I removed a 10 inch limb 9 years ago from a 40 inch diameter silver maple, taking care not to damage the branch collar. The cut seems to closing in and healing okay, there is only about a 4 inch hole left and will be closed over in the next year. The problem is that over the last few years, there is a hollow area developing behind the cut, it's about 10 inches back and 18 inches down behind the cut.
I'm concerned the hollow area will weaken the tree or allow disease, if it hasnt already. The rest of the tree seems fine. I've seen trees where folks have filled in hollow areas with concrete. Is there something that should be done to fill in this hollow area, or should it be left alone.