The cross-sectional area ratio of a specific part of the flexor pollicis longus tendon- a stable sonographic measurement for trigger thumb: a cross-sectional trial.

Background: Trigger thumb is a pathologic condition of the digital pulleys and flexor tendons. To find a cutoff value of the cross-sectional area ratio of specific parts of the flexor pollicis longus tendon to diagnosis trigger thumb in the high-frequency ultrasound examination.

Methods: We evaluated 271 healthy volunteers and 57 patients with clinical diagnosis of trigger thumb. The cross-sectional area of the metacarpophalangeal joint of flexor pollicis longus tendon (C1) and the cross-sectional area of the midpoint of the first metacarpal of flexor pollicis longus tendon (C2) were analyzed.

Results: There is no difference between gender, age and left and right hands in the ratio of C1 to C2 (C1/ C2). The mean of C1/ C2 in the healthy thumb was 0.983 ± 0.103, which was significantly smaller in comparison to the diseased thumb (P < 0.05). Based on the receiver operating characteristic curve, we chose the diagnostic cut-off value for the C1/ C2 to be 1.362 and 1.153 in order to differ a trigger thumb from children and adults.

Conclusions: The C1/ C2 of the healthy thumb was relatively stable, with a mean value of 0.983 ± 0.103. The cutoff value of C1/C2 to distinguish healthy thumb from diseased thumb in children and adults were 1.362 and 1.153, respectively.