Hi Izzy. We've treated many cases with surgery -depends if the pad is swollen, as a few don't develop and can be left. The puss inside is always a solid lump in our experience and builds up unless removed. It requires good close vision and a very steady hand. You need to cut around the sore, releasing it from the pad and squeeze out the puss, without cutting flesh and starting bleeding. We then pack the wound with Teatree cream and bind it up with gauze and micropore tape. Needs to be kept dry, checked after a few days and re-bound. We have never needed antibiotics, but have them to hand. I use a stanley knife blade held in my fingers. The pads are tough and a fine scalpel may break. Obviously a two person job.
You need to establish the cause. Too high perches in the coop (depends on the breed and size of coop) or sharp bedding or sharp stuff in the ground.