Reducible valgus flat-foot: assessment of posterior subtalar joint surface displacement by posterior arthroscopy during sinus tarsi expansion screwing.

INTRODUCTION: Subtalar arthroereisis corrects childhood and adult reducible valgus flat-foot in certain indications. Inserting an expansion screw in the sinus tarsi simultaneously corrects the calcaneal valgus of the talocalcaneal divergence and first-ray pronation if these are reducible. The displacement induced in the posterior subtalar joint (decoaptation, translation, rotation) is, however, poorly known. The present study involved arthroscopic assessment of posterior subtalar joint surface displacement during insertion of a talocalcaneal arthroereisis screw, with the hypothesis that displacement varies in three dimensions according to screw size.

MATERIAL AND METHOD: Eight specimens were used for the study. All ankles were supple, taken from adult subjects. A 4.5-mm arthroscope was used and measurements were taken with a graduated palpator in the posterior subtalar joint. Three sinus tarsi expansion screws of incremental diameter were assessed. Before and after insertion measurements were made of posterolateral and posteromedial talar exposure on the calcaneus, anteroposterior and lateromedial translation, and talocalcaneal joint-line opening.

RESULTS: Medial rotation, varization and anterior translation of the calcaneus were comparable in all cases. Mean lateral opening of the posterior subtalar joint was 0.88 mm with 8-mm screws and 1.25 mm with 16-mm screws. Significant differences between 8 and 16 mm screws were found for lateral subtalar joint opening (P=0.028) and for lateromedial translation (P=0.004).

CONCLUSION: Sinus tarsi expansion screwing corrects hindfoot valgus and talocalcaneal divergence by inducing medial translation of the calcaneus under the talus and talar medial rotation and varization, proportional to screw size (medial translation and lateral opening of the subtalar joint).

LEVEL OF EVIDENCE: III.