[TChad]

The first thing I would do in your situation is to try to find out more about the mail you received using an online search engine.

Find a text string in the invitation that is likely to be unique, and do a Google search for that probably-unique string BETWEEN QUOTES.

Back pain is so common that you might just be the target of a company's ad campaign that sent the invitations to the general population, without knowledge of the medical histories of the invitations' recipients. If so, you will probably figure that out from the results of the Internet search.

[cva66snipe]

I might have an answer. If you have ordered any orthopedic appliances {braces,canes etc} your insurer or your place of purchase could be the source. I remember years ago my wife had a colostomy done and for years she would get occasional unsolicited free samples in the mail for pouches. My bet? Insurance company. Likely in line whatever part whatever of exemption whatever in third party information sharing exemption sub part whatever. They have the most lawyers and look for the most ways to turn a buck using your info.

Second possibility less likely is social media if you mentioned having the condition and you were traced back that way. They do have programs which look for key words to advertise to you. I'd bet more on insurance though because they have the most to gain if they can point you to information in treatment which can cut their cost. Doctors have too many legalinese headaches just trying to keep their practice open without inviting potential trouble. The fact you said in one post your name was not on the letter inside but just on the envelope sounds more like you got on a mailing list for persons with that ailment.

[Osage Orange]

I believe it's HIPAA...fwiw-

I actually work in the healthcare field........