If part of the bone is eaten away you have bone infection. I have had to deal with bone infection twice on my own horses and a couple of times with horses belonging to others.
One of mine was from a hot nail and the infection went in instead of breaking out because a Vet really messed it up by digging instead of just letting it come to the coronet like it should have. I still have this horse some 15 years later after finally getting the infection cleared up. I have also had several joint infections that I have also cleared up.
The other coffin bone infection was much worse. It was a nail puncture where the dirty nail actually hit the coffin bone. That horse was eventually put down.
If you have any chance of clearing up a bone infection or a joint infection, it will take aggressive antibiotic treatment with a very powerful antibiotic. I have cleared mine up with injectable Gentmicin or with Chloramphenicol. I gave the first horse 2 IV shots a day of Gentomicin (or Gentocin) for 35 or 40 days. Gentocin is supposed to be very hard on the liver and kidneys, so I took blood serum into the Vet clinic once a week to check blood values. Luckily, I had no toxicity problems. Prolonged use of this drug can cause complete liver or renal failure in some individuals. I did not have anything to lose (nor did the horse), but frequent serum tests will show it before it gets serious. This horse's bone infection cleared up completely and the black spot on the coffin bone even filled back in over the years. She went on to be a top heading horse in the roping pen.
Chloramphenicol is quite another matter. In some places it has been completely outlawed because it is very persistent in milk and meat. It would actually be my 'drug of choice' if you can get it. I can only get in on a Vet prescription from a compounding Pharmacy. I do not know if that is even an option where you are. If it is, you can get it made into a paste and can give 1 gram 2X a day orally. I have had it clear up every joint infection i have worked with. Fantastic stuff but very difficult to get. It takes someone going the extra mile for you. I got a University Vet School to get it for me on the condition that I reported back to them on results, etc. I also cleared up Pseudomonas infections in both stallions and broodmares with it. They were supposed to untreatable according to the Vet School and the horses were scheduled to be put down.
This kind of infection (bone infection or joint infection) will never clear up with ordinary antibiotics like Penicillin or Sulfa. This kind of infection will only respond to a very powerful antibiotic that that is specific to gram negative rod infections.