Two contrasting presentations of localised pigmented villonodular synovitis of the knee.

Localised pigmented villonodular synovitis (LPNS) is a rare condition which most commonly affects the knee. We report two contrasting cases highlighting their different clinical presentations, investigations and subsequent treatments. Arthroscopic diagnosis was performed and used as a therapeutic tool. Pre-operative MRI was also an important contributor to diagnosis. LPNS should be included in the differential diagnosis of a young adult presenting with mechanical symptoms within the knee.