Painful Intra-articular Collagen Fibroma of the Shoulder: A Case Report.

Collagen fibroma is a rare benign fibroblastic tumor. It typically arises in the subcutaneous tissue or fascia and is usually asymptomatic; intra-articular occurrence and painful presentation are extremely rare. We report a case of collagen fibroma arising within the shoulder joint and associated with severe pain. A 62-year-old man presented with resting and motion-related pain in the left shoulder. Magnetic resonance imaging performed at another institution demonstrated an intra-articular lesion; however, it was overlooked due to its signal intensity similarity to that of the surrounding subscapularis muscle and joint capsule. When the patient was referred to our institution six years later, the shoulder pain had markedly worsened, with a visual analog scale score of 100 mm. The tumor enlarged between the scapula and the subscapularis muscle, markedly compressing the anterior joint capsule. We performed surgical excision, and histopathological examination confirmed the diagnosis of collagen fibroma. Both resting and motion-related pain resolved rapidly after surgery, and the patient remained recurrence-free during a five-year follow-up period. This case demonstrates that collagen fibroma can arise intra-articularly and cause severe pain, which may lead to diagnostic difficulty. For patients with unexplained shoulder pain, it is essential to perform careful imaging evaluation with consideration of intra-articular tumors.