Tarsal Coalitions: Radiographic, CT, and MR Imaging Findings

Abstract

Background: Tarsal coalitions affect up to 13% of the population and can be a cause of chronic ankle and hindfoot pain. They can be subdivided as osseous, cartilaginous, or fibrous types, each with unique radiographic, CT, and MR imaging findings. In particular, MR imaging offers the unique ability to determine the exact type of tarsal coalition that is present as well as whether any associated soft tissue abnormalities are present.

Questions/purposes: The purposes of this paper were to (1) review the anatomy of the hindfoot; (2) review the radiographic, CT, and MR imaging findings of tarsal coalitions; and (3) review the imaging appearance of the specific types of tarsal coalitions.

Methods: Online searches were performed using Google Scholar with the search criteria of "tarsal coalition," "hindfoot anatomy," and "subtalar coalition," and limiting the searches to papers published in the last 10 years in major radiology journals.

Results: The anatomy of the hindfoot is complex but essential to understand. There are various radiographic, CT, and MR imaging findings that can be consistently noted in cases of tarsal coalition. The specific types of tarsal coalition demonstrate characteristic imaging findings.

Conclusions: Knowledge of the normal anatomy of the foot, in particular the hindfoot, combined with the knowledge of the imaging characteristics of different histologic subtypes of coalitions (osseous, cartilaginous, and fibrous) is essential for interpreting radiographic, CT, and MR images of the ankle and foot.

Keywords: calcaneonavicular coalition; subtalar coalition; tarsal coalition.

Fig. 1

Search strategy and criteria.

Fig. 2

Normal anatomy of the foot. Schematic of the hindfoot and midfoot from a superior view demonstrating the normal bones and ligaments.

Fig. 3

Normal anatomy of the foot. Schematic of the calcaneus from a superior view demonstrating the relationship of the three subtalar joints.

Fig. 4

Forty-two-year-old man with chronic ankle pain. Lateral radiograph of the foot demonstrates the “C sign” (arrows), talar beak (asterisk), and failure to visualized a normal subtalar joint. The findings are consistent with middle subtalar joint osseous coalition.

Fig. 5

Thirty-seven-year-old man with chronic ankle pain. a Lateral radiograph of the ankle demonstrates the “C sign” (arrows) and failure to visualize middle facet, known as the absent middle facet sign. The findings are consistent with middle subtalar joint osseous coalition. b Normal lateral radiograph of the contralateral ankle demonstrates absence of the “C sign” and a normal appearing middle subtalar joint (arrowheads). It is important to remember that visualization of the middle subtalar joint depends on true lateral technique. If the lateral view is technically limited, the middle subtalar joint may not be visualized due to technical error. In particular, positioning of the foot in abduction or inversion may obscure the middle facet joint space due to the overlap of the lateral process of the talus and/or the sustentaculum tali.

Fig. 6

Fifty-two-year-old man with chronic ankle pain and decreased range of ankle motion. Coronal reconstructed image through the level of the middle subtalar joint demonstrates joint space narrowing, sclerosis, bone proliferation, and irregularity of subchondral bone plate (arrow). The findings are consistent with a non-osseous middle subtalar joint coalition.

Fig. 7

Sixty-four-year-old woman with chronic hindfoot pain. Sagittal (a) and coronal (b) CT reconstructed images through the middle subtalar joint demonstrate bony coalition (asterisk). The findings are in keeping with an osseous middle subtalar joint coalition.

Fig. 8

Thirty-two-year-old female with vague ankle discomfort. Sagittal (a) and coronal (b) T1-weighted images through the level of the middle subtalar joint demonstrate an osseous coalition as evidenced by continuity of bone marrow signal across the expected region of the middle subtalar joint space (arrows).

Fig. 9

Fifty-nine-year-old man with ankle swelling and pain. a Sagittal T1-weighted image through the middle subtalar joint demonstrates narrowing of the joint with an irregular cortical edge/subchondral bone plate (arrow). Also note the talar beak (asterisk). Coronal T1-weighted (b) and T2-weighted fat-suppressed (c) images through the middle subtalar joint show the bony proliferation of the sustentaculum tali (arrowhead), the abnormal narrowing and oblique orientation of the joint (curved arrow), and associated bone marrow edema. There is intermediate signal at the level of non-osseous coalition, denoting a cartilaginous coalition of the middle subtalar joint.

Fig. 10

Twenty-one-year-old man with chronic heel pain. Sagittal T2-weighted fat-suppressed image through an abnormal middle subtalar joint (arrow) demonstrates narrowing of the joint space, irregularity of the bone edges, and thin bands of decreased signal intensity. The findings are in keeping with a fibrous middle subtalar joint coalition.

Fig. 11

Thirty-six-year-old man with chronic posterior ankle pain. a Sagittal reformatted CT image demonstrates an area of osseous proliferation, approximation, and irregular borders at the subtalar coalition between the posteromedial process of the talus and the posterior surface of sustentaculum (arrow). The findings are consistent with a non-osseous extra-articular talocalcaneal coalition. Note the normal middle subtalar joint (arrowhead). b Axial CT image through the area of coalition on the medial aspect (arrow) shows the bony abnormality and proliferation that bulges into the tarsal tunnel region. Given its location, it may be a cause of tarsal tunnel syndrome. c Surface rendered three-dimensional image, viewing from medial to lateral, demonstrating the area of abnormal coalition (arrow) and the normal middle subtalar joint (arrowhead). d Surface rendered three-dimensional image, viewing from posterior to anterior, demonstrating the area of abnormal coalition (arrow).

