Addressing The Biomechanics Of Stage II Adult-Acquired Flatfoot

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Understanding The Progression Of The Adult-Acquired Flatfoot Deformity

1. Preexisting flatfoot deformity
2. Pronated position of the subtalar joint creates increased friction and gliding resistance of the posterior tibial tendon
3. Posterior tibial tendon gradually attenuates and ruptures
4. Pronated subtalar joint creates increased mobility of the forefoot on the rearfoot, increasing strain on the supportive ligaments
5. Sequential ligament rupture occurs beginning with the spring ligament and followed by the long and short plantar ligaments as well as the superficial and deep deltoid ligaments
6. Progressive flatfoot deformity occurs and is characterized by hindfoot valgus, lowering of the medial longitudinal arch and forefoot abduction

Douglas Richie, Jr., DPM, FACFAS, FAAPSM

   The clinician can gain appreciation for the biomechanical changes by asking the patient with stage II adult-acquired flatfoot to perform a single foot heel rise test. The heel rise test is recommended for individuals with posterior tibial tendon dysfunction (PTTD) to detect a partial or complete rupture of the posterior tibial tendon.31,32 Weakness of the posterior tibialis muscle theoretically contributes to the inability to perform a heel rise task or abnormal kinematics during a heel rise task.33 Clinically, one would observe an abnormal heel rise test when the individual cannot perform a heel rise or performs the heel rise with hindfoot eversion (fails to invert on rising), suggesting that the posterior tibialis muscle no longer is acting to invert the hindfoot.5,34

   The normal combined action of the posterior tibialis and triceps surae muscles in theory produces ankle plantarflexion with inversion during a heel rise task.16,35,36 The failure of a patient with stage II adult-acquired flatfoot to perform the heel rise is not directly attributed to ankle plantarflexion weakness. The tibialis posterior is not an effective plantarflexor of the ankle, even in healthy patients. Instead, the heel rise test requires a stable arch and midfoot in order for the triceps to actively plantarflex the ankle and the entire foot across the metatarsal heads. Houck and co-workers demonstrated that patients with stage II adult-acquired flatfoot show greater ankle joint plantarflexion at midstance and reciprocal dorsiflexion of the first ray, indicating breakdown of the midfoot.30 Researchers have shown increased flexibility of the forefoot on the rearfoot in other studies of patients with stage II adult-acquired flatfoot, suggesting that this demonstrates loss of ligamentous stability.31,32

Assessing The Evidence On Treatment Interventions For Stage II Adult-Acquired Flatfoot

Non-operative interventions for adult-acquired flatfoot include longitudinal arch supports, custom foot orthotic devices, ankle braces and custom ankle-foot orthoses (AFOs). To be effective, these devices must offload the supportive ligaments that have been damaged by the progression of deformity and shorten the length of the posterior tibial tendon if it is still intact. How these devices accomplish these treatment goals remains somewhat obscure.
Flemister and colleagues speculate that rupture of the spring ligament results in greater hindfoot eversion and plantarflexion of the talus.37 They speculate that foot orthoses that support the medial longitudinal arch while controlling hindfoot valgus may compensate for this ligament failure.37

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