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The Mechanism of the Navicular Bone Turning Inward in a Pronated Foot

The inward rotation of the foot's navicular bone in a pronated foot (a flatfoot condition characterized by excessive inward rolling) is primarily caused by posterior tibial tendon (PTT) dysfunction and the collapse of the foot arch.

Principle of Medial Displacement of the Navicular Bone in Pes Planus (Flatfoot/Pronated Foot)
Structural Characteristic of Pes Planus/Pronation

Pronation means the foot excessively rolls inward (medially), leading to a flattening of the arch (a functional flatfoot deformity).

In this state, the ankle bone (talus) drops and rotates inward, affecting other bones that form the medial longitudinal arch.

Role and Position of the Navicular Bone

The navicular bone is located between the talus and the cuneiform bones, serving as a crucial bone in the foot arch and acting as its apex.

The Posterior Tibial Tendon (PTT), a key tendon that supports and maintains the arch, attaches to the navicular bone.

Posterior Tibial Tendon Dysfunction

In a pronated foot, the excessive inward tilt places abnormal stress and tension on the PTT.

Persistent stress can lead to inflammation (tendinitis) or weakening of the PTT (dysfunction), causing it to lose its ability to pull the arch upward.

Medial and Inferior Displacement of the Navicular Bone

When the PTT fails to function properly, the arch loses its support. The navicular bone, being at the center of the arch, is unable to withstand the weight.

It sinks downward (inferior displacement) and twists inward (medial displacement/subluxation).

Consequently, the navicular bone appears to bulge out on the inner side of the foot. This can cause pain in the navicular area, especially if an accessory bone (accessory navicular) is present (accessory navicular syndrome).