Talonavicular coalition
Overview
Tarsal coalition is a condition in which 2 or more bones in the midfoot or hindfoot are joined. The most common types of coalitions are those between the calcaneus and either the talus or the navicular bones. Patients with this congenital condition usually present during late childhood or adolescence, but presentations in adulthood have been reported. The condition usually presents as recurrent sprains and pain in the midfoot and has been associated with peroneal spastic flatfoot, fixed flatfoot, and other abnormalities of the foot (also see the eMedicine article Acquired Flatfoot and the Medscape article Flat Feet). Radiographic evaluation has been revolutionized by the use of CT scanning. Early treatment usually involves conservative modalities, including immobilization, while surgical intervention includes resection of the coalition or arthrodesis of degenerated painful joints
Treatment
Plain anteroposterior (AP) and lateral radiographs of the foot are usually not diagnostic but may be suggestive of tarsal coalitions. Classic secondary signs of tarsal coalition are mainly seen in the lateral view. These include talar beaking seen on the anterior talar side of the talonavicular junction, broadening and rounding or flattening of the lateral talar process, and narrowing of the posterior facet. The last 2 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.23 A study by Sakellariou et al examined lateral radiographs of 20 patients with clinical and radiographic diagnosis of talocalcaneal coalition and compared them to 22 asymptomatic volunteers.24 CT scans were 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.25 Calcaneonavicular coalition may be suggested by the presence of the anteater-nose sign on the lateral view corresponding to an elongated anterior calcaneal process.26 This radiographic sign may be found on the lateral radiograph of patients aged 9 years or older with tarsal 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 2 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.27 In cases in which the middle and posterior facets are not parallel, 2 different angles would have to be measured and 2 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. In the past, tomograms have been 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 scans have become the criterion standard in the evaluation of tarsal coalitions. In 1986, Herzenberg et al correlated the use of the coronal CT views to evaluate tarsal coalitions to 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.10 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 CT scanners and advanced image reconstruction software has prompted some to accept coronal reconstructions of noncoronal CT views, but no study has demonstrated equal diagnostic ability. MRIs have been advanced as another tool to evaluate cases of fibrous or cartilaginous coalitions that may not be well seen in CT scans, but no good study has been performed demonstrating significant diagnostic utility over CT scans. Although MRI has been found to be very good at detecting tarsal coalition, CT scanning is still considered to be the criterion standard and to be more cost-effective than MRI. Bone scintigraphy has been advanced as a way to confirm suspected coalitions that are not well seen in plain radiographs or as a screening tool. However, the decreasing expense of CT scans and CT scans' ability to depict detail has reduced the utility of scintigraphs