Aim: To describe two cases of intra-articular nodular fasciitis (NF) which developed within the knee joint and were associated with the expression of the MYH9-USP6 gene fusion. Patients and methods: Two women, 30 and 56 years of age, with no history of joint disease or knee joint trauma, are presented in our cases. We report these cases describing the clinical presentation, assessment, histopathological examination, gene expression, and clinical management. Results: Both patients presented with knee pain and limitation in the range of flexion. We diagnosed our two cases as intraarticular nodular fasciitis based on histological findings and by the detection of the MYH9-USP6 gene fusion. The transcript of MYH9-USP6 gene fusion was identified by RT-PCR and direct sequencing in both cases. Conclusion: We report the first cases of intra-articular NF involving the knee joint, with identification of a MYH9-USP6 gene fusion by RT-PCR. NF should be considered in the differential diagnosis of intra-articular lesions.
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