Interosseous membrane window size for tibialis posterior tendon transfer—Geometrical and MRI analysis

Background Tibialis posterior (TP) tendon transfer through the interosseous membrane is commonly performed in Charcot-Marie-Tooth disease. In order to avoid entrapment of this tendon, no clear recommendation relative to the interosseous membrane (IOM) incision size has been made. Objective Analyze the TP size at the transfer level and therefore determine the most adequate IOM window size to avoid muscle entrapment. Methods Eleven lower extremity magnetic resonances were analyzed. TP muscle measurements were made in axial views, obtaining the medial-lateral and antero-posterior diameter at various distances from the medial malleolus tip. The distance from the posterior to anterior compartment was also measured. These measurements were applied to a mathematical model to predict the IOM window size necessary to allow an ample TP passage in an oblique direction. Results The average tendon diameter (confidence-interval) at 15 cm proximal to the medial malleolus tip was 19.47 mm (17.47–21.48). The deep posterior compartment to anterior compartment distance was 10.97 mm (9.03–12.90). Using a mathematical model, the estimated IOM window size ranges from 4.2 to 4.9 cm. Conclusion The IOM window size is of utmost importance in trans-membrane TP transfers, given that if equal or smaller than the transposed tendon oblique diameter, a high entrapment risk exists. A membrane window of 5 cm or 2.5 times the size of the tendon diameter should be performed in order to theoretically diminish this complication.