The symptomatic os subfibulare

Clinical History

The patient presented with lateral ankle pain associated with tenderness at the tip of the lateral malleolus. She suffered a sprain of the left ankle one week before. The pain was worse with weight-bearing. There was no evidence of joint instability. The patient was treated with immobilization for 4 weeks and anti-inflammatory agents.

Imaging Findings

Anteroposterior and lateral radiographs of the ankle joint demonstrated an ossicle below the lateral malleolus. The ossicle which was separate from the fibular tip appeared divided in two parts (Figs 1a, b). Coronal T1-weighted MR images showed an os subfibulare bearing a transverse fracture through its midportion (Fig. 2). The superior fragment of the ossicle was of low signal intensity, indicating bone marrow oedema associated with the preceding sprain injury. The anterior talofibular ligament, which is the main lateral stabilizer of the ankle joint, appeared intact with low signal intensity.
On the coronal STIR MR images, the accessory ossicle appeared fragmented, with its upper portion showing high signal intensity (Fig. 3). On the sagittal STIR MR images a marked marrow oedema was seen involving mainly one of the fragments, as well as a small joint effusion (Fig. 4). Axial T2-weighted MR images displayed the ovoid shaped os subfibulare (Fig. 5).

Discussion

The os subfibulare is a normal anatomic variant that represents either an unfused accessory ossification centre or a supernumerary bone [1]. The ossicle is located under the tip of the lateral malleolus [2]. It appears toward the end of the first year of life and fuses with the metaphysis between the ages of 15 and 17 years [3]. Such ossicles rarely persist beyond skeletal maturation with a reported prevalence of 1- 2.1% [4, 5]. The os subfibulare is usually round, oval, or comma-shaped.
The os subfibulare most commonly remains asymptomatic, however, it may cause pain, sustain or simulate a fracture, or it may even precipitate arthrosis in response to overuse and trauma [5]. Arthroscopic and operative findings have shown that os subfibulare is embedded partially or completely within the fibres of the anterior talofibular ligament, with some parts exposed in the talofibular joint or covered by a synovial membrane. Ossicles may also be connected to the fibres of the posterior talofibular ligament [6]. It has been postulated that symptoms are associated with disruption to the fibrous or cartilaginous attachments of the ossicle resulting in a fracture, fibrous union, or pseudarthrosis. Mechanical irritation or joint instability may cause pain and recurrent ankle sprains [3, 7]. Furthermore, the ossicle may cause limitation of the range of motion of the ankle joint resembling avulsion fractures of the lateral malleolus. The latter are commonly caused by inversion injuries in the clinical setting of ankle sprains.
In this case, the uppermost fragment of the ossicle was of abnormal low signal intensity on T1-weighted MR images with corresponding high signal intensity on T2-weighted and STIR images, indicating bone marrow oedema due to post-traumatic fracture. No contrast medium was administered to the patient to investigate injury.
Despite its rare occurrence, it is important to distinguish os subfibulare from an acute avulsion fracture of the lateral malleolus to avoid delayed fracture diagnosis that may lead to complications as malunion, nonunion, or a chronically painful ankle. To assist the differential diagnosis, os subfibulare has a round shape and smooth, well-defined cortical margins, whereas an acute malleolar avulsion fracture fills the missing part of the distal tip of the lateral malleolus, with irregular cortical margins facing the fibular tip.
Asymptomatic patients can be treated effectively with immobilization, anti-inflammatory agents, physiotherapy and modified footwear [8]. Excision of the os subfibulare should be reserved for patients with recurrent sprains and persistent pain.

Differential Diagnosis List

Final Diagnosis

Fracture of the os subfibulare

URL:	https://eurorad.org/case/12186
DOI:	10.1594/EURORAD/CASE.12186
ISSN:	1563-4086