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

• Despite several modifications of this technique, at the present time no clear information relative to the interosseous membrane window size is available in order to avoid some potential complications.
• The combined diameter of the tibialis posterior unit at 15 cm was 19.47 mm (17.47–21.48).
• The distance between the original position of the tibialis posterior unit to its new expected position in the anterior compartment was at 10 cm: 12.27 mm (10.71–13.83).
• Taking the anterior to posterior compartment distance (9–13.8 mm), the maximum combined tendon diameter (2.1 cm) and the total transferred tendon distance (7 cm), the estimated transferred tendon angle range is 25–30°.
• Assuming a maximum estimated combined tendon diameter (2.1 cm), and the smallest transfer angle possible (25°) in order to maximally estimate the membrane window size, its length should measure approximately 5 cm.

BackgroundTibialis 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.ObjectiveAnalyze the TP size at the transfer level and therefore determine the most adequate IOM window size to avoid muscle entrapment.MethodsEleven 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.ResultsThe 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.ConclusionThe 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.