Tarsal Coalition Workup

Updated: Feb 23, 2021
  • Author: Louis P Vu, MD; Chief Editor: Vinod K Panchbhavi, MD, FACS, FAOA, FABOS, FAAOS  more...
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Workup

Plain Radiography

Standard plain anteroposterior (AP) and lateral radiographs of the foot do not visualize tarsal coalition well and thus are usually not diagnostic; however, they may be suggestive of this condition on the basis of secondary signs such as the talar beak or the anteater-nose sign.

Classic secondary signs of tarsal coalition are mainly seen in the lateral view. These include the following:

  • Talar beaking seen on the anterior talar side of the talonavicular junction
  • Broadening and rounding or flattening of the lateral talar process
  • Narrowing of the posterior facet

The last two are signs of degenerative changes. Any rotation of the foot may result in the appearance of a tarsal pseudocoalition on the lateral view secondary to off-plane view of the subtalar joint facets.

Some have suggested that the C-sign of Lateur, a C-shaped line composed of the dome of the talus and the inferior outline of the sustentaculum tali, is pathognomonic for subtalar coalition. [36]  A study by Sakellariou et al examined lateral radiographs of 20 patients with clinical and radiographic diagnosis of talocalcaneal coalition and compared them with those of 22 asymptomatic volunteers. [37]  Computed tomography (CT) was used as the diagnostic standard. They concluded that the C-sign was highly sensitive and specific for the diagnosis.

However, in a retrospective review of 48 patients who had lateral ankle radiographs and CT scans for nontraumatic indications, Brown et al found that the C-sign was specific but not sensitive for a flatfoot deformity and was neither specific nor sensitive for talocalcaneal coalition. [38]

Calcaneonavicular coalition (see the image below) may be suggested by the presence of the anteater-nose sign on the lateral view corresponding to an elongated anterior calcaneal process. [39]  This radiographic sign may be found on the lateral radiographs of patients aged 9 years or older with tarsal coalition.

Plain radiograph (Slomann view) showing typical ap Plain radiograph (Slomann view) showing typical appearance of calcaneonavicular coalition.

Oblique 45º views of the foot demonstrate a calcaneonavicular coalition 90-100% of the time. Only approximately 10% of the cases demonstrate a frank osseous coalition, with the remainder demonstrating increased proximity of the two tarsals, indistinct juxtaposed cortices, hypoplasia of the head of the talus, and flattening or narrowing of the navicular as it approaches the anterior calcaneus process.

A talocalcaneal coalition is best seen with the Harris-Beath axial or "ski-jump" view. This is taken with the patient standing on the cassette, bending 10º at the ankle. Harris and Beath recommended a 45º beam view originally, but they later expanded their views to beam angles of 30º, 35º, and 45º. Cowell recommended first taking the 45º beam view and then, if the middle and posterior facets are not well visualized, using a standing lateral view to measure the appropriate angle for the axial view. [40]

In cases where the middle and posterior facets are not parallel, two different angles would have to be measured and two corresponding axial views taken. If a middle facet coalition is present, the coalition is seen on the medial side. If the coalition is osseous, no joint is visualized. If it is fibrous or cartilaginous, the joint appears irregular and angled inferior-medial. In normal feet, the middle facet is usually parallel to the posterior facet on axial views.

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Computed Tomography

In the past, tomograms were necessary to demonstrate anterior facet coalitions and to confirm the presence of more difficult middle or posterior facet coalitions. Since the mid-1980s, however, coronal CT has become the criterion standard in the evaluation of tarsal coalitions, particularly for assessing talocalcaneal coalitions. [41]  (See the image below.)

CT scan appearance of talocalcaneal coalition. CT scan appearance of talocalcaneal coalition.

In 1986, Herzenberg et al correlated the use of coronal CT views to evaluate tarsal coalitions with cadaver specimens. In their study, the patient's feet were placed in a plantar position on the gantry with the CT ring in the neutral position. [9]  Other studies have used a special apparatus to dorsiflex the foot and rotate the ring to maintain a coronal axis of the subtalar joint. The advent of high-speed spiral (helical) CT scanners and advanced image reconstruction software prompted some to accept coronal reconstructions of noncoronal views, but equal diagnostic ability has yet to be conclusively established.

Upasani et al evaluated three-dimensional (3D) multiplanar CT images of calcaneonavicular coalitions and adjacent tarsal relationships in 74 feet (37 patients). [41]  In 32 of the 37 patients (86%), bilateral involvement (69 coalitions) was present. Coalitions were categorized into four types as follows:

  • Type I (forme fruste), 28%
  • Type II (fibrous), 23%
  • Type III (cartilaginous), 45%
  • Type IV (osseous), 4%

The average shape of the coalition was found to be a curved wedge, which was, on average, 16 mm wide dorsally, 7 mm wide on the plantar surface, 10 mm in length, and 25 mm in depth.

According to the authors, the shape of the cuboid correlated with the extent of ossification. [41]  In type I or type II coalitions, the cuboid extended medially plantar to the fibrous connection; in more complete type III and type IV coalitions, the cuboid was squared off and remained lateral to the osseous bridge. The authors noted that it is important to understand 3D anatomy when diagnosing milder forms of coalitions and during resection so as to avoid iatrogenic injury to the calcaneus, the head of the talus, or the cuboid.

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Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) has been advanced as another tool for evaluating cases of fibrous or cartilaginous coalitions that may not be well seen on CT; however, the evidence to date has not proved it to have a significant diagnostic advantage over CT. Although MRI has been found to be very good at detecting tarsal coalition and determining the exact type (as well as identifying associated soft-tissue abnormalities), [42]  CT is still considered to be the criterion standard and to be more cost-effective than MRI. [43]

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Bone Scanning

Bone scintigraphy has been advanced as a way of confirming suspected coalitions that are not well seen on plain radiography or as a screening tool. However, the decreasing expense of CT and the ability of CT to depict detail has reduced the utility of scintigraphy.

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Histologic Findings

The tissue of a tarsal coalition may be osseous, cartilaginous, or fibrous. Histopathologic analysis of resected nonosseous coalitions has revealed no evidence of neural elements. This absence of nerve tissue argues against the abnormal coalition tissue acting as a primary pain generator. Microfractures and histologic signs of bone remodeling near a coalition's boundary with normal bone have been identified and are likely pain generators via periosteal nerve fibers.

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