The talonavicular and subtalar joints: the "calcaneopedal unit" concept.

The talonavicular (TN) joint and the three subtalar (ST) joints are linked anatomically and functionally. Together they form the subtalar joint complex, where movement occurs between the calcaneopedal unit (CPU) (entire foot except the talus) and the talotibiofibular unit (talus held tightly by the ankle mortise). Many are unaware of the TN joint's dual membership: it is a component of the subtalar joint complex (talocalcaneonavicular joint) and also the transverse tarsal joint (with the calcaneal-cuboid joint). The anatomy of the articulating surfaces, movement of the CPU when unloaded, shifts and changes in CPU shape with weight bearing, application to clinical tests and X-ray interpretation, and the pathophysiology applications to pes cavovarus, pes planovalgus and congenital talipes equinovarus (club foot) will be reviewed here. The CPU concept corresponds to a horizontal segmentation of the foot. This is a useful supplement to the two other segmentation methods: frontal (hindfoot, midfoot and forefoot) and sagittal (medial and lateral columns). This horizontal segmentation solves the issues with the ST joint complex, which straddles the hindfoot and midfoot, and also the issues with the dual membership of the TN joint. This concept makes it easier to understand foot deformities, better interpret the clinical and radiological signs and deduce logical treatments.