Congenital distal tibiofibular synostosis.

Congenital distal tibiofibular synostosis is a very rare anomaly. The cases of congenital tibiofibular synostosis reported in the literature so far have been confined to the proximal tibiofibular joint. We present an unusual case of tibiofibular synostosis that involves the distal part of the tibia and fibula based on characteristic X-ray, CT, and MRI findings.

Case report

A 25-year-old male presented to the orthopedics outpatient department with a 12-year history of increasing deformity on the lateral aspect of his right lower leg. There was no prior history of trauma, surgery, or infection. Apart from deformity and vague discomfort, he had no complaints. Clinical examination showed prominence and deformity at the posterolateral aspect of the lower fibula near the ankle. The leg lengths were equal, and no knee varus or valgus deformities were noted. There was no tenderness in the compartments of the lower leg. There was no neurological deficit or muscle wasting. Both the knee and ankle joints revealed a full range of painfree movement when compared to the normal side.

Initially, both anteroposterior and lateral x-rays of the ankle joint (including the lower tibia and fibula) showed a bony bridge causing complete synostosis of the distal tibial and fibular metadiaphysis (Fig. 1). After 1.5 years, computed tomography (CT) and magnetic resonance imaging (MRI) were done. CT demonstrated the complete fusion of the distal tibia and fibula, with fat attenuation areas at the site of synostosis (Fig. 2A). Three-dimensional CT showed that the bony bridge extended along the interosseous membrane, leading to complete fusion (Fig. 2B). MRI further confirmed the distal tibiofibular synostosis with fatty changes (Figs. 3A and B). No underlying marrow edema or osteochondroma was detected, and no features of degenerative changes appeared at the ankle joint. Surgery was offered to the patient but was refused.

Figure 1

25-year-old male with congenital distal tibiofibular synostosis. Anteroposterior and lateral radiograph of right ankle including the lower tibia and fibula show bony bridge (open arrows) causing complete synostosis of distal tibiofibular joint.

Figure 2

25-year-old male with congenital distal tibiofibular synostosis. A: CT axial sections through the lower tibia and fibula demonstrate the complete bony fusion (open arrow) with central areas of fat attenuation (black arrow) as compared to normal contralateral side. B: 3D CT shows the bony bridge along the interosseous membrane leading to complete fusion (black arrow).

Figure 3

25-year-old male with congenital distal tibiofibular synostosis. A, T1-weighted, and B, T2-weighted fat-suppressed serial axial MR images depict the bony fusion (white arrows) with fatty changes at the site of fusion (black arrow). There is no abnormal marrow signal intensity or underlying lesion.

Discussion

Tibiofibular synostosis is rarely described in orthopedic literature and has diversified causes. Proximal synostosis is usually congenital, while the distal joint involvement is acquired (1, 2). Trauma is the most common cause of distal tibiofibular synostosis (1). It may be secondary to iatrogenic causes or underlying conditions like kissing osteochondroma, fibrodysplasia ossificans progressive (FOP), subperiosteal haemorrhage (as seen in scurvy and hemophilia), and flourosis (1). There are very few isolated case reports of congenital distal tibiofibular synostosis (3). Several hypotheses regarding this congenital origin include intrauterine trauma, infection, focal inflammation, or developmental arrest shortly after joint cavitation (1).

Most cases of distal tibiofibular synostosis are acquired and occur secondary to ankle fracture (4). It generally develops 6 to 12 months after an eversion ankle sprain with disruption of the interosseous membrane (4, 5). It can occur after open reduction and internal fixation for an ankle fracture (6) or can be a complication of suture button syndesmosis fixation (7).

On both conventional and advanced imaging, synostosis appears as a bony bridge connecting the two bones with or without deformity. Imaging has a preoperative role in excluding the underlying neoplastic and non-neoplastic conditions like hereditary causes (FOP and hemophilia) or metabolic conditions (scurvy and flourosis) (1). Neoplastic conditions like osteochondroma and ossifying parosteal osteosarcoma can be excluded (8, 9).

Our patient had no history of trauma or surgery, and the long duration of the progressively increasing deformity indicated that underlying cause was likely to be congenital. CT and MR did not reveal any underlying abnormality to account for synostosis. The longstanding clinical history, along with the absence of underlying causes on imaging, suggested a congenital origin.