I have had two MRI's and they revealed bulging discs in my upper and mid back. These bulges can hit certain nerves and cause the numbness you are mentioning. I see a chiropractor, and he adjust these areas and also does procedures that help the bulging discs, when I go as I should, I do not have flare ups, however, if I am lax with my treatments, I get all kinds of feelings like buzzing in my neck, numbness in my hands and arm, etc. I also have Thoracic Outlet Syndrome. This is where the thoracic nerve is pinched and this causes lots of hand and arm pain as well as certain pains in the cervical area, you might have a mild case of that also.
It could be arthritis but the only way to know for sure is to have a blood test done. I would think that it would show up on the MRI if it was serious though. The numbness sounds like you may have a pinched nerve. I would ask the doctor to be referred to a neurologist to have a nerve conduction study done, This can help to find where the numbness is coming from. Either way you are still having symptoms and not feeling any better so maybe you should seek a second opinion. It never hurts to get one, I hope this helps. Good luck.