Sesamoiditis – What does it mean? Following on from the last two articles on lameness in yearlings this article will focus on the complex condition called Sesamoiditis.

A sesamoid is a bone that lies within a ligamentous or tendinous structure that crosses a joint. The most commonly recognised sesmoid bone is at the back of the fetlock joint. The navicular bone that lies within the foot or patella in the stifle is also a sesamoid bone. This bone is under extreme forces during fast work and a common cause of lameness. The lameness can be a result of "tearing" of the ligamentous attachment to the bone or due to fracture. Like any bone in the horse skeleton the sesamoid bone adapts to the forces placed on it during exercise and a balance between "conditioning" and injury must be achieved to maintain soundness.

When assessing yearling radiographs the significance of changes associated with the sesamoid bone can be difficult to determine. As with all radiographic findings an assessment of the significance of the changes should include an evaluation of the actual horse. When assessing yearling radiographs we are being asked to predict the likelihood of lameness and gathering of all available information is important to provide an informed opinion. However, it is the sesmoid bone that is often the culprit of differing opinions on yearling radiographs as many other factors, such as personal experience, training techniques and conformation can influence the ultimate opinion of risk. There are a number of studies assessing the significance of specific sesamoid changes which help us to objectively evaluate this bone, but even large studies cannot provide all the information we need to predict injury.

The radiographic changes commonly seen with the sesamoid bone are vascular channels, spurs, lysis or fractures. Vascular channels are common and can be thin, wide, short, regular, irregular and vary as to their exact location in the bone, although most are on the edge of the bone. They may represent an area of inflammation at the attachment of a ligament and are commonly referred to as Sesamoiditis, which means inflammation of the sesamoid. The other changes of lysis or spurs can also represent sesamoiditis. Sesamoiditis or sesamoid fracture can be a career ending injury and so some degree of caution is often placed on any radiographic change of the sesamoid.

These are images of the front fetlock with the sesamoid bone being pointed to by the white arrow. The image on left has a number of thin black lines at the edge of the sesamoid that are called vascular channels. It is thought thin channels with parallel walls have minimal clinical significance. The image on the right has one wide, irregular vascular channel and this would be regarded as having an increased risk of lameness.