Acquired unilateral pes planus in a child caused by a ruptured plantar calcaneonavicular (spring) ligament.

A traumatic cause for acquired pes planus in children is uncommon and it is not often considered as a primary diagnosis. We describe a patient, who ruptured her plantar calcaneonavicular (spring) ligament and presented late with an acquired unilateral flat foot similar to a tibialis posterior rupture. Corrective surgery in the form of calcaneal osteotomy with iliac graft lengthening and medial advancement of tibialis posterior achieved excellent results with patient remaining asymptomatic and returning to routine activities. In the presence of an acute traumatic onset of unilateral pes planus, spring ligament and tibialis posterior tendon rupture should be considered in the differential diagnosis.