The posterior tibial tendon and the tarsal sinus in rheumatoid flat foot: Magnetic resonance imaging of 40 feet

Objective. To investigate the role of the posterior tibial tendon in the fl at foot deformity of rheumatoid arthritis (RA). Methods, Eleven patients with hyperpronated feet and 9 without hyperpronati on underwent magnetic resonance imaging (MRI) of the feet and ankles. Radia l oblique double echo images provided cross sectional views of the posterio r tibial tendon as it coursed behind and under the medial malleolus and ins erted beneath the midfoot, Results. Complete tears (Type III) of the posterior tibial tendon were seen in one patient with hyperpronation and in one without hyperpronation. Howe ver, partial tears (Types I and II) of the posterior tibial tendon were com mon, and when Type I-III posterior tibial tendon tears were grouped togethe r, they were seen in 68% of flat feet and in 43% of feet that were not flat . Abnormalities of the tarsal sinus, including cortical erosions and replac ement of the normal fat signal with intermediate signal soft tissue, were s een in 74% of flat feet and in 5% of feet that were not flat. Conclusion. Posterior tibial tendon tears are common in RA flat feet, but a re usually incomplete and are not solely responsible for the flat foot defo rmity.