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

   Most clinicians favor AFOs (ankle braces) over foot orthoses to address the severe biomechanical forces they see with stage II adult-acquired flatfoot.38 In another study focusing on three different designs of ankle braces to treat stage II adult-acquired flatfoot, Houck and coworkers found that controlling hindfoot eversion was the most insignificant treatment effect.39 They noted that improvements in medial longitudinal arch alignment and correction of forefoot abduction were the most significant measureable treatment effects, but only found these improvements with certain designs of ankle braces. The custom articulated AFO provided the best improvement of the height of the medial longitudinal arch and correction of forefoot abduction in comparison to a custom solid AFO. The researchers concluded that “The consistent finding in this patient of improved kinematics with the articulated versus solid ankle design underscored the importance of allowing ankle movement for foot function.”39

   One dilemma when treating stage II adult-acquired flatfoot is the presence of muscle weakness. Studies have demonstrated not only weakness of the tibialis posterior but overall general weakness of all of the ankle plantarflexors in patients with adult-acquired flatfoot.37,38 This is why Flemister and Houck concluded that “ankle plantarflexion weakness may account for functional impairments and gait disturbances reported by patients with PTTD. Orthoses that restrict ankle motion (solid AFO), while very popular, may induce plantarflexor weakness and increase dependence on the orthosis for support.”39

   With rupture of the spring ligament, excessive adduction and plantarflexion of the talus occur across the talonavicular joint. While foot orthoses may be able to provide some support to this joint, the patient often does not tolerate the pressure of the device against the skin of the foot in this region. Since the talus is locked within the ankle mortise, one can best accomplish control of adduction by controlling tibial rotation via an ankle-foot orthosis. This is why the most significant change measured with ankle bracing in a patient with stage II adult-acquired flatfoot was improved alignment of the medial longitudinal arch.37

   To this date, there are seven studies published in the medical literature that validate positive treatment effects for stage II adult-acquired flatfoot.17,40-45 The most impressive of these studies utilized a combination of strengthening exercises with AFO therapy. In both the studies by Alvarez and Lin, over 80 percent of the patients treated with articulating AFOs and exercise were able to avoid surgery and remain brace-free with a follow up of one to eight years.42,43

   Both strengthening exercise programs and surgical reconstruction of the tibialis posterior tendon have demonstrated recovery of muscle function in patients with stage II adult-acquired flatfoot.17,46 To this date, there are no studies documenting healing of ruptured ligaments when one uses bracing to treat stage II adult-acquired flatfoot. However, the fact that studies have shown that a majority of patients treated with bracing of stage II adult-acquired flatfoot are able to discontinue using their AFOs after a period of use suggests that ligament integrity must have been restored to achieve this level of function.

Final Words

The stage II adult-acquired flatfoot deformity represents a failure of the posterior tibial tendon and key supportive ligaments of the ankle and hindfoot. Clinicians can address the early presentation of this deformity with muscle strengthening and foot orthoses with modifications to decrease strain on the medial ankle structures. When the spring ligament ruptures, one can obtain control on talus adduction by controlling tibial rotation with ankle-foot orthoses.

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