MRI of tarsal coalition: frequency, distribution, and innovative signs.

OBJECTIVE: Tarsal coalition is uncommon, but not rare. Since it may present with chronic symptoms and since prior studies have shown that magnetic resonance (MR) has a high accu- racy for diagnosis, we evaluated the prevalence, frequency, location, and types of coalition, as well as secondary signs, as seen in a large clinical MR population.

SUBJECTS AND METHODS: Initially a "word search" program was used to ind all diagnoses of "tarsal coalition" in our RIS database as part of 27,483 ankle MRIs performed over a ten-year period. We revaluated 101 of 169 identiied cases for location (calca- neonavicular vs. talocalcaneal/subtalar) and type (osseous, cartilaginous, or ibrous, utilizing previously described MR criteria) of coalition. Additional imaging indings, including marrow edema and fractures, and associated secondary signs, such as talar "beaks," the "anteater" (hyperplastic anterior calcaneal process) sign, and the innovative "reverse anteater" (hyperplastic navicular) and "drunken waiter" (hypo/dysplastic sustentaculum tali) signs, were reported as well. As a second part to the study, in order to more ac- curately determine prevalence in a population requiring an MRI of the ankle, two observers reviewed 667 consecutive ankle MRIs performed on 640 patients. Patients less than 12 years of age (N = 20, 3.0%) or with nondiagnostic exams (N = 73, 10.9%) were excluded. Note was made: 1. if a coalition was present, 2. if it was deinite or possible, 3. of the location, 4. of the type, 5. of secondary signs, and 6. the frequency of bilaterality.

RESULTS: In the initial retrospective study, coalitions were identiied in 0.6% of exams performed, including 78 calcaneonavicular coalitions. Seven fractures were seen, all in the ibrous calcaneonavicular subgroup. We found only eight cases (10.3%) of the "anteater" sign, while identifying 19 cases (24.4%) of the "reverse" anteater sign in the calcaneonavicular group. Talar "beaks" were seen in 15 cases (19.2%) of the calcaneonaviuclar group and nine (39.1%) of the subtalar group. Sixteen (69.6%) of the subtalar cases demonstrated the "drunken waiter" sign, a dysplastic sustentaculum. We saw a dramatic difference in frequency of tarsal coalitions when we applied a prospec- tive approach; 11.5% of all patients (66/574) had a coali- tion, while 12.2% of studies (70/574) demonstrated tarsal coalitions: 25.7% (18/70) were subtalar, 71.4% (50/70) were calcaneonavicular, and 2.9% (2/70) had both. The calcaneonavicular coalitions were roughly evenly divided between the cartilaginous (56%) and ibrous (44%) subtypes, without any osseous coalitions being identiied. The subtalar coalitions were also nearly evenly divided between osse- ous/partially osseous (33.3%), cartilaginous (27.8%), and ibrous (38.9%). Talar "beaks" were seen in 25 cases (50%) of the calcaneonavicular group and ive (27.%) of the sub- talar group. We identiied six cases (12%) of the "anteater" and nine cases (18%) of the "reverse anteater" signs in our calcaneonavicular group.

CONCLUSION: Tarsal coalitions ap- pear to be more common than previously described. This is likely to be because ibrous and cartilaginous coalitions are infrequently recognized by plain radiography. The frequency of tarsal coalitions may be as high as 11%. We also found a disproportionately higher relative frequency of calcaneo- navicular coalitions, either because subtalar coalitions are more subtle on MRI or because calcaneonavicular coalitions can be overly diagnosed. Calcaneonavicular coalitions tend to be overwhelmingly nonosseous, while there is a more even distribution for subtalar coalitions. Newly described signs, such as the "reverse anteater" and "drunken waiter," are commonly seen in calcaneonavicular and subtalar coali- tions, respectively.