Fig. 12

Thirty-nine-year-old man with chronic posterior ankle pain. a Coronal T1-weighted image through the ankle demonstrates intermediate signal intensity between the talus and sustentaculum tali (arrowhead) with associated irregularity of the bony margins. b Sagittal T2-weighted image with fat saturation demonstrates an extra-articular cartilaginous coalition between hypertrophied posteromedial process of talus and posterior surface of sustentaculum tali. Note the bony irregularity and the intermediate-to-slightly increased signal between bony surfaces denoting cartilaginous coalition (arrowhead). Small cystic changes are also present in the posteromedial process of talus.

Fig. 13

Twenty-eight-year-old woman with palpable abnormality along the medial aspect of the ankle. Sagittal T1-weighted image through the medial aspect of subtalar joint (a) and coronal T1-weighted image posterior to subtalar joint (b) demonstrate low to isointense T1 signal along the posterior aspect of the middle subtalar joint between the posterior medial process of the talus and posterior aspect of sustentaculum tali (arrow). The findings are consistent with a fibrous extra-articular talocalcaneal coalition. Also note the normal middle subtalar joint more anteriorly (arrowhead).

Fig. 14

Forty-three-year-old woman with pain in the region of the distal Achilles tendon. Sagittal proton density-weighted image at the level of the middle subtalar facet demonstrates an accessory articular facet between the posterior margin of the sustentaculum and the posteromedial process of the talus (arrow). This facet is a normal variant and is usually continuous with the medial aspect of the posterior subtalar joint. In contradistinction to tarsal coalitions, note that there is no irregularity associated with the margin of the bones. The middle subtalar joint (asterisk) appears normal. The Achilles tendon demonstrated findings of paratendinitis (not shown).

Fig. 15

Twenty-six-year-old man with chronic ankle pain. a Oblique radiograph of the right hindfoot demonstrates narrowing of the calcaneonavicular space (circle). The findings are consistent with a non-osseous calcaneonavicular coalition. b Comparison view of the contralateral foot demonstrates the normal wide space between calcaneus and navicular bones (dashed circle).

Fig. 16

Thirty-seven-year-old woman with chronic ankle pain. Lateral radiograph of the ankle demonstrates elongation of the anterior process of the calcaneus (dashed line) with associated blunting of the tip of the anterior process of the calcaneus (asterisk), known as the anteater sign. The finding is consistent with an osseous calcaneonavicular coalition.

Fig. 17

Fifty-one-year-old man with chronic midfoot pain. Internal oblique radiograph of the foot demonstrates prominence of the posterolateral aspect of the navicular bone (asterisk). Prominence of the posterolateral aspect of the navicular is known as the reverse anteater sign. Also note that on this oblique view the calcaneus and navicular surfaces are approximated with space narrowing and irregularity of the bone margins. The findings are consistent with a non-osseous calcaneonavicular coalition.

Fig. 18

Forty-five-year-old woman with chronic ankle pain. a Axial T2-weighted image with fat suppression demonstrates bone marrow edema in the proximal aspect of the navicular (arrowhead). Increased signal between navicular and calcaneus surfaces denotes a cartilaginous calcaneonavicular coalition. In contradistinction, note that decreased signal intensity on fluid sensitive sequences would be consistent with a fibrous coalition. b Axial T1-weighted image demonstrates approximation of the joint surfaces and irregularity of the bone margins. There is hypertrophy of the proximal portion of the lateral aspect of the navicular bone (arrow), which correlates with the radiographic reverse anteater sign. c Sagittal T1-weighted image demonstrates a prominent anterior process of calcaneus (circle), the MR equivalent to the radiographic anteater sign.

Fig. 19

Seventeen-year-old boy with chronic ankle pain and decreased range of motion of the ankle. Sagittal T1-weighted (a) and T2-weighted with fat suppression (b) images demonstrate a low-signal intensity band between calcaneus and navicular (arrows) with irregular bone edges and slight adjacent edema, consistent with a fibrous calcaneonavicular coalition.

Fig. 20

Forty-year-old man with chronic midfoot pain. Oblique radiograph of the foot demonstrates joint space narrowing and irregularity of the bony margins between the navicular and cuboid (circle), consistent with a non-osseous naviculocuboid coalition.

Fig. 21

Forty-four-year-old man with vague ankle pain. Axial (a) and sagittal (b) T1-weighted images demonstrate a low-signal intensity band between the navicular and cuboid bones (asterisks), consistent with a fibrous naviculocuboid coalition. Also note the irregularity of the bone margins in the region of the coalition (arrows). c Axial T1-weighted image of a normal naviculocuboid joint in another patient. Note the normal articular cartilage and subchondral plate (circle).

Fig. 22

Fifty-year-old woman with chronic medial ankle pain. a Coronal proton density-weighted image demonstrates a low-signal band between the navicular and cuboid bones (arrow) with associated irregularity of the bone margins. Sagittal (b) and axial (c) T2-weighted images at the level of the naviculocuboid joint demonstrates joint space narrowing, irregular bony margins, and a low-signal band spanning the joint space (arrowheads). The findings are consistent with a fibrous naviculocuboid coalition. Also note the adjacent bone marrow edema (asterisks).

Fig. 23

Thirty-one-year-old man with posterior heel pain. Sagittal (a) and axial (b) proton density-weighted images demonstrate thickening of the naviculocuboid ligament (asterisks), a normal variant. Note that there are no osseous changes or irregularity at joint interface. The patient had retrocalcaneal bursitis (not shown).