Isolated Idiopathic Intra-articular Necrotizing Granuloma in a Native Knee Joint: A Rare Case Requiring a Diagnosis of Exclusion.

Idiopathic necrotizing granuloma localized to a native knee joint is an exceedingly rare entity. While non-necrotizing granulomas as defined by organized epithelioid histiocytes and multinucleated giant cells without central necrosis are more commonly associated with autoimmune, foreign body, or inflammatory conditions, the presence of necrosis raises concern for more aggressive pathology, particularly infectious etiologies such as mycobacterial or fungal disease. Accordingly, idiopathic necrotizing granuloma remains a diagnosis of exclusion, requiring systematic evaluation to rule out infectious, post-procedural, autoimmune, and systemic granulomatous causes. This case report presents a 54-year-old man with a necrotizing granuloma within the suprapatellar pouch of the right knee noted at the time of arthroscopy for a medial meniscal tear and osteoarthritis. The patient did complain of pain directly in the area corresponding to the mass, the peripatellar region, as well as the medial joint line. He was initially treated with conservative measures including acetaminophen, non-steroidal anti-inflammatory, methylprednisolone, a knee sleeve, and physical therapy. After failure to improve, the patient requested to follow up with our practice due to increasing pain and difficulty with ambulation for surgical consideration. Radiographs and MRI revealed joint space narrowing consistent with osteoarthritis and a medial meniscal tear without mention of a mass. The patient requested surgical treatment secondary to pain with activities and difficulties with his vocation as a laborer. All physicians must be mindful that necrotizing granulomas indicate more aggressive tissue injury and should raise immediate concern for infection or other aggressive pathology mandating a comprehensive diagnostic evaluation to rule out systemic conditions and to prevent a delay in diagnosis or treatment. Knowledge that necrotizing granuloma can present as a painful mass within the knee without prior surgery, prior injection, infection, or systemic disease to our knowledge has not been described.