Hi
I have always been told leg, hoof injuries require tetnus shots and PCN. (other cuts injuries too) Not sure how bad your mare is hurt, but blood flow is typically slower in the legs making it harder for them to heal. I have had them step on roofing nail and had to pull it out. I have poured iodine in the hole with the other two things above mentioned. I give the tetnus antitoxin (fast acting) even when the horses are vacinated regularily. This is what the vets do too when they come. Good luck, it may start hurting more as time goes by as well.
Beth