The foot is the region distal to the ankle and consists of three main parts: the tarsus, the metatarsus and the phalanges. The top of the foot is referred to as the dorsal surface, whilst the underside of the foot is known as the plantar surface. The tarsus, or proximal foot, connects the tibia and fibula with the metatarsus and consists of seven bones. Like in the carpus of the hand, the bones are arranged into a proximal and distal row. However, in the tarsus there is also an intermediate bone interposed between the distal and proximal row on the medial side, referred to as the navicular bone. This article will discuss the anatomy of the navicular bone along with its articulations, vascular supply, innervation and ossification followed by any related clinical pathology.
The navicular is a boat-shaped bone, which has an important role in the maintenance of the medial longitundinal arch of the foot. Proximally, the navicular bone articulates with the head of the talus and with the three cuneiform bones at its distal end. The smooth areas on the navicular bone, that articulate with other bones, are known as articular facets. The concave proximal surface of the navicular bone is ovoid in shape and articulates with the head of the talus. The transversely convex distal surface consists of three aritcular facets, which articulate with the cuneiform bones. The medial articular facet is larger and articulates with the medial cuneiform. It is roughly triangular in shape and consists of a medial rounded apex and a curved lateral base. The two remaining facets, which articulate with the intermediate and lateral cuneiforms, are also triangular in shape with plantar apices. The lateral facet, however, may be shaped as a wide crescent or a semicircle instead of a triangle.
Both the convex dorsal and concave plantar surfaces of the bone are roughened and give attachment to different ligaments. The medial surface is also rough and contains a prominent tuberosity, the navicular tuberosity, which is palpable 2.5 cm distal to the medial malleolus. This tuberosity is separated medially from the plantar surface by a groove. The lateral surface is irregular and sometimes contains a facet for articulation with the cuboid.
Attachments, Vascular Supply and Innervation
The tibialis posterior muscle attaches mainly to the navicular tuberosity but tendinous slips run through the groove to reach all the three cuneiforms, cuboids and middle three metatarsals. The plantar calcaneonavicular ligament attaches lateral to the groove near the proximal aspect of the bone whilst the calcaneonavicular part of the bifurcated ligament attaches to the lateral surface. There are plantar and dorsal cuneonavicular ligaments that join the three cuneiforms with the navicular bone. On the lateral side of the navicular bone, plantar and dorsal cuboideonavicular ligaments are present. The dorsal talonavicular is another ligament, which is broad and thin and attaches to the neck of the talus with the dorsal surface of the navicular bone.
The medial plantar artery supplies the plantar surface of the navicular bone whilst the anastomosis between the medial plantar and dorsalis pedis arteries supplies the tuberosity. The dorsal aspect of the bone is supplied directly or from a branch of the dorsalis pedis artery. The medial plantar and deep fibular nerves innervate the navicular bone.
Endochondrial ossification is a bone remodelling process where new bone is laid down by osteoblasts, cells that synthesise bone. It is an important process in bone development and involves transforming cartilage into bone. The location where this type of ossification begins is referred to as the ossification centre.
The ossification centre of the navicular bone appears during the third year of life. In 5% of the population, an accessory navicular bone is also present. This anatomic variant arises from a different ossification centre, located near the navicular tuberosity. There are three types of accessory navicular bone: type I, type II and type III. Type I is a 2-3 mm sized sesamoid bone, also referred to as os tibiale externum and is located at the level of the inferior calcaneonavicular ligament within the tibialis posterior tendon. Type II is an accessory bone, also referred to as prehallux, connected to the navicular by a fibrocartilage or hyaline cartilage (synchondrosis). Type III is a fused variant of type II, also known as the cornuate navicular, and is joined to the navicular bone by a bony ridge. On the rare occasion, the navicular bone can be bipartite, in two parts, due to its development from two ossification centres, which can lead to premature degeneration. Another accessory bone that may be present is an os talonaviculare dorsale, located within the dorsal aspect of the talocalcaneonavicular joint.