I am wondering if we could say the original sentence as the revised ones, and if there is any difference in meaning among them?
A. When he broke his leg, he made a crutch for himself.
B. Once he broke his leg, he made a crutch for himself.
Original:
Once, when he broke his leg, he made a crutch for himself.
Well, yes, no and sort of... But wee need to examine the three sentences:
Original:
Once, when he broke his leg, he made a crutch for himself.
This sentence gives two kinds of information, one temporal, one temporal or causal:
Your first variety:
When he broke his leg, he made a crutch for himself.
A simple sequence of events - break leg, make crutch. Not much different from the original, just simpler. The emphasis has shifted slightly to the cause.
Your second variety:
Once he broke his leg, he made a crutch for himself.
Now here something nasty happens. Written like this, the once shifts his meaning to "as soon as". This sounds as if the person in the sentence had made some effort to break the leg (possibly on purpose) and once he managed to do so, he made the crutch.
If you re-write your second example, it doesn't help either:
Once he broke his leg. He made a crutch for himself.
Once gets his original meaning of "at some point in time" back, but the two sentences loose their logic connection, so no alternative either.
Conclusion:
Stick with the original or your first variety, ditch the second.
It is perfectly correct to say:
A. When he broke his leg, he made a crutch for himself.
B. Once he broke his leg, he made a crutch for himself.
(Although A. is much preferable. B sounds like 'he was waiting for the moment his leg breaks, so that he could make himself a crutch'.
"Once, when he broke his leg, he made a crutch for himself" is like using the same word twice in a sentence. Here 'once' & 'when' mean the same and very is pretty much like repeating a word.
