MRI-based extensor carpi ulnaris tendon cross-sectional area as a quantitative biomarker for extensor carpi ulnaris tenosynovitis

Background Extensor carpi ulnaris tenosynovitis (ECUT) is a common cause of ulnar-sided wrist pain and is often associated with tendon hypertrophy. Although extensor carpi ulnaris tendon thickness (ECUTT) has been used to assess tendon enlargement, a one-dimensional measurement may not fully reflect morphological changes of the tendon. We therefore investigated the diagnostic value of extensor carpi ulnaris tendon cross-sectional area (ECUTCSA) measured on magnetic resonance imaging (MRI) as a quantitative imaging biomarker for ECUT. Methods In this retrospective case-control study, MRI examinations from 26 patients with clinically diagnosed ECUT and 26 control subjects were analyzed. Axial T1-weighted wrist MR images were reviewed by two musculoskeletal radiologists who were blinded to clinical information. ECUTCSA and ECUTT were measured at the level demonstrating maximal tendon hypertrophy. Group comparisons were performed using unpaired t-tests, and diagnostic performance was evaluated using receiver operating characteristic (ROC) analysis. Interobserver agreement was assessed using intraclass correlation coefficients (ICCs). Results The mean ECUTCSA was significantly greater in the ECUT group than in the control group (10.86 ± 2.32 mm 2 vs. 7.25 ± 2.52 mm 2 , p < 0.001). The optimal ECUTCSA cutoff value was 8.80 mm 2 , yielding a sensitivity of 84.6%, specificity of 84.6%, and an area under the ROC curve (AUC) of 0.89. The optimal ECUTT cutoff value was 1.98 mm, with an AUC of 0.88. Interobserver agreement was excellent for ECUTCSA (ICC = 0.92) and ECUTT (ICC = 0.88). Conclusion ECUTCSA is a reproducible quantitative MRI parameter associated with ECU tenosynovitis and demonstrated good diagnostic performance in this study. These findings support the potential role of ECUTCSA as an imaging biomarker for quantitative assessment of tendon hypertrophy. Further multicenter studies with larger cohorts are warranted to validate its clinical utility.