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.
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.
Fibroma of the tendon sheath (tenosynovial fibroma)
MRI shows a well-defined soft tissue mass below the patellar tendon of the right knee joint. On T1-weighted images, it predominantly demonstrates iso- to slightly low signal intensity, while on T2-weighted images, it appears with medium to slightly high signal intensity. No clear destructive changes are noted in the adjacent bony structures. The lesion is located near the tendon or synovial sheath, with mild soft tissue swelling but no obvious bone involvement. On contrast-enhanced scans, the mass exhibits moderate heterogeneous enhancement.
Considering the patient’s age, the slowly growing local mass, the low-to-moderate MRI signal intensity, the pattern of contrast enhancement, and the pathological findings (lack of giant cells and hemosiderin deposition, demonstrating spindle or myofibroblastic proliferation), the most likely diagnosis is Fibroma of the Tendon Sheath (FTS). If there is any doubt, complete surgical excision followed by pathological examination can confirm the diagnosis.
The rehabilitation plan should be tailored to the patient's functional status before and after surgery. The following example assumes the patient undergoes surgical removal and then enters the rehabilitation phase:
Throughout the entire rehabilitation process, knee pain, swelling, and range of motion should be continuously assessed. If discomfort or worsening symptoms occur, seek medical evaluation promptly to adjust the training plan. For patients with underlying medical conditions or compromised cardiopulmonary function, exercise intensity and frequency should be determined more conservatively under the guidance of a specialist or rehabilitation therapist.
Disclaimer: The above report is for reference only and should not serve as the sole basis for final medical diagnosis or treatment. Please consult a specialist or professional medical institution for specific diagnosis and treatment.
Fibroma of the tendon sheath (tenosynovial fibroma)
