Biomechanical Evaluation of the Flexor Digitorum Longus and Flexor Hallucis Longus Transfer Used for the Treatment of Adult Acquired Flatfoot Deformity: A Finite Element Study

<b>Introduction:</b> Management strategies for stage II tibialis posterior tendon dysfunction are centered on tendon transfers and osteotomies. One of the most commonly used tendon transfers is flexor digitorum longus (FDL) tendon to navicular, but its superiority over transfers to other locations or transfers of other tendons, along with the role of spring ligament and tibialis posterior tendons, have not been objectively evaluated. <b>Aims:</b> We aimed to quantify both the location and magnitude of secondary stresses that develop as a consequence of the initial pathology. <b>Methods:</b> In this study, we used a computational model to study flat foot development and evaluate the effects of various tendon transfers and failures of passive structural elements, as well as their effect on the biomechanics of the foot. <b>Results:</b> We found that both FDL and FHL transfers have biomechanical advantages and disadvantages. Neither of these transfers decrease the stress on the tibialis posterior tendon if the underlying pathologies such as spring ligament failure are not addressed. <b>Conclusions:</b> Of the tendon transfers evaluated, FDL transfer to the navicular had the most profound effect on reducing the stresses on the spring ligament.