Sidebone is a common condition of horses, characterized by the ossification of the collateral cartilages of the coffin bone.
These are found on either side of the foot protruding above the level of the coronary band.
Rarely, severe lameness can be caused by fracture of an ossified cartilage, or by ossification which deviates enough to impinge on the short pastern bone.
Many horses compete actively in demanding sports with sidebone, and are not hindered in any way.
Discovery of sidebone on an equine prepurchase exam without signs of lameness or local sensitivity should not affect the purchaser's opinion of the horse.