Despite great home care, i have had significant bone loss at a young age. no periodontist knows why. who is the best periodontist in the world?

Best Periodontist: There is no one "best periodontist in the world" I'm sure you can find a number of highly qualified & respected local Periodontists. Speak to a number of local General Dentists, Prosthodontists or Oral Surgeons. They know who are the ones they would go to for themselves. As a last resort, go to a local Hospital Affiliated Dental Residency program, Dental School or call your local dental society.

Must be a cause: It's terrific that you have great home care, but there must be some other factor or factors that have contributed to your bone loss. May I suggest you seek treatment in a university setting where an entire department can examine you. They may have other resources at their disposal.