Hi Laura,

Sidebone is a term used for ossification of the collateral cartilage of the coffin bone, I believe it can be caused by working a horse on a hard surface or can be hereditary or even due to poor conformation. I think for the most part it doesn't bother the horse, but in some extreme cases where it does cause lameness to the horse it is difficult to treat. Stall rest and bute would be the best bet, or having a neurectomy done on the horse (in really extreme cases only!). I hope this helps, maybe somebody else can add more knowledge on the topic?