Congenital discoid cartilage; report of a case of discoid medial cartilage, with an embryological note.

Discoid cartilage was first described in 1889 by Young, and for long was found affecting only the lateral cartilage. The “ snapping knee “ was described as a clinical sign of this condition by Kroiss in 1910. In 1930 Watson Jones described a ring-shaped medial cartilage in a thirty-four-year-old man with clicking at the inner side of his knee ; there had been no history of injury. The cartilage was circular, with a thick margin all round its circumference and a central perforation with thin margins. Watson Jones suggested that there had been an old “ bucket-handle “ tear, the inner segment of which had been carried in towards the centre of the joint. A similar specimen was found in class-room dissecting and was described by Basmajian (1952). In this case the other knee was found to have normal cartilages. Apart from these two cases we have found four cases of true discoid medial cartilage in the literature. Cave and Staples described two cases in 1941, Dwyer and Taylor one in 1945, and Smillie one in 1948. From the descriptions it seems likely that three of these belong to the category which Smillie has termed “ primitive.” Such a cartilage is a complete plate with a thick free margin which may be convex. Three of the discoid medial cartilages referred to had longitudinal tears and the fourth showed cystic degeneration and horizontal cleavage. Horizontal cleavage, which according to Smillie occurs characteristically in the primitive type, was also present in one of the three with longitudinal tears.