Purpose: Computed tomography (CT) is the current imaging standard for confirming the diagnosis of a calcaneonavicular coalition. The purpose of this study was to evaluate three-dimensional (3D) multi-planar CT images of calcaneonavicular coalitions and adjacent tarsal relationships, compared to controls, in order to obtain an understanding of the 3D anatomy for classification and as an aid to surgical resection.
Methods: All patients who underwent CT of bilateral feet at our institution to evaluate a suspected calcaneonavicular coalition between 9/2005 and 3/2008 were retrospectively reviewed. Seventy-four feet (37 patients) were included in this analysis. In addition, 12 patients with a presumed normal calcaneonavicular relationship, who underwent CT of bilateral feet for various other diagnoses, were included as a control cohort. Seven measurements were performed on the 3D reconstructions to quantify the shape of the coalition (dorsal and plantar widths of the calcaneal and navicular contributions, coalition depth of the calcaneal and navicular contributions, and coalition length). The coalitions were classified into four types based on their appearance and their relationship to adjacent tarsal bones.
Results: 32/37 patients (86%) were noted to have bilateral involvement (69 coalitions). Coalitions were categorized into four types: Type I (forme fruste)-28%, Type II (fibrous)-23%, Type III (cartilaginous)-45% and 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.
Conclusions: 3D CT reconstructions enabled the classification of the spectrum of calcaneonavicular coalitions. The shape of the cuboid was found to correlate with the extent of ossification. In Type I or II coalitions, the cuboid extended medially plantar to the fibrous connection. In more complete Type III or IV coalitions, the cuboid was "squared off" and remained lateral to the osseous bridge. An understanding of the 3D anatomy is important when diagnosing milder forms of coalitions, and during resection in order to avoid iatrogenic injury to the calcaneus, head of the talus, or cuboid.