Calcaneonavicular Coalition

52 yo with L foot pain

Coalition (tarsal) is an abnormal bony, cartilaginous, or fibrous union between two or more bones (hind- and midfoot). It is a fairly common condition, generally estimated to affect up to 1% of the population. Talocalcaneal and calcaneonavicular coalitions account for approximately 90% of the cases of tarsal coalition. Bilateral coalitions occur in approximately 50% of cases, and they are most often associated with calcaneonavicular coalition. Disagreement exists regarding the prevalence of the 2 coalitions. While some groups report an almost equal number of each, others report that either talocalcaneal or calcaneonavicular coalition is more common.
Calcaneonavicular coalition occurs in between the anterolateral process of the calcaneus and dorsolateral margin of the navicular bone. In patients with fibrous and cartilaginous coalitions, irregularity and narrowing of bone interfaces may occur in addition to sclerosis and eburnation. A solid bony bar forms a bridge if an osseous coalition is present. Calcaneonavicular coalition is viewed best on a 45° internal oblique radiograph.
This type of coalition is usually seen in pts between 8-12 years of age when cartilaginous coalition that results from embryologic failure of tarsal segmentation undergoes ossification. Symptoms occur because the ossification limits subtalar motion, which is required for normal walking. The recent orthopedic and radiologic literature emphasizes the need to use CT and MRI in diagnosing coalition. With CT scans, both feet are assessed simultaneously after proper uniform positioning in the CT gantry. Proper assessment of tarsal coalitions requires both axial and coronal views of the ankle and foot. A section thicknesses of 3 mm or less are optimal for evaluation. On axial views, 2 abnormalities are noted: Medial broadening of the anterior and dorsal aspects of the calcaneus can occur at the navicular interface. On coronal views, lateral bridging with protrusion of an abnormal bony mass and rounding of the talus may be present.
With MRI three views of the feet and ankle are ideal: axial, coronal, and sagittal. Bone marrow, soft tissue edema, and inflammatory changes are best evaluated by using fat-suppressed sequences such as short-tau inversion recovery (STIR) or fat-suppressed T2-weighted sequences. Osseous structures, ligaments, and tendons are evaluated with T1-weighted, T2-weighted, and fast spin-echo proton density–weighted images. Sagittal and axial images are most useful in the evaluation of calcaneonavicular coalitions. Sagittal views are particularly advantageous because of the oblique alignment of a calcaneonavicular bridge. Coronal views are valuable for assessing talocalcaneal coalitions. A continuous bone marrow bridge is visible at the junction of an osseous coalition. Marginal reactive changes can be noted. STIR, fat-saturated T2-weighted, or proton density–weighted imaging may show edema at the articulation. In nonosseous coalitions, findings such as articular narrowing and irregularity of joint space signal intensities may be present. With fibrous and cartilaginous coalitions, a more precise interpretation is obtained using MRI compared to other imaging modalities.

Suggested Reading;

Crim JR and Kjeldsberg KM. . Radiographic Diagnosis of Tarsal Coalition. AJR 2004; 182:323-328.

Newman JS andN ewberg AH. Congenital Tarsal Coalition: Multimodality Evaluation with Emphasis on CT and MR Imaging. Radiographics. 2000;20:321-332.

Lysack JT and Fenton PV. Variations in Calcaneonavicular Morphology Demonstrated with Radiography. Radiology, February 1, 2004; 230(2): 493 - 497.