Synostosis may be seen between any two tarsal bones, but the most commonly encountered coalitions are calcaneonavicular and talocalcaneal.
| A |
B |
C |
The presence of a calcaneonavicular coalition (also known as calcaneonavicular bar) is not uncommon. It is best viewed in the medial oblique view (A), but calcaneonavicular coalition can also be recognized in the lateral view alone as an extension or elongation of the anterior calcaneal process superiorly (B). This finding has been referred to as the "anteater nose" . The presentation of the calcaneonavicular coalition in the medial oblique view varies, depending on the type of union. The superomedial aspect of the anterior calcaneal beak and inferolateral aspect of the navicular are continuous as one bony structure if there is osseous coalition. However, the fibrous or cartilaginous calcaneonavicular coalition is more frequently encountered. In the latter cases, the two bones will be in close apposition and appear to articulate with one another (C). The margin of each bone may be quite irregular and sclerotic, resembling degenerative arthritis. In contrast, the two bones may be found in close anatomic relationship to one another, yet no obvious articulation or marginal sclerosis is identified. Though this latter presentation represents variant form, it is questionable whether or not it truly represents calcaneonavicular coalition. It has also been suggested that the os calcaneus secundarius, found at the same site, is related to the formation of the calcaneonavicular coalition .
Identification of a calcaneonavicular coalition is best visualized if the foot is obliqued approximately 45 degrees relative to the x-ray film (i.e., the medial oblique view). If less than 45 degrees is attained, there will be superimposition of the anterior calcaneus and navicular, mimicking a calcaneonavicular coalition. Proper foot position can be determined by examining the relationship of the cuboid and third cuneiform bones in the radiograph. If the two bones are separate and distinct (such that the cuboid is wholly isolated), the oblique position was performed properly. If not, a distinct space will not be seen between the two bones. The latter case should also not be misinterpreted as coalition.
The talocalcaneal coalition receives the most attention in the literature of all the tarsal coalitions. Coalition of the middle talocalcaneal joint region is the most frequently encountered of the three possible talocalcaneal coalitions, anterior, middle, and posterior. Anatomically, the middle talocalcaneal coalition is either extra-articular (located between the posterior aspect of the sustentaculum tali and the talar body) and/or intra-articular . It may either be complete, nearly complete, incomplete, or rudimentary . Therefore, varying radiographic findings can be attributed to variant presentations. Additionally, the presence of an os sustentaculum tali has, like the os calcaneus secundarius, been suggested to contribute to the formation of coalition .
Radiographically, the complete middle talocalcaneal coalition is best appreciated in the axial (Harris Beath) view (A, below). (B) is an example of a normal axial for comparison:
A  |
B  |
The joint space will be obliterated if osseous coalition is present; gross joint space narrowing and subchondral sclerosis are seen if fibrocartilaginous. In the latter case, the middle subtalar joint appears obliquely (approximately 45 degrees) oriented. An os sustentaculum tali may rarely be identified in association with middle talocalcaneal coalition.
Though it is not obvious in the lateral foot view, specific radiographic findings may be identifed, leading to its diagnosis:
A  |
B  |
Normally the talar lateral process is triangular shaped. When middle talocalcaneal coalition is present (A), this landmark frequently appears rounded or even flattened inferiorly. Also, the shadows of the sustentaculum tali and the talar posteromedial process are normally separate and distinct (B). In contrast, the inferior margins of these structures will be continuous with one another if there is middle talocalcaneal coalition (A). This finding has recently been coined the "C sign" . Less frequently, one may see an osteoarthritic articulation between these two anatomic landmarks. An exostosis or "beak" along the superior aspect of the talar neck has also been associated with talocalcaneal coalition. But, this latter finding is not a reliable indicator of coalition since it can be seen in its absence.
 |
 |
© Copyright 1998, Robert A. Christman, D.P.M.
These articles and figures may not be published, reposted, or redistributed without permission from Dr. Christman.
This page was updated May 5, 1998.