We report a case of a thirty-four-year-old woman who presented with a three-year history of recurrent hemarthroses with atraumatic pain in the right knee. The patient was diagnosed with neurofibromatosis type 1 (NF1) at 5 years of age. She had several café-au-lait spots and cutaneous neurofibroma. Examination showed a soft mass in the infra-patellar region.
The lateral conventional radiograph of the right knee revealed a lobular soft tissue density within Hoffa's fat pad (Fig. 1). The ultrasonography of this region demonstrated a homogeneous hypoechoic mass (Fig. 2). The MRI examination was performed. It showed an infiltration of the Hoffa’s fat pad by a not well defined nodular lesion measuring 12x38x39 mm, with an isosignal on T1-weighted and moderate high signal on proton density fat suppressed images (Fig. 3a, b), a bright enhancement on post-gadolinium injection (Fig. 4). Moreover, a 12mm superficial cutaneous neurofibroma was also seen at the popliteal fossa having the same characteristics of this mass (Fig. 5a-c). The diagnosis was performed after histological analysis.
NF1 is one of the most common autosomal dominant disorders affecting humans. It is estimated to affect 1 in 3, 000 newborns and 1, 000, 000 people worldwide [1]. It results from a genetic defect on chromosome 17 and is far more common than neurofibromatosis type 2 (NF2) [2].
The clinical features of NF1 include: café-au-lait spots, Lisch nodules, axillary freckling, optic gliomas and peripheral neurofibromas [3].
Neurofibroma (including variants) belongs to the benign group of nerve sheath tumours which are now included in the 2013 WHO soft tissue classification. Nerve sheath tumours include all neoplasms occurring in both cutaneous and deep-seated locations. This section describes the new category of hybrid nerve sheath tumours, which bear hybrid features of more than one type of conventional nerve sheath tumour. Hybrid neurofibroma/schwannomas are more common in NF1 patients [4].
Neurofibromas are considered to be nerve sheath tumours arising from nonmyelinating Schwann cells [2]. They are most commonly seen in patients aged between 20 and 30 years and demonstrate no sex predilection [5, 6].
Three types of neurofibroma are classically described, including localized (90%), plexiform, and diffuse lesions. Localized neurofibromas are painless slow-growing masses measuring less than 5 cm [4, 7]. They are the most common type seen with NF1. They occur anywhere in the body where nonmyelinating Schwann cells are present, both superficial and deep, in bone and soft tissue [8]. But intra-articular involvement appears quite rare, limited to a few case reports. The rare cases of hip or knee joint involvement have occurred in patients with NF1 by a process of intra-articular penetration from neurofibromas originating in extra-articular locations [9, 10].
However, our patient had a limited intraarticular neurofibroma without extension from an extra-articular neurofibroma location.
On MR images, neurogenic tumours (schwannomas and neurofibromas) are unspecific. We usually find low signal intensity on T1-weighted sequences, very high signal intensity on T2-weighted sequences, homogeneous and intense enhancement after intravenous contrast administration. We can suggest this type of tumours by a fusiform morphology that follows the course of the nerve. The “target sign” may help in the diagnosis. It consists of a central area of hypointensity with a peripheral ring of high signal intensity on T2-weighted sequences. This finding corresponds to a fibrous tissue centrally, and myxoid tissue peripherally. Unlike schwannomas, neurofibromas are not encapsulated [11].
Neurofibroma in Hoffa’s fat pad.
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Based on the provided X-ray, ultrasound, and MRI images, a soft tissue mass can be observed in the right knee joint below the patellar tendon (inferior to the patella):
Imaging suggests that the lesion is confined within the joint, with no large-scale extension outside the joint or clear invasive connection to surrounding tissues.
Considering the patient’s known history of Neurofibromatosis Type 1 (NF1), combined with imaging and clinical presentations, the following diagnoses or differential diagnoses are considered:
Based on the following:
The most likely diagnosis is: Localized Intra-Articular Neurofibroma.
Definitive confirmation still requires histopathological examination (surgical or biopsy) to rule out other pathologies.
Based on the patient’s condition and presumed diagnosis, the preliminary treatment approach may include:
This report is for reference only and cannot replace a face-to-face consultation or professional medical opinion. Specific treatment plans should be formulated by a specialist based on the patient’s actual condition.
Neurofibroma in Hoffa’s fat pad.
