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First of all, it is very important to define the  "word root" in order to analyse the word "osteoarthritis" appropriately. So it might be said that a "word root" is the word part that is the core of a word and it contains the fundamental meaning of the word itself.
In the case of "
osteoarthritis", it can be found the root "arthritis" which makes references to the inflammation of the joint and the root "osteo" which is associated with the the word bone. So the term would make reference to the inflammation of the bone joint. 

Answer: Three words

osteo - of the bone

arthr - joint

itis - inflammation

Explanation:

The root word refer to the primary lexical unit of a word, and of a word family.  It is the core of a word and it contains the fundamental meaning of the word beyond which the word cannot be further broken down

Osteoarthritis is derived from the Greek words osteo which means of the bone and arthr which means joint and itis which means inflammation