Diagnosis of flexor hallucis longus tenosynovitis using the cross-sectional area of the flexor hallucis longus tendon.

Background: Flexor hallucis longus tenosynovitis (FHLTS) is the inflammation of the sheath through which the flexor hallucis longus tendon (FHLT) passes. The FHLT thickness (FHLTT) is correlated with ankle pain. Thus, the FHLTT is considered a characteristic FHLTS feature. However, asymmetrical thickening of the FHLT can occur at any point. Therefore, we used the FHLT cross-sectional area (FHLTCSA) as a new morphological parameter to diagnose FHLTS.

Methods: 26 participants in the control group (Group 1) and 26 in the FHLTS group (Group 2) underwent magnetic resonance imaging. The FHLTCSA was calculated for the ligament based on the images showing the maximum hypertrophic changes. The FHLTT was measured based on the images showing the FHLT with the maximum thickness.

Results: The mean FHLTCSA was 7.86 ± 1.88 mm2 and 13.11 ± 2.31 mm2 in Groups 1 and 2, respectively. The average FHLTT was 2.35 ± 0.55 mm and 2.91 ± 0.37 mm in Groups 1 and 2, respectively. The FHLTCSA (P < 0.001) and FHLTT (P < 0.001) in Group 2 were significantly greater than those in Group 1. Receiver operating characteristic (ROC) curve analysis revealed that the optimal cutoff value for the FHLTCSA was 10.37 mm2, with a sensitivity of 92.3%, a specificity of 92.3%, and an area under the curve (AUC) of 0.97. The optimal cutoff value for the FHLTT was 2.71 mm, with a sensitivity of 76.9%, a specificity of 76.9%, and an AUC of 0.84.

Conclusion: The FHLTCSA is a sensitive morphological parameter for assessing FHLTS.