I agree with all of the above. Tendinitis is preferred.
But tendonitis has become accepted.
I always have to laugh a little bit about this word because I am sure that it has been misspelled so often ("tendonitis" instead of "tendinitis") that everyone is just giving up ----- and letting "tendonitis" have its way.
This is the way the English language changes.
If enough people use a word for a long enough period of time ---it generally finally becomes acceptable.
And -- there have been many many posts about this word on this board before.....