Tendinitis is the inflammation of a tendon. So why is it spelled with “din,” rather than “don”?
According to my dictionary it’s spelt tendonitis.
According to Merriam-Webster, “tendinitis” is the preferred spelling, with “tendonitis” as a variant. Its etymology comes from “tendin-”, which is the Latin combining form. “Tendon” ultimately comes from “tendere”, which means “to stretch”, which tendons do. (They connect bone to muscle.)
Whatever the spelling, tendinitis hurts like hell and takes forever to go away.
Robin
According to my medical billing statements, it’s tendinitis. It makes no sense to me. It makes cents to them, though, and as long as they pay the bills, I’m not griping.
I was going to say…
I’ve had tendinitis on and off for years, and I think it’s spelled both ways in my records. I think it depends on the preference of the doctor involved.
Robin
Just pulled a record on a dog I knew was diagnosed with bicipital tendinitis, and I see that I wrote tononitis.
MsRobyn cites the reason that I understood for the seemingly silly spelling. Those whacky latin conjugations!
[aside]That reminds me of my session with capsulitis. I couldn’t raise my arm above half-way between waist high and shoulder high. The doctor sent me to physical therapy and the therapost said:
[/aside]
What you wrote was a therapost. What you visited was a therapist. 
Stupid keyboard. The “i” and “o” are side by side.