To investigate the role of the posterior tibial tendon in the flat foot deformity of rheumatoid arthritis (RA).
Eleven patients with hyperpronated feet and 9 without hyperpronation underwent magnetic resonance imaging (MRI) of the feet and ankles. Radial oblique double echo images provided cross sectional views of the posterior tibial tendon as it coursed behind and under the medial malleolus and inserted beneath the midfoot.
Complete tears (Type III) of the posterior tibial tendon were seen in one patient with hyperpronation and in one without hyperpronation. However, partial tears (Types I and II) of the posterior tibial tendon were common, and when Type I-III posterior tibial tendon tears were grouped together, 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 replacement of the normal fat signal with intermediate signal soft tissue, were seen in 74% of flat feet and in 5% of feet that were not flat.
Posterior tibial tendon tears are common in RA flat feet, but are usually incomplete and are not solely responsible for the flat foot deformity.