Both, "Ellbogen"", and "Ellenbogen" are German synonyms for elbow. All dictionaries I consulted, including English-German dictionaries, don't say that one or the other is correct but they sometimes offer only one of both.
Even in the corresponding Wikipedia "Ellbogen" article both expressions are used, sometimes within a single parapraph. Therefore I believe we can not say which of both versions is the "better" one.
Are there indeed no expressions, be it in the figurative, or the literal meaning of "Ellbogen" where we prefer one over the other? Are both variants entirely interchangeable? Was the letter 'n' introduced, or was it omitted?