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.
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).
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.
Fracture
1. X-ray findings: A bony structure with a radiographic density similar to bone can be seen in front of the calcaneus, located at the anterior superior edge of the calcaneus (i.e., the anterior process). A separated or small bony fragment may be visible, with relatively clear contour boundaries; part of it may show a distinct cortical edge (suggesting an accessory bone/additional bone), or a fracture line may be present.
2. MRI findings: Mild to moderate soft tissue edema or effusion can be observed around the anterior portion of the calcaneus. If it is an acute injury, local bone marrow edema may be observed on T2-weighted or STIR images. A relatively regularly shaped bone structure with well-defined borders (considered as Os Calcaneus Secundarius) or a wider-based fracture fragment can also be seen.
3. Adjacent joints: No obvious collapse or displacement is noted in the subtalar and calcaneocuboid joint surfaces. Mild to moderate soft tissue swelling is present, especially on the lateral side of the foot and near the distal fibular (lateral) malleolus, where effusion and signs of inflammation are visible.
Considering the patient is a middle-aged female (with increased risk factors potentially related to wearing high heels or sustained plantar flexion), has a history of foot inversion sprain, persistent pain, and localized swelling, along with imaging findings demonstrating a suspected fracture line or fragment at the anterior superior margin of the calcaneus accompanied by surrounding bone marrow and soft tissue edema, an anterior calcaneal process avulsion fracture (Anterior Calcaneal Process Fracture) is the most likely diagnosis.
If there is only a small, well-marginated bony shadow without marked surrounding edema, an asymptomatic or mildly symptomatic Os Calcaneus Secundarius may be considered. However, given the clinical symptoms (pain and swelling) and the injury mechanism in this case, a fracture is more likely.
1. Treatment Strategy
(1) Conservative Treatment:
This report is based on the clinical information and imaging results provided by the patient and is intended to offer medical reference. All diagnoses and treatment plans should be integrated with in-person specialist evaluations and further examinations. This report cannot substitute for an official diagnosis or treatment recommendations by professional healthcare institutions. If you have any doubts, please seek medical attention promptly.
Fracture
