Os calcaneus secundarius mimicking anterior process fracture: Diagnostic pitfall in acute ankle trauma

Abstract

The os calcaneus secundarius is an accessory ossicle arising from secondary ossification centers and located near the anterior process of the calcaneus. Although usually asymptomatic, it may become clinically relevant following ankle trauma and can closely mimic an anterior calcaneal process fracture on conventional radiographs. We report the case of a 38-year-old male who presented with right ankle pain after an inversion injury sustained during a football match. Initial anteroposterior and lateral radiographs of the foot were unremarkable, while an oblique view revealed a small, well-defined ossified structure adjacent to the anterior process of the calcaneus, raising suspicion of a fracture. Computed tomography of the ankle demonstrated a well-corticated, smoothly marginated oval ossicle measuring approximately 15 mm, located between the anterior process of the calcaneus, the cuboid, and the talonavicular joint, with intact surrounding cortical outlines, allowing a definitive diagnosis of os calcaneus secundarius. The patient was treated conservatively with nonsteroidal anti-inflammatory drugs and rest, with complete resolution of symptoms within 30 days. This case emphasizes the essential role of imaging, particularly computed tomography, in distinguishing accessory ossicles from acute fractures and avoiding unnecessary treatment.

Introduction

Accessory ossicles of the foot are developmental variants that arise from secondary ossification centers [1]. Although usually asymptomatic, they may occasionally become clinically relevant, particularly following trauma, where they can mimic fractures or other pathological conditions [2].

The calcaneus secundarius (CS) is an uncommon accessory ossicle located near the anterior process of the calcaneus. Reported prevalence ranges from 0.6% to 7% in anatomical and radiologic series, depending on the population studied and the imaging modality used [3]. While generally asymptomatic, it can present with ankle pain or restricted hindfoot motion and is frequently mistaken for an avulsion fracture of the anterior calcaneal process [1,4]. In some cases, it may even mimic calcaneonavicular coalition [3].

Although not exceptionally rare, it remains underrecognized in the setting of acute ankle trauma, where misinterpretation may lead to unnecessary additional investigations or inappropriate management.

We report a case of calcaneus secundarius initially suspected to be a fracture, highlighting the key imaging findings that allow accurate diagnosis.

Case presentation

A 38-year-old male with no significant past medical history was admitted to our institution complaining of right ankle pain following an inversion injury sustained during a football match. On physical examination, swelling and tenderness were noted over the lateral ligament complex and the dorsum of the foot, with painful limitation of ankle motion and an antalgic gait.

Anteroposterior and lateral radiographs of the right foot were unremarkable (Fig. 1). However, on the oblique (¾) view, a small, well-defined ossified structure was observed adjacent to the anterior process of the calcaneus, raising suspicion of a possible fracture fragment (Fig. 2).

Fig. 1.

Anteroposterior and lateral radiographs of the right foot showing no abnormal findings.

Fig. 2.

Oblique (¾) radiograph of the foot demonstrating a small, well-defined ossified structure adjacent to the anterior process of the calcaneus.

A computed tomography (CT) scan of the right ankle, performed with axial and sagittal reconstructions, revealed a well-corticated, smoothly marginated oval bone fragment measuring approximately 15 mm in its longest axis. The fragment was located between the anterior process of the calcaneus, the cuboid, and the talonavicular joint. There was complete cortical continuity, and the adjacent cortical outlines of the calcaneus and cuboid were intact, without any cortical disruption or step-off, confirming the diagnosis of an os calcaneus secundarius (Fig. 3).

Fig. 3.

Axial (A) and sagittal (B) CT reconstructions demonstrating a well-corticated, smoothly marginated oval ossicle measuring approximately 15 mm in its longest axis, located between the anterior process of the calcaneus, the cuboid, and the talonavicular joint.

The patient was treated conservatively with the administration of nonsteroidal anti-inflammatory drugs and rest. His symptoms markedly improved within 10 days, and he was completely pain-free and able to resume full mobilization after 30 days.

Discussion

The os calcaneus secundarius (OCS) is an accessory ossicle located at the anterior aspect of the calcaneus, typically positioned between the calcaneus, cuboid, talus, and navicular bones [5]. It represents a developmental variant resulting from the incomplete fusion of secondary ossification centers during skeletal growth [4]. Anatomical studies report a prevalence ranging from 0.6% to 7%, though the ossicle is usually asymptomatic [2].

Although frequently incidental, OCS may become clinically significant after trauma or repetitive mechanical stress. Its location within the subtalar and midtarsal region can lead to altered biomechanics or impingement, particularly in athletes and physically active individuals [6]. Symptomatic presentations typically involve anterior hindfoot pain, exacerbated by weight-bearing and motion [2].

A key diagnostic challenge is differentiating OCS from an anterior process fracture of the calcaneus, which commonly occurs following inversion and plantarflexion injuries. On standard radiographs, OCS may be misinterpreted as a fracture fragment, particularly on anteroposterior or lateral views. Oblique radiographic views may improve visualization, but advanced imaging is often necessary for definitive diagnosis [5,6].

Computed tomography (CT) is the preferred modality for characterizing OCS, providing precise evaluation of cortical integrity, ossicle morphology, and anatomical relationships. CT typically demonstrates a well-corticated, smoothly marginated, ovoid ossicle, in contrast to fracture fragments, which show irregular margins and incomplete corticalization [6,7]. The presence of complete cortical margins on CT is a key distinguishing feature that reliably excludes an acute fracture. Magnetic resonance imaging (MRI) can be valuable in selected cases by detecting bone marrow edema, synchondrosis inflammation, or associated soft tissue injury, which may indicate recent trauma [2,6].

Beyond fractures, the differential diagnosis should include calcaneonavicular coalition, os cuboideum secundarium, and other accessory ossicles in the midfoot. Accurate identification of OCS prevents misdiagnosis and unnecessary immobilization or surgery [4,6].

Management of symptomatic OCS generally begins with conservative measures, such as rest, activity modification, and nonsteroidal anti-inflammatory drugs [2,6]. Surgical excision is reserved for refractory cases or when the ossicle causes mechanical impingement [4,6].

Conclusion

The os calcaneus secundarius is an accessory ossicle that may closely mimic fractures of the anterior calcaneal process on conventional radiographs. Recognition of its well-corticated, smoothly marginated morphology and typical anatomical location is crucial to avoid misinterpretation. Advanced imaging, particularly CT and, when necessary, MRI, plays a pivotal role in confirming the diagnosis, guiding appropriate management, and preventing unnecessary immobilization or surgery. This case highlights the essential role of radiology in differentiating anatomical variants from acute traumatic lesions of the ankle.

Patient consent

Consent was obtained from the patient for publication of this case report and accompanying images.