The Nature of Enlarged “Vascular Channels” in the Navicular Bone of the Horse

The morphologic features of the areas along the distal margin of the navicular bone, previously termed vascular channels, and their relationship to blood vessels in this area are described. Alterations in these channels have long been considered one of the pathognomonic signs of navicular disease and suggestive of vascular change. This study of the navicular bones of 4 normal yearlings and 12 adult horses from the pathology service demonstrate that the vascular channels are lined not with blood vessels or vascular endothelium, but rather with synovial tissue displaying a normal vascular supply. The channels represent invaginations of synovial tissue from the synovial fossae into the bone and therefore are more correctly termed “synovial invaginations.” The process by which this invagination occurs remains unknown.