Adult acquired flat foot is when a previously healthy-arched foot quietly collapses, usually because the posterior tibial tendon (the tendon on the inside of the ankle that holds the arch up) has stretched and torn. Caught early, it responds to bracing, an arch support, and physiotherapy. Once a fixed deformity has developed, surgical reconstruction is the answer. The classic operation is a combined procedure: tendon transfer (the flexor digitorum longus, a neighbouring tendon, is repurposed to do the job the posterior tibial can no longer do) plus a calcaneal osteotomy (the heel bone is cut and shifted to restore the alignment of the back of the foot under the leg). In severe cases, a midfoot fusion is added to lock in correction at the level where it has collapsed most. For Stage III rigid flat foot, a fusion of the hindfoot joints (subtalar, talo-navicular, calcaneo-cuboid — a ‘triple fusion’) becomes the right operation because the soft-tissue options no longer have anything flexible to work with.

Flat foot is best understood in stages. Early on the tendon is inflamed but the foot still moves normally, and bracing and physiotherapy often hold things comfortably. As the tendon fails, the deformity becomes first flexible and then fixed — and the operation needed changes with it. This is why timing matters: a flexible flat foot can usually be rebuilt with a soft-tissue reconstruction, whereas a rigid one needs a fusion. The interval between those two outcomes is sometimes as little as a year, which is why a foot that is slowly flattening is worth assessing sooner rather than later.