An eleven-year follow-up of a growing osteochondroma of the fibula causing tibiofibular synostosis

Clinical History

The patient presented with a lump and vague pain in the posterolateral aspect of her proximal calf. The lump had been growing in the region for 11 years. Physical examination revealed a hard and tender mass in the lateral aspect of the right upper calf. Routine laboratory studies were normal.

Imaging Findings

Plain radiographs of the involved calf showed a large osseous excrescence arising from the external surface of the upper fibula and containing spongiosa and cortex that were continuous with those of the parent bone (Fig. 1). The bony outgrowth was sessile with a broad, flat base. On MR images, a solitary osteocartilaginous exostosis was seen (Figs. 2-4). There was continuity of the cortical and medullary bone in the lesion with that of the parent bone, a finding merely diagnostic of an osteochondroma. The cap of the osseous protuberance displayed the imaging characteristics of cartilage, showing high signal intensity on T2-weighted spin echo MR images. Due to the large size of the osteochondroma, tibiofibular synostosis (bony bridge) was produced with pressure deformity in the nearby tibia. There was bowing of the fibula, and minimal diastasis of the proximal tibiofibular joint.

Discussion

Osteochondroma, also known as osteocartilaginous exostosis, is the most common benign bone tumour, representing 10-15% of all primary bone tumours and up to 50% of benign tumours [1]. Osteochondroma may develop after irradiation or may arise spontaneously. Lesions are composed of cortical and medullary bone with an overlying cartilage cap. The radiologic hallmark of osteochondroma is continuity of lesion with the marrow and cortex of the underlying parent bone.
Osteochondromas occur most commonly around the knee (40% of cases). The femur is the most-affected bone (30%), while tibial lesions account for 15-20% of cases [2]. Osteochondromas may be solitary or multiple as part of the hereditary multiple exostoses syndrome (HME) [1]. Complications are more common with multiple osteochondromas and include deformity (cosmetic and bone), fracture, vascular injury, neurologic compromise, overlying bursa formation, and malignant transformation. Bone deformity includes osseous bowing, malalignment and extrinsic pressure erosion of adjacent bones. When paired bones of the forearm or lower leg are involved, scalloping of the cortex may ensue.
The size of osteochondromas varies between 1-10 cm. Continued lesion growth after skeletal maturity, and a cartilage cap greater than 1.5 cm suggest malignant degeneration of an osteochondroma [3]. Solitary osteochondromas are usually asymptomatic with a male predilection ranging from 1.6-3.4 to 1. Small asymptomatic or minimally symptomatic lesions should be followed-up, while symptomatic lesions should be resected. Patients with HME require continued clinical and imaging surveillance to evaluate progression of lesions, or complications. Measurement of thickness of a cartilage cap can be accomplished with ultrasound, while CT or MR imaging are best suited for the evaluation of complex anatomic areas [1].
On radiographs, osteochondroma appears as a sessile or pedunculated bone excrescence, containing both spongiosa and cortex that are continuous with those of the parent bone. The cartilaginous cap has a variable degree of calcification.
Typically, osteochondromas tend to point away from the adjacent joint. Widening of the involved metaphysis of tubular bones may be noted. MR imaging often reveals yellow marrow centrally (which has high signal intensity on T1-weighted images and intermediate signal intensity on T2-weighted images) with a low-signal intensity cortex. The cartilage cap has low to intermediate signal on T1-weighted images and high signal on T2-weighted images. Mineralized portions in the cartilage cap remain low signal intensity with all MR sequences. After the intravenous administration of contrast, peripheral and septal enhancement in the cartilage cap can be seen.

Differential Diagnosis List

Final Diagnosis

Osteochondroma of the fibula

URL:	https://www.eurorad.org/case/12150
DOI:	10.1594/EURORAD/CASE.12150
ISSN:	1563-4086