This 28-year-old female presents with ankle and foot pain, with no recent injury and no history of surgery. Have a look at images 1 through 5, and see if you can come up with the rare diagnosis in this case.
On the images provided, note apparent absence of the tarsal navicular bone, except for a small accessory ossicle dorsally (image 6, arrow). The talar neck appears elongated and the talar head enlarged in size (better seen on images 7 and 9).
The rare diagnosis is congenital solid osseous talonavicular coalition. Note that the enlarged talar head directly articulates with the cuneiform bones. On images 7 and 10, the arrow indicates fusion of the middle cuneiform bone and base of the second metatarsal, as an additional anomaly. Note also multifocal, prominent bone marrow edema, seen as gray T1 marrow signal in the talus and calcaneus adjacent to the posterior subtalar joint on image 6 (arrows), and bright marrow signal on the PD fat sat coronal images 8 and 9 (arrows), consistent with osseous stress reaction involving body of talus and calcaneus and the lateral cuneiform and cuboid bones.
One of the rarest of tarsal coalitions, talonavicular coalition was first described by Anderson in 1879. Tarsal coalition represents a failure of embryologic mesenchymal segmentation, with calcaneonavicular and talocalcaneal coalition being more common types of tarsal coalition. The incidence of tarsal coalition in the general population has been thought to be at around 1 to 2%, but the incidence of the rare talonavicular coalition seen in this case is unknown.
Talonavicular coalition has shown a strong familial or genetic predisposition, and appears to be inherited in an autosomal dominant pattern with high penetrance and variable expression. Like other coalitions, this anomaly is frequently bilateral, and is associated with other skeletal anomalies including symphalangism, clinodactyly, clubfoot and peroneal spastic flatfoot. On exam, these patients typically demonstrate pes planus. Often, these patients are relatively asymptomatic, compared to patients with the more common calcaneonavicular or talocalcaneal coalitions which affect hindfoot mobility. When symptomatic, conservative treatment with orthotics and footwear modification is most common, although surgical treatment may be indicated for chronic painful flatfoot if conservative treatment fails.
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