Case of the Month

22 year old male with recurrent ankle sprains.

Diagnosis: Fibrous Calcaneo-navicular Coalition.

Investigation

Left Image: Lateral radiograph shows elongation of the anterior process of the calcaneus.

Centre Image: Oblique radiograph shows thin, broad and irregular joint space between the calaneus and navicular.

Right Image: Sagital MRI (proton density) confirms fibrous coalition.

 

Discussion

Tarsal coalition involves the abnormal union of two or more bones of the hind-foot or mid-foot by an osseous, cartilaginous or fibrous junction. Only approximately 10% of coalitions are osseous.

Tarsal coalitions generally present in the 2nd to 3rd decades of life, and are slightly more common in males.

Coalitions occur with an incidence of between 1 and 2 %.

Presentation includes pain, limitation of subtalar and midtarsal joint motion, laxity of the ankle ligaments, recurrent ankle sprains, peroneal spastic flat-foot and fixed flat-foot. Patients may also be asymptomatic.

Calcaneo-navicular and talo-calcaneal coalitions are the two most frequent forms of tarsal coalition, occurring in approximately equal frequency and accounting for at least 90% of tarsal coalition patterns.

Coalitions are bilateral in approximately 50% of cases. 39% of relatives of people with symptomatic coalitions have asymptomatic coalitions.

Plain radiographs can detect a large proportion of calaneo-navicular coalitions, but are less sensitive in the detection of talo-calcaneal coalitions. Osseous coalitions demonstrate a bony bridge between the two bones, while fibrous and cartilaginous coalitions are characterised by morphological abnormality of the bones involved, with cortical irregularity and narrowing of the articulation.

Calcaneo-navicular coalition occurs between the anterior process of the calcaneus and the lateral margin of the navicular. These are best evaluated on an oblique mid-foot view and show elongation of the anterior process of the calcaneus with either a bony bridge or alternatively cortical irregularity and flattening of both the calcaneus and navicular. On the lateral ankle view, the anterior process of the calcaneus may appear elongated or the talar neck shortened.

Talo-calcaneal coalition may show narrowing of the posterior facet of the subtalar joint or the so called ‘C sign’, a continuous line between the sustentaculum tali of the calcaneus and the talar dome.

A dorsal talar beak is an indirect sign of a coalition, secondary to limitation of movement at the abnormal joint and compensatory excessive motion at the talo-navicular joint.

MRI and CT may be required for confirmation of the presence of a tarsal coalition, aid preoperative planning or to exclude the condition when there is a high clinical suspicion. CT is excellent at diagnosing osseous coalitions, but may be equivocal in the detection of non osseous coalitions.

MRI is optimal for evaluating both osseous and non osseous coalitions.