This is the primary cause of acquired flat foot in adults. Essentially the underlying pathomechanics is that the posterior tibial tendon and muscle just give up - they can no longer do their job. Why do they give up? My understanding and experience is that all that all those with posterior tibial tendon dysfunction have a medially located subtalar joint axis. If this is the case, then it means that the posterior tibial tendon will have a much shorter lever arm to the joint axis. This would mean that in these people the muscle would have to work substantially harder than if the tendon had a longer lever arm if the subtalar joint axis was closer to a more normal or average position. After a life time of having to work so much harder, no wonder the tendon and muscle complex just give up. This is becoming more common in runners who start forefoot striking.
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