Limitations of lumbar spine MRI in the diagnosis of ankylosing spondylitis.

OBJECTIVES: To assess the value of inflammatory and fatty lesions in the lumbar spine on magnetic resonance imaging (MRI) in differentiating ankylosing spondylitis (AS) from non-inflammatory chronic back pain. METHODS: We reviewed the lumbar spine MR images of 192 consecutive AS patients and 208 non-AS subjects with non-inflammatory chronic back pain. Lesions including vertebral corner inflammatory lesions (CIL), inflammation in posterior elements (PE) of the spine, and fatty deposition lesions (FDL) seen on lumbar spine MRI were scored in a blinded manner. RESULTS: The frequencies of CIL and FDL in AS patients were higher than that in non-AS patients (both p<0.01), but there was no significant difference in the positive rate of inflammation in PE of the spine between two groups. AS patients had higher scores of all three types of lesions than non-AS patients (all p<0.01). Positive likelihood ratio increased as the cut-off score for distinguishing AS from other diseases increased (ranged from 1.14 to 18.42). But the biggest value of area under the receiver operating characteristic curve of all types of lesions was only 62.58%. We also summarised some features of these lesions that may help to distinguish AS from non-inflammatory chronic back pain. CONCLUSIONS: Our study found that the value of inflammatory and fatty lesions (including CIL, inflammation in PE and FDL) seen on lumbar spine MRI in the diagnosis of AS was limited. But the diagnosis of AS would be more convincing if patients had high scores of these three types of lesions (CIL ≥16, and/or inflammation in PE of the spine ≥5, and/or FDL ≥2).