Fibroma of tendon sheath (FTS) is a rare and benign soft-tissue tumor. It is predominantly found in the fingers, hands, and wrists of young, adult males. We describe a series of three cases all presenting in the atypical knee location. We also review the literature on FTS, located both in the knee and elsewhere, looking for common clinical, imaging, and histologic patterns to help differentiate it from similar knee lesions. FTS typically presents as a painless, slow-growing, solid nodule. In the knee though, 71% of lesions present with pain/discomfort and 31% present with a palpable mass. Physical exam for knee FTS commonly reveals painful range of motion (50%), decreased range of motion (42%), and a palpable non-tender mass (33%). MRI of FTS usually reveals a well-defined soft-tissue mass, with low signal on T1, variable signal on T2, and variable enhancement. Histologically, the tumors are composed of a dense fibrocollagenous stroma with scattered spindle-shaped fibroblasts and narrow slit-like vascular spaces. Most FTS are removed by marginal excision, with 24% of lesions subsequently recurring. No lesion has ever metastasized. Despite its rarity, this lesion should be included in the differential diagnosis of a knee mass on physical exam or imaging, especially if it is painful, benign appearing, and present in a middle-aged male.
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