Why is a problem with tendons called tendinopathy?

The etymological path of of the word tendon is fairly winding. It ultimately comes from Ancient Greek τένων (transliteration: tenōn), meaning "tendon", which was pronounced with no d-like sound after the n. The stem of this Greek word, found in forms like the genitive singular τένοντος (transliteration: tenontos), was τενοντ- "tenont-". There are a few English words built on the Greek stem, such as tenontoplasty, tenontodynia, tenontitis, tenontomyoplasty.

English tendon is from a Medieval Latin noun tendo or tendon which was irregularly adapted from the Greek word. The stem of this Latin noun is either tendōn- or tendin-. (Note: when citing Latin forms, I'm going to omit vowel length markers on the final syllable, as the length of o in this position varied in Medieval Latin.)

The insertion of d in the Latin word is quite irregular, and the OED says it is probably due to analogy with the Latin verb tendere "to stretch". In contrast, the adaptation of the end of the word from Greek -ων to Latin -o, and the use of either -ōn- or -in- in the Latin stem, follows certain patterns that are common in Latin nouns.

For some reason, native Latin nouns with stems ending in -ōn-, and some with stems ending in -in-, developed the ending -ō, without n, in the nominative singular form (this form was used as the lemma or citation form of Latin nouns). Because of this, Greek nouns that ended in -ων -ōn in the nominative singular often were taken into Latin as noun the ending -ō in the nominative singular, and with stems in -in- or -ōn-. An example of a borrowing from Greek that was adapted with a stem in -ōn- is Latin leo, leōnis "lion", from Greek λέων leōn.

So the change of o to i in tendinopathy is not completely arbitrary, but it isn't completely predictable either. It's a little odd, and as far as I know, there's no particular reason why the word couldn't have been formed as "tendonopathy" instead; or, on the other hand, why the English noun couldn't have taken the form "tendin" (like margin, origin, virgin) rather than "tendon".