CASE 8890 Published on 09.02.2011

An unusual mass in Hoffa\'s fat pad of the knee


Musculoskeletal system

Case Type

Clinical Cases


Schubert R
Radiologie am Europa-Center, Berlin, Hermann Herbst, Institute of Pathology, Vivantes Klinikum Neukölln, Berlin.


44 years, female

Clinical History
A middle-aged, otherwise healthy woman was referred for an MRI of the right knee. She complained of a mass below the knee cap, that she had already noticed a year ago and that had become moderately tender within the last few weeks. A history of trauma was denied.
Imaging Findings
MRI showed a 20 x 36 x 37 mm, smoothly marginated mass in the lateral infrapatellar fat pad of the knee with intimate contact to the dorsal surface of the patellar tendon. On T1-weighted images, it appeared isointense to muscle and displayed a concentric, onion-layer-like pattern. On T2-weighted images, it appeared heterogeneously hyperintense, with some oedema in the adjacent subcutaneous fat tissue (Fig. 1a). There were no susceptibility artifacts on gradient echo sequences (Fig. 1b). Contrast medium was not administered. The lesion was surgically excised in toto without prior biopsy. Histology showed a hypocellular tissue with abundant collagen, some hyalinization and slit-like vascular channels. The spindle-shaped cellular components were arranged in fascicles. There were no giant cells or siderophages (Fig. 2). The specimen also included parts of a synovial bursa (probably the deep infrapatellar bursa).
Fibroma of the tendon sheath (FTS), also known as tenosynovial fibroma, is a poorly recognised, benign proliferation of myofibroblasts surrounded by a dense fibrous stroma. It was first established as a histological diagnosis in 1936 [1]. Whether it represents a true neoplasm or a reactive fibrosis, is still unknown. Macroscopically, FTS typically appears as a solitary, firm nodule of up to 5.5 cm, attached to a tendon or synovial sheath. More than seventy-five percent have been described in the extremities, mostly the fingers, hands and wrists [2]. The knee and other large joints are rarely affected [3].
The main differential diagnosis is the more common giant cell tumour (GCT) of the tendon sheath (or localised pigmented villonodular synovitis (PVNS) in joints). MRI findings of FTS and GCT are quite similar in that they often show low signal intensities on native images. However, contrast enhancement seems to be less in FTS [4]. Theoretically, FTS should also appear brighter on T2-weighted images than GCT, due to its lower cellular density, and lack susceptibility artifacts on GRE due to the absence of hemosiderin deposits [5]. This view is unproven, but supported by the present case (Fig. 1), and by some scattered images in the literature [4]. Giant cells and hemosiderin-laden macrophages are typically absent in FTS. In its current classification of soft-tissue tumours, the WHO puts FTS and GCT into completely different categories [6].
FTS must also be distinguished from more rapidly growing, rare fibrous joint masses, e.g. nodular fasciitis, and potentially malignant lesions, above all, fibrous histiocytoma. This differentiation can only be made by histology [7].
The treatment of FTS consists of surgical removal, with recurrence rates depending on the accuracy of the excision [3]. Malignant transformation has never been described.
In conclusion, an "educated guess" at the diagnosis may be possible if a tenosynovial joint mass fulfils certain MRI criteria. However, the final diagnosis still requires excisional biopsy.
Differential Diagnosis List
Fibroma of the tendon sheath (tenosynovial fibroma)
Tenosynovial giant cell tumour or PVNS
Nodular fasciitis
Fibrous histiocytoma
Final Diagnosis
Fibroma of the tendon sheath (tenosynovial fibroma)
Case information
DOI: 10.1594/EURORAD/CASE.8890
ISSN: 1563-4086