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
Addressing The Biomechanics Of Stage II Adult-Acquired Flatfoot
Authors have commonly reported that most patients presenting with stage II adult-acquired flatfoot deformity have a history of preexisting flat feet for most of their life.7 In many cases, the patient has already had treatment for symptoms of flat feet and is already wearing foot orthoses. The significant change that causes the patient to seek treatment is the fact that in one foot, the symptoms and deformity have changed and the level of disability has worsened. At the present time, there is no evidence that any intervention earlier in life will prevent eventual rupture of the posterior tibial tendon. However, researchers have suggested that foot orthotic intervention may have a protective benefit in patients with pronated feet.
A lifetime of walking on a pronated foot may lead to overload and ultimate failure of key supportive structures. Uchiyama and coworkers were among the first to document an increased gliding resistance of the posterior tibial tendon when the human foot is pronated and a flatfoot deformity is present.8 These same researchers later verified that a pronated foot position uniquely overloads the posterior tibial tendon rather than the other tendons of the ankle.9 They further recommended that orthoses or surgical procedures that realign the hindfoot and ankle can reduce the load on soft tissue structures and prevent further degeneration.
Since we know that certain patients are at risk for developing adult-acquired flatfoot, particularly those with preexisting flatfeet and overweight individuals, it appears that early intervention with foot orthotic therapy may have a protective benefit. While no studies have determined the benefits of early intervention, the biomechanics of soft tissue overload certainly validate any effort to protect certain structures in the human foot that are at risk for rupture later in life.
Several studies have demonstrated the benefit of certain foot orthotic modifications to reduce internal inversion subtalar joint moments. This will theoretically reduce load on the posterior tibial tendon.
Munderman and colleagues showed that varus posting significantly reduced the inversion moment of the ankle and hindfoot.10 Williams and co-workers studied the effects of the inverted cast correction for custom foot orthoses and found that this modification can reduce the peak inversion moment of the rearfoot by 54 percent in comparison to not wearing orthoses.11 Bonanno and colleagues showed that 4 mm and 6 mm medial heel skive modifications to custom foot orthosis fabrication could increase pressure on the medial calcaneus, and speculated that this may improve pronation control of the subtalar joint in flat, pronated feet.12 Murley and co-workers showed that various foot orthotic designs could reduce peak electromyographic activity in the tibialis posterior, a finding that could validate the use of these devices as a protective mechanism for adult-acquired flatfoot.13