It really depends on why your vet wants to inject your horse's coffin joints. It seems to be a fairly common joint to inject with jumping horses. As with any joint injections, generally the first time you inject it you see the greatest improvement.
If the joint is fairly intact and there isn't much arthritis, then you'll probably see a lot of improvement with the injections. Once a joint has a lot of degenerative arthritis, joint injections don't help that much.
Huge difference seen in about a week, seems to have dwindled in the past couple months but still better than before. If they are suggested by a qualified vet and seem necessary to you as well I would go for it. Not everyone will agree with injections in general though.
My horse has a bone spur on his LF coffin so as maintenance, he gets it injected. Really does help him. I think your Vet would be able to help you weigh the pros and cons and make a decision on if it's it right for your horse.
Had it done with 2 of them. Helped tremendously. Both had navicular issues. One was a leased high mileage 17 year old veteran going in wedgies and full pads. The other about 10 and plain shod or just rim pads.
This was, like, between 1994 and 2000 so it was an emerging treatment. Probably even better now.
I would caution good diagnostics are in order to be sure of what you are dealing with...and I could not watch this procedure...BIG long needle there....bleech.
When opportunity knocks it's wearing overalls and looks like work.