Systematic assessment of inflammation by magnetic resonance imaging in the posterior elements of the spine in ankylosing spondylitis

Objective

Magnetic resonance imaging (MRI) is sensitive for scoring inflammatory lesions in the spine, but attention has primarily focused on vertebral bodies, and no study has systematically examined the posterior elements. We aimed to systematically determine the frequency and distribution of inflammatory changes in the posterior elements of the spine using MRI, and to assess the reliability of their detection and their impact on discrimination of spinal MRI.

Methods

We scanned 32 patients recruited to placebo‐controlled trials of anti–tumor necrosis factor therapy. Inflammatory lesions were detected by systematic review of consecutive sagittal STIR slices of the entire spine. Two readers evaluated pretreatment and posttreatment scans, blinded to treatment and time point. Inflammation was scored dichotomously (present/absent) in each posterior structure. Reproducibility was assessed by calculating random model variance components and generalizability coefficients, and discrimination by using Guyatt's effect size.

Results

Most patients (87.5%) had ≥1 lesion in the posterior elements (mean ± SD number of affected spinal levels per patient 6.7 ± 5.3), and they were detected most frequently in the thoracic spine. Interobserver reproducibility for total lesion count was very good to excellent for lesions in the thoracic spine and transverse and spinous processes. The addition of a simple dichotomous method for scoring posterior element inflammation substantially enhanced the discrimination observed using established MRI methods for scoring vertebral body inflammation.

Conclusion

Inflammatory lesions in the posterior elements were present in the majority of patients with AS, and standard MRI protocols of the spine should be modified to ensure adequate visualization of posterolateral structures.