Well if your horse has thrush you can try treating it with Sudocrem balls.
First clean your horse's foot and if you wish flush/spray with something completely innocuous like dilute cider vinegar. Then stuff any cracks/crevices with the Sudocrem balls. Pictures below.
Pull off a small amount of cotton wool - about the size of a 10 pence piece or small gum ball (remember my hands are quite small)
Roll it around in the Sudocream. Make sure your hands are clean. You can make loads in advance and either store them in the tub or in a clean plastic bag.
I did say it gets messy.
Take one clean foot with thrush (see grotty central sulcus).
Stuff Sudocrem ball in the crevice and you are done. Plug all the holes/crevices and leave in situ.
Repeat daily if possible until the thrush has cleared. This may take a long time. But at least you know with the crevice plugged it will stop any more dirt and painful stones wedging in there.