To be honest I do not remember the reason but it was done on two occasions for the same lameness; it may have been something with the navicular bursa. It was five years ago and the horse did not improve.
They did give him an injection for pain before they gave the other injection. I believe it had hyaluronic acid, cortisone, and something else in the injection. In my opinion, although it was painful for him, the risk for laminitis was not great. His hoof was wrapped near the top with vetrap to keep it clean.
I believe with CT scans now, the vets can better diagnose those lamenesses.