Congenital absence of the medial meniscus associated with lipoma arborescens.

Congenital abnormalities of the medial meniscus are extremely rare and have been reported commonly with other deformities. We report an isolated aplasia of the medial meniscus. A 37-year-old man presented with a slow-growing painless swelling, accompanied by intermittent effusion of his left knee. Magnetic resonance imaging demonstrated frond-like proliferations of fatty synovium. He was initially diagnosed with synovial chondromatosis, but later found to have lipoma arborescens. While an arthroscopic synovectomy was being performed, congenital absence of the medial meniscus was discovered as an incidental finding.