Sesamoiditisis refers to proliferative or degenerative bony changes of the horse's proximal sesamoid bones, which are the paired teardrop shaped bones located behind the fetlock joint. There are two general forms of sesamoiditis which have been described in horses, a 'periostitis' form and an 'osteitis' form.

• Periostitis form: This form of sesamoiditis occurs as a result of damage to the bone-ligament interface with the sesamoid bones. This can result from injuries to the palmar/plantar annular ligament of the fetlock, distal sesamoidean ligaments of the fetlock, or the suspensory ligament branches.
• Osteitis form: Occurs as a secondary result of horses with arteriosclerosis, thrombosis, and local ischaemic necrosis.

The clinical signs observed in horses with sesamoiditis include moderate to severe lameness with swelling, heat and pain. In some cases the area of the injury may feel enlarged on palpation.

Treatment of horses with sesamoiditis depends on the underlying cause, associated with the form developed, severity, and whether horses are acutely or chronically affected. Recent studies suggest that horses with severe sesamoiditis are five times more likely to develop suspensory ligament branch injury (SLBI) in yearlings which are clinically normal at the time of the sale.