Presentation
Progressively increasing painless swelling in the posterior aspect of the left knee.
Patient Data
A pedunculated multilobulated osseous outgrowth is seen from the posteromedial aspect of the fibula metadiaphysis. A lobe is seen projecting into the posterior soft tissues of the upper leg while another is seen growing away from the joint. The proximal fibular is widened and distorted.
The tibia bone and adjacent femorotibial articulations are preserved. The overlying soft tissues bulge posteriorly without evidence of inflammatory changes. The subcutaneous tissue planes are intact.
Findings are consistent with solitary fibular osteochondroma.
A large multilobulated osseous mass is seen arising from the posteromedial aspect of the proximal fibula bone, with a portion extending away from the joint but the main part extends to the posterior muscles displacing the popliteus muscle and compressing the lateral and medial heads of the gastrocnemius muscles.
The mass measures approximately 59 x 45 x 68 mm. The cartilage cap measures approximately 4.4 mm thick with intermediate to low signal on T1 and high signal on T2 weighted images. Post contrast administration, enhancement within the osseous component of the mass is observed.
No fracture of the osseous component and no bursa is seen. There is no soft tissue mass. No joint space narrowing or effusion.
Features of solitary fibula osteochondroma without evidence of malignant transformation.
Case Discussion
Osteochondromas are relatively common bone tumors accounting for 10-15% of all bone tumors. Frequently asymptomatic as in this case in which the patient presented with a painless palpable lump, other clinical presentation may include sequellae of neurovascular compression, bursal formation and bursitis.
The most common bones and regions affected are the distal femur (30%), followed by the proximal tibia (15-20%), in the upper limb, the humerus (10-20%). This present case is seen in the proximal fibular which is an uncommon presentation.
Although they are regarded as a benign lesion, they have low malignant potential which is approximately 1% (if solitary and sporadic) and 5-25% (in the setting of hereditary osteochondromatous syndromes). Features that may suggest malignant transformation include pain at the site of the mass (other than traumatic), surrounding soft tissue involvement, cortical bone destruction and thickened cartilage cap greater than 15mm which were all absent in this case.