The role of magnetic resonance in the diagnosis of the os trigonum syndrome

PURPOSE: We investigated the yield of Magnetic Resonance Imaging (MRI) in hindfoot conditions, particularly the os trigonum syndrome, which are very difficult to diagnose clinically. MATERIAL AND METHODS: We examined 7 consecutive patients complaining of hindfoot pain for more than 4 months (male:female = 1:6; age range 16-22 years, average 18.6). Three patients practiced competitive sports and 4 ballet. We performed conventional radiography (orthogonal projections) and then MRI with a 0.5 T superconductive unit with surface coils; MR images were acquired with T1-weighted spin-echo (SE), T2-weighted gradient-echo (GRE), and fast inversion recovery (FIR) fat-suppressed sequences with 4 mm thickness and 0 mm gap. RESULTS: In 2 cases the os trigonum had irregular margins with subchondral sclerosis and widened synchondrosis. In 3 patients we found flexor hallucis longus tenosynovitis, likely caused by tendon compression and displacement within its sheath; there were neither os trigonum marrow edema nor synchondrosis widening. One patient had os trigonum hypertrophy, mild synchondrosis widening and marrow edema, in the os trigonum and the posterior aspect of talus. One patient had the os trigonum, but no signs referable to the os trigonum syndrome. CONCLUSIONS: In the posterior impingement syndrome, our objective is to show inflammatory changes in the posterior capsule of the ankle joint, adjacent ligaments, tendons and chondrosynovial surface. In these cases, the yield of conventional radiography and CT is rather poor, while MRI provides important information on soft tissues involvement, synovial reaction, chondral and subchondral bone injuries and the association of flexor hallucis longus synovitis, if present. MRI also yields detailed information for correct therapeutic approach. In conclusion, for the (differential) diagnosis of hindfoot pain in clinically suspected os trigonum syndrome, MRI appears to be the technique of choice, after conventional radiography, thanks to its noninvasiveness, multiplanarity, and high spatial and contrast resolution.