Osselets refers to the inflammation of the connective tissue that is around the cannon bone and the fetlock joint.
Osselets is caused by stress on the fetlock, which results in a stretching of the joint capsule.
The early stage is called green osselets, and is characterized by a hot, soft swelling at the fetlock.
Horses with short, upright pasterns are predisposed to develop osselets, as this conformation promotes concussion of the fetlock joint.
[6] Other contributors to the horse contracting the osselets include uneven tensions in the fetlock joint (usually due to poor or unbalanced footwear), irregular terrain and hasty conditioning.
The latter often causes muscle fatigue and does not protect the horse's joints from tripping or "bad steps" while working.