Keys To The Biomechanical Evaluation Of The Symptomatic Adult-Acquired Flatfoot

Douglas Richie, DPM, FACFAS, FAAPSM

Given the relatively common nature of adult-acquired flatfoot, this author offers practical pearls and emerging insights from the literature on key biomechanical findings that can influence decision making for both conservative and surgical treatment.

The symptomatic adult-acquired flatfoot, also known as posterior tibial tendon dysfunction, continues to be an intriguing subject that has received considerable attention from researchers and clinicians over the past decade. Recent publications from multiple disciplines have enabled podiatric physicians to modify or even change their approach to evaluation and treatment of this common disorder.

   Adult-acquired flatfoot is defined as a symptomatic, progressive deformity of the foot caused by a loss of dynamic and static supportive structures of the medial longitudinal arch.1 Although the condition begins with a loss of dynamic support from the posterior tibial tendon, the sequential ruptures of key ligaments in the ankle and hindfoot are the more important events leading to collapse of the arch and progressive disability of the patient.2

   The adult-acquired flatfoot almost always begins with a preexisting flatfoot and has a predilection to affect females over the age of 40.3,4 When the foot functions in a valgus position in the hindfoot and carries excessive body mass, the posterior tibial tendon has increased friction and gliding resistance along the medial malleolar gliding pulley.5 The combination of this mechanical strain with other metabolic factors will lead to a progressive attenuation and rupture of the posterior tibial tendon. However, visible change or collapse of the foot will not occur with simple rupture of the posterior tibial tendon. Instead, the loss of the posterior tibial tendon will cause a dysfunction of the foot during gait, which will then place progressive strain on key ligamentous structures in the hindfoot.

   Increased load and strain will lead to rupture of the spring ligament, the interosseous talocalcaneal ligament and the long and short plantar ligaments.2 This will lead to a subluxation and triplane rotation of various joints that characterize the adult-acquired flatfoot: valgus alignment of the hindfoot, collapse of the medial longitudinal arch and abduction of the forefoot.

Emerging Concepts In The Staging Of Adult-Acquired Flatfoot

The challenge facing the clinician in the initial evaluation of the adult-acquired flatfoot is determining the stage or severity of deformity in order to make appropriate treatment decisions. Currently, the accepted staging system for adult-acquired flatfoot relies on some variation of the original Johnson and Strom classification, which was originally published in 1989.6

   There are numerous pitfalls with this classification primarily due to the reliance on subjective evaluation rather than objective findings. This classification defines Stage I adult-acquired flatfoot as tenosynovitis of the posterior tibial tendon with no evidence of collapse of the foot in comparison to the contralateral side. In Stage II, the posterior tibial tendon is attenuated or fully ruptured. Visible change in alignment of the foot has occurred but the deformity is still flexible. Stage III deformity is a rigid, non-reducible flatfoot deformity with complete rupture of the posterior tibial tendon. Myerson later added a Stage IV deformity, which is a valgus malalignment of the talocrural joint due to rupture of the deep deltoid ligament.7

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