MR morphometry of posterior tibialis muscle in adult acquired flat foot.

We conducted magnetic resonance imaging of the posterior tibial (PT) and flexor digitorum longus (FDL) muscle bellies in 12 patients undergoing surgical treatment for unilateral posterior tibial tendon (PTT) dysfunction. All patients had atrophy of the PT muscle compared to the normal leg (mean 10.7%, p = 0.008). In those patients with a complete rupture of PTT there was replacement of the PT muscle by fatty infiltration. Conversely, the FDL muscle showed a compensatory hypertrophy (mean 17.2%, p < 0.002). We support the use of FDL as an appropriate tendon for augmentation of PTT in stage II disease. This study also demonstrates that in the presence of a complete rupture, excision of the PTT is a reasonable surgical procedure and pure tenodesis will be more likely to fail because the PT muscle belly undergoes fatty infiltration. In patients with a diseased but intact PTT there was no fatty infiltration and the muscle volume was at least 83% of the normal side in all cases. We therefore suggest that in the presence of an intact PTT the PT muscle belly may provide some useful function if used to augment the FDL transfer when the diseased tendon is excised.