Fracture of the anterosuperior calcaneal process or an os calcaneus secundarius?

Clinical History

The patient presented with midfoot pain and discomfort on weight-bearing following an inversion injury to the left foot, while descending stairs. There was moderate swelling and ecchymosis along the lateral side of the foot and pain distal to the lateral malleolus. The patient was treated with immobilization for 6 weeks and anti-inflammatory agents.

Imaging Findings

The lateral radiograph of the left foot showed a radiolucent line coursing across the anterosuperior process of the calcaneus (Fig. 1). An additional medial oblique radiograph depicted a broad-based fracture line involving the anterosuperior calcaneal process (Fig. 2). MR imaging was then performed to better assess injury. Sagittal T1-weighted MR images revealed a fracture of the anterosuperior calcaneal process, with a slightly irregular interface between the calcaneus and the bony fragment (Fig. 3). Axial T1-weighted MR images revealed to better extent the broad-based bone fragment with mild hypointensity, reflecting bone marrow oedema (Fig. 4). On the STIR MR images, marked marrow oedema was appreciated involving both the fragment and adjacent bone (Fig. 5).

Discussion

The anterior calcaneal process, also known as the anterior facet or promontory, is a saddle-shaped bony projection at the superior aspect of the calcaneus. The bifurcate ligament inserts on the process, connecting the calcaneus to the cuboid and navicular bones.
Anterior calcaneal process fractures usually occur during inversion with the foot in plantar flexion causing stretching of the bifurcate ligament. With the foot in plantar flexion, as occurs with wearing high-heeled shoes, there is higher incidence of this fracture in women [1]. The characteristic clinical finding is pain distal to the lateral malleolus and 3-4 cm anterior to it.
The os calcaneus secundarius (OCS) is an accessory ossicle of the anterior calcaneal process at the interval between the anteromedial aspect of the calcaneus, the cuboid, the talar head and the tarsal navicular [2]. It is a rare ossicle with a prevalence of 0.6% to 7% [1, 3]. OCS is more common in children, however, with an estimated prevalence of 6% to 11%, most likely due to supernumerary ossification centres fuse during childhood [4]. The OCS is attached to a crescent notch in the anterior calcaneal facet by a fibrocartilaginous band [1, 5]. It has been suggested that the OCS derives from fractures of the anterosuperior calcaneal process. These fractures are avulsion injuries of the bifurcate ligament caused by inversion and forced plantar flexion of the foot. Rarely, fractures may be caused by eversion on a dorsiflexed foot [1].
Although the OCS is considered a clinically insignificant normal anatomic variant, it may cause pain, sustain or simulate fracture, or precipitate early arthrosis due to repetitive trauma or chronic stress [5]. Furthermore, the ossicle may cause limitation of the range of motion of the subtalar joint resembling calcaneonavicular coalition [5]. Treatment of a symptomatic OCS includes non-steroidal anti-inflammatory drugs and rest.
Fractures of the anterosuperior calcaneal process may be particularly subtle and can be easily missed on conventional radiographs. Special projections (i.e., medial oblique view) are required to detect suspected fracture. When radiographic or CT findings are unclear and there is clinical suspicion of fracture, MR imaging is warranted. On MR imaging, an ovoid small and well-corticated ossicle favours the diagnosis of OCS [3]. Larger bony structures with a wider proximal base and adjacent bone marrow oedema suggest fracture.
Familiarity with the presence of OCS is important to avoid delayed fracture diagnosis that may result in malunion or nonunion, or unnecessary immobilization.

Differential Diagnosis List

Final Diagnosis

Fracture

URL:	https://www.eurorad.org/case/12109
DOI:	10.1594/EURORAD/CASE.12109
ISSN:	1563-4086