Nodular fasciitis with intra-articular presentation within the knee.

A female in her early 30s presented with worsening right knee pain with atraumatic onset. Imaging revealed a heterogeneous, contrast-enhancing, T1 hypointense mass in the retropatellar fat pad. Initial differential diagnosis favoured localised tenosynovial giant cell tumour. Malignancy was also considered. These alternative pathologies require more aggressive treatment modalities and demonstrate significant recurrence rates. The patient ultimately underwent open excision and histological analysis which revealed intra-articular nodular fasciitis (NF). Detection of ubiquitin-specific protease 6 (USP6) on histopathological analysis was crucial in establishing diagnosis. At the final follow-up, knee extension had improved, she was ambulating without aids, and she had returned to work. Our case emphasises that intra-articular NF is a rare entity that can mimic more common intra-articular tumours. The detection of USP6 on histopathological analysis allows accurate diagnosis and treatment.