Saphenous Nerve Schwannoma: A Rare Differential Diagnosis of Knee Pain in Children

Abstract

Schwannomas are uncommon benign tumors of the peripheral nerves with a low risk of malignant transformation. They rarely affect children, can affect any part of the body but rarely occur in the lower extremity and typically present with a palpable mass, pain or neurological signs. Imaging helps to orient the diagnosis and anatomopathological examination helps to confirm it. We report a case of a 12-year-old girl who presented with left knee pain with subcutaneous mass overlying the tibial tuberosity medially. Clinical examination revealed a positive Tinel's sign. Magnetic resonance imaging (MRI) of the knee was performed, which revealed an encapsulated subcutaneous soft tissue mass overlying the tibial tuberosity medially, eccentric to the course of the infrapatellar branch of the saphenous nerve. The patient was operated with total intracapsular excision of the lesion and the anatomopathological study of the surgical specimen came back in favor of a schwannoma. Postoperatively, the patient showed a good recovery with disappearance of pain and swelling.

Keywords: Child; Diagnosis; Imaging; Saphenous nerve; Schwannoma.

Figure 1.

X-ray of the left knee, face and profile, showing the presence of an opaque, oval, well-limited, subcutaneous soft tissue mass with clear contours, respecting the adjacent bony structures, opposite the upper end of the anterior tibial tuberosity.

Figure 2.

MRI of the left knee in T1 sagittal section, T2 sagittal and axial sections and T1 sagittal section after injection of gadolinium showing the presence opposite the anterior tibial tuberosity, a mass at the expense of the saphenous nerve with eccentric development, lateralized on the right, oval, appears encapsulated, in T1 isosignal (compare to the muscle) in heterogeneous T2 hyper signal, intensely and heterogeneously enhanced after injection of gadolinium, containing cystic areas, associated with a discreet infiltration of the peri lesion fat, without involvement of the bone structures in front of it, evoking first a schwannoma of the infra patellar ramus of the saphenous nerve.

Figure 3.

(A) Spindle cell tumor made of short intersecting bundles (HE × 100) and (B) tumor cells are monomorphic neuroid in appearance showing a palisading arrangement in places (HE × 200).