Most likely a tendon tear where tension on the tendon must be reduced/removed. Almost certainly the horse will be on box rest, as the shoe is being used as a splint.
Horse had a severed deep digital flexor (DDTF)tendon after it was injected into it's coffin joint and got infected (meep!) and has reverse rotation because of the severed tendon. He used the wood so he could reduce the angle easily as time went on.