Illustrated Encyclopedia of Human Anatomic Variation: Opus V: Skeletal Systems: Leg

Ankle and Foot: Tarsals, Metatarsals and Phalanges

Ronald A. Bergman, PhD
Adel K. Afifi, MD, MS
Ryosuke Miyauchi, MD
Peer Review Status: Internally Peer Reviewed

The tarsal bones are variable in size, form, and number. About 30 accessory tarsal ossicles have been described. The diversity has been related partially to their function in weight bearing. The tarsal bones are commonly reduced to six through the fusion of the calcaneous and the navicular.

Additional elements to the normal seven may occur in the tarsus. The most frequent is the presence of a triangular bone (os trigonum), which may result from lack of fusion between the lateral tubercle of the posterior process of the talus and the primary ossification center of the bone. Its frequency is reported variously as 2.5-11.4%. The next most frequent variation involves the independent ossification of the plantar half and the dorsal half of the medial cuneiform bone (os cuneiforme mediale bipartum). Additional elements may be found, probably related to developmental problems of the calcaneus and navicular bones. Some of these additional elements are found (a) in relation to the medial cuneiform and the first and second metatarsals (os intermetatarsum is found in 10% of feet), (b) on the medial side of the cuneonavicular joint (os paracuneiforme), and (c) between the fifth metatarsal and cuboid (os vesalianum). The tuberosity of the navicular may be a separate bone (os tibiale externum). A secondary cuboid has been described arising from the navicular as a projection that separates from the lateral side of the plantar surface of the bone. At the posterior end of the sustentaculum there may be an unusual bone called os sustentaculi proprium. The os calcaneum secundarium is a rare small bone present on the dorsum between the calcaneus, cuboid, navicular, and the head of the talus.

On the dorsum, at the proximal ends of cuneiforms I and II, a free intercuneiform bone (os intercuneiforme) may occasionally be found in the intercuneiform fossa.

The lateral portion of the navicular may be united with the cuboid, the lateral portion thus being separated from the remainder of the navicular. This results in the articulation of the cuboid with the talus.

Congenital talonavicular synostosis has been reported. An additional facet has been observed on the lateral surface of the calcaneus anterior to the peroneal trochlea in about 7% of cases.

The accessory navicular is the largest supernumerary bone of the foot, and the reported incidence found in normal feet is about 10-21% (Kiter). Three types of accessory navicular bone (AN) have been described: Type 1 or tibiale externum is a sesamoid bone of 2x3 mm in diameter and embedded within the distal portion of the tibialis posterior tendon. The Type 2 AN is a triangular or heart-shaped bone measuring up to 12 mm. Type 3 AN is a prominent navicular tuberosity and is thought to represent a fused type 2 AN. Types 2 and 3 have clinical importance (Kiter). The tibialis posterior may insert directly into an accessory navicular without extending to the sole of the foot.

The metatarsals may be reduced in number to four elements.

Sesamoid bones occur at joints other than the first metatarsophalangeal joint, but these are more rare than those found in the hand. In a study of 246 feet, Pfitzner found two sesamoids in the fifth toe in about 6%, and one sesamoid in the second toe in about 2%. Sesamoids also were found at the interphalangeal joint of the great toe in 55% and in the same joint of the second toe in about 1%.

Phalanges may be increased or decreased in number and size. The number of digits may be increased or decreased. Octodactyly has been reported. The digits may be fused together or absent.

A critical review of tarsal variations has been provided by R. O'Rahilly.

An Anomalous Tarsal Bone.

Septodactyly (manus) and Octodactyly (pedis).

Tarsal Accessoria. Tarsal Fusions.

Supernumerary Tarsal Bones.

Duplication of the Great Toe. Duplication of the Little Toe. Hexadactyly (manus).

Polydactylism.

Os Vesalianum.

Duplication of the Great Toe.

Supernumerary Tarsal Bones.

Duplication of the Fifth Toe from Bifid Fifth Metatarsals.

Doubling of the First Toe, Part 1 of 2

Doubling of the Toe, Part 2 of 2

Trigonum, Part 1 of 8

Trigonum, Part 2 of 8

Trigonum, Part 3 of 8

Trigonum, Part 4 of 8

Trigonum, Part 5 of 8

Trigonum, Part 6 of 8

Trigonum, Part 7 of 8

Trigonum, Part 8 of 8

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