Peroneus Longus overload caused by soft tissue deficiencies associated with early adult acquired flatfoot: A finite element analysis.

Background: Peroneus Longus tendinopathy has been related to overload from cavus and ankle instability. The etiology of isolated Peroneus Longus tendon synovitis has not been elucidated. Loss of foot arch integrity as a cause of isolated Peroneus Longus overload is difficult to establish using cadaver modeling. Our objective was to analyze Peroneus Longus stress changes in pathological scenarios related to flatfoot development.

Methods: A three-dimensional finite element foot model which included the foot bones and main soft tissues that maintain the arch was used. Simulations were performed in midstance of gait. Tendon's maximum principal stress and von Mises were calculated in scenarios where the plantar fascia, spring ligament and the posterior tibial tendon were weakened.

Findings: Decreasing plantar fascia stiffness thus weakening arch integrity increases Peroneus Longus stresses by over three times. Additional failure of tissues that support arch, such as the spring ligament and tibialis posterior tendon further overloads this tendon. The absence of Peroneus Longus also affects stresses in tissues that maintain the arch. Stress concentrations increase in the plantar component of the Peroneus Longus.

Interpretation: Results offer an explanation into isolated Peroneus Longus overload synovitis. Recognition of failing medial arch structures that occur in early acquired flatfoot as a cause of Peroneus Longus overload could help in its treatment. We caution the practice of transfer of peroneus brevis to longus in surgical treatment of flatfoot as it may further overload an overloaded tendon and focus should be on restoration of arch stability to offload stresses within it.