Cysts of the semilunar cartilage of the knee. A light and electron microscopic study.

The semilunar cartilages of the knee of six cases showing various degrees of mucoid degeneration with cyst formation were studied with the light microscope. Three cases were selected for electron microscopic observation which revealed that some of the cystic spaces filled with mucoid material were lined by synovial cells. Masses of mucinous, alcian bluc positive material (acid mucopolysaccharides) were observed also among degenerated and intact bundles of collagen. It is concluded that the accumulation of acid mucopolysaccharides in meniscial cysts is at least in part a product of secretion of synovial cells. This observation explains the high incidence of recurrence of meniscial cysts after incomplete surgical excision and supports the concept that it is an active rather than a degenerative process.