The history of orthodontics has been intimately linked with the history of dentistry for more than 2000 years. Dentistry had its origins as a part of medicine.
Addressing an envelope to a post office box address is just like addressing envelopes to any other type of address. Of course, before you can address the envelope, you need the name of the recipient, the post office box number and the city, state and ZIP code.
You can send Certified Mail to a P.O. Box in the United States. USPS Certified Mail letters are addressed and delivered to the PO Box with no changes in mailers process. In other words you just address the letter using the PO Box instead of the street delivery address.