The Third Cuneiform Bone (os cuneiforme tertium; external cuneiform), intermediate in size between the two preceding, is wedge-shaped, the base being uppermost. It occupies the center of the front row of the tarsal bones, between the second cuneiform medially, the cuboid laterally, the navicular behind, and the third metatarsal in front.
- The anterior surface, triangular in form, articulates with the third metatarsal bone.
- The posterior surface articulates with the lateral facet on the anterior surface of the navicular, and is rough below for the attachment of ligamentous fibers.
- The medial surface presents an anterior and a posterior articular facet, separated by a rough depression: the anterior, sometimes divided, articulates with the lateral side of the base of the second metatarsal bone; the posterior skirts the posterior border, and articulates with the second cuneiform; the rough depression gives attachment to an interosseous ligament.
- The lateral surface also presents two articular facets, separated by a rough non-articular area; the anterior facet, situated at the superior angle of the bone, is small and semi-oval in shape, and articulates with the medial side of the base of the fourth metatarsal bone; the posterior and larger one is triangular or oval, and articulates with the cuboid; the rough, non-articular area serves for the attachment of an interosseous ligament. The three facets for articulation with the three metatarsal bones are continuous with one another; those for articulation with the second cuneiform and navicular are also continuous, but that for articulation with the cuboid is usually separate.
- The dorsal surface is of an oblong form, its postero-lateral angle being prolonged backward.
- The plantar surface is a rounded margin, and serves for the attachment of part of the tendon of the Tibialis posterior, part of the Flexor hallucis brevis, and ligaments.
The third cuneiform articulates with six bones: the navicular, second cuneiform, cuboid, and second, third, and fourth metatarsals.