Presentation
One year history of progressive painless swelling of the superolateral aspect of the left leg with numbness extending from the fibular head up to the dorsal foot. The manual muscle test revealed weakness of the extensor hallucis longus muscle.
Patient Data
Large well-circumscribed lobulated intermuscular soft tissue mass, surrounding the proximal fibula. It shows a homogeneous high signal on T1 and T2, attenuated on fat sat sequences with no thickened septa, enhancement or evidence of local invasion. There is an inter-tibiofibular and retrofibular extension encasing the common fibular nerve and its superficial and deep branches. Moderate thickening of the adjacent cortical fibula.
Annotated images showing the encasement of the common fibular nerve (red arrow) and it's deep (blue arrow) and superficial (yellow arrow) branches.
Case Discussion
The clinical presentation and the MRI features are most consistent with parosteal lipoma compressing the common fibular nerve.
Parosteal lipomas are rare benign fat-containing tumors, comprising just 0.3% of all lipomas. Typically occur in 40- to 60-year-old patients. Clinically, patients present with a slowly enlarging, painless soft tissue mass closely related to a bone +/- nerve palsy. Complete surgical resection may be performed in cases with nerve entrapment.