent Class Analysis (LCA) and 2) Investigate whether these subgroups differ in their demographic and clinical characteristics.Methods: The sample included 1,037 patients aged 18-40 years with persistent low LBP. LCA was applied to MRI findings of the spine and SIJs. The resulting subgroups were tested for differences in self-reported demographic and clinical characteristics.Results: A five-class model was identified and the subgroups were labelled the following way: 1.‘No or few findings’, n=1162.‘Mild spinal degeneration’, n=5403.‘Moderate to severe spinal degeneration’, n=2294.‘Moderate to severe spinal degeneration with mild SIJ findings’, n=685.‘Mild spinal degeneration with moderate to severe SIJ findings’, n=84The two SIJ subgroups (Subgroup 4 and 5) had a higher median activity limitation score (Roland Morris Disability Questionnaire calculated as a proportional score): 65(IQR 48-78)/65(48-78)), compared with Subgroups 1-3 (48(35-74)/57(39-74)/57(39-74)), a higher prevalence of women (68%(95%CI 56-79)/68%(58-78)) compared with Subgroups 2 and 3 (51%(47-55)/40%(33-46)), a higher prevalence of being overweight (67%(95%CI 55-79)/53%(41-65)) compared with Subgroup 1 (36%(26-46)) and a higher prevalence of previous LBP episodes (yes/no: 81%(95%CI 71-91)/79%(70-89)) compared with Subgroup 1 (58%(48-67)). Subgroup 5 was younger than Subgroup 4, median age 29 years (IQR 25-33) versus 34(30-37) and had a higher prevalence of HLA-B27 (40%(95%CI 29-50)), compared with the other subgroups (Subgroup 1-4) (12%(6-18)/7%(5-10)/6%(3-9)/12%(4-20)). Across the subgroups with predominantly spinal findings (Subgroups 1-3), median age, prevalence of men, being overweight, and previous LBP episodes were statistically significantly lower in Subgroup 1, higher in Subgroup 2 and highest in Subgroup 3.Discussion and conclusions: Five distinct subgroups of MRI findings in the spine and SIJs were identified, and the demographic and clinical differences between these subgroups contribute to an aetiological understanding of these MRI findings and their role in the clinical presentation of back pain. Firstly, the results indicate that SIJ MRI findings not only can be seen as a part of the spondyloarthritis disease entity but also are associated with age, gender and being overweight. Moreover, the results indicate that (i) LBP patients with SIJ MRI findings are more disabled compared with patients without SIJ MRI findings, and (ii) moderate to severe spinal degeneration and/or SIJ MRI findings may be associated with recurrent pain.Arnbak et al. Arthritis Research & Therapy (2016) 18:237