Comparison of ultrasonography and magnetic resonance imaging for the assesment of clinically defined knee enthesitis in spondyloarthritis

Objectives: ultrasonography (US) and magnetic resonance imaging (MRI) are increasingly used imaging techniques for visualising entheses, however few studies have made a direct comparison of each. This study aimed to compare each technique for the detection of enthesitis in patients with spondyloarthritis (SpA) related knee swelling in order to make a lesion by lesion comparison. Methods: consecutive SpA patients with knee synovitis were recruited: each had clinical assessment for enthesitis at 8 sites in the involved knee joint followed by US and MRI examinations. Inflammatory and structural changes at tendon and ligament insertions were scored and a lesion by lesion comparison was made. Results: 21 patients were recruited. Clinically defined involved knee joint enthesitis was evident in 18 of 21 (86%) patients in 61 of 168 (36%) evaluated sites. Clinical enthesitis was associated with more hypoechogenicity (16 vs. 4%, p=0.007) and thickening (16 vs. 6%, p=0.03) by US compared to non-tender sites. Considering all MRI findings only increased signal in the surrounding tissues was higher at tender sites (41 vs. 20%, p=0.01) and the insertions points themselves showed little abnormality. The positive agreements between individual lesions by both methods was very low (10-26%) with low kappa values (0.06-0.18) with no correlations between the MRI and US scores (r²= 0.059). Conclusions: the difficulty in procuring `gold standard` histological validation is synovial joints makes the assessment of enthesitis using clinical and current imaging protocols of limited utility for diagnostic purposes.