Could you escape a cuff around your ankle by breaking your ankle? Or breaking your hand to get out of handcuffs?
No, or at least, probably not. You could break your hand or ankle trying to escape from cuffs (thought that’s rare), but actually breaking one to escape relies on a couple things to go wrong. First, either the cuff itself has to be defective, or the person who cuffed your character didn’t know what they were doing.
Thing is, I can’t find any credible reports of this working. Which doesn’t mean that it’s impossible, just that it’s extremely unlikely. I suspect the answer is that, most people who’d find themselves in a situation like this are going to have better options open to them.
There are a number of ways to get out of handcuffs. Some rely on the person putting them on screwing up, such as where the escape artist will insert a small metal sham into the cuff’s teeth, to separate the locking mechanism and open the cuffs. Others, such as picking the cuffs, is very doable with a small amount of practice.
Normally lock picking (at least in the modern world) is a fairly finicky skill. You need time, the right tools, and a lot of practice. That doesn’t hold true when it comes to handcuffs. The locks on these are fairly simplistic, out of necessity, and as a result, they’re fairly easy to open, even without dedicated tools. In fact, most handcuff picking tutorials you’ll find online suggest using things like straightened paperclips or bobby pins. Yes, really.
The critical thing to understand about most handcuffs is that they’re not, really, locked. At least not in the way your front door or car locks. The key is just a standardized design that operates interchangeably between almost all commercially available cuffs. (There are some heavy manacle designs that do use a more secure key, but these are a rarity.) The keys even work across manufacturers in most cases.
The hard part is simply getting a tool which will interact with the lock mechanism itself. Again, this can be achieved with a paperclip in seconds. Literally, in seconds.
Now, if you’ve got someone who’s willing to seriously mangle their hand or foot, they might be able to get out of a properly configured cuff. But, we’re talking about self-mutilation to the point where that appendage is never working right again.
So, this leads us back to the big problem with this approach. If your escape hinges on destroying one of your limbs, it needs to be something you won’t need to actually escape. Breaking your heel and foot into enough pieces that you could slip out of a cuff would basically mean it’d be impossible to escape.
If you’re thinking of one of these, “you have time to cut off your foot before the car’s fuel line goes up,” kinds of situations, then serious dismemberment is justifiable, but it’s still unnecessary given how easy it is to pick a handcuff. But, unless you’re working off some kind of torture porn scenario (like Saw), there’s really no reason to do this.
There is another reason why this is a bad idea. Whenever you’re writing, you need to keep an eye on what will happen next. Especially when your characters are making plans. If someone’s in a situation where they need to escape from a pair of cuffs, it’s very likely those cuffs aren’t the only thing they’ll need to deal with. Even if it just means running a few hundred feet into the woods, that’s something which will require their feet. If they’ll need to deal with doors or operate machinery, that will require their hands.
When you’re making plans, you need to make sure you don’t do anything that will invalidate later steps. There is some validity to a, “we’ll cross that bridge when we come to it,” approach, and some people really do live that way, but in a situation like this, failing to account for what happens next can be fatal (for your character.)
This doesn’t mean you have to abandon the idea entirely, and it is possible for a character to see this as the only possible way out. But, it’s extremely unlikely, they’d be able to actually get their hand of foot free, even if they did break some bones in the attempt.
There is a legitimate point to characters making sacrifices, sometimes extreme ones. There are compelling moments to be had when characters make incredibly self destructive decisions because it’s something they won’t need to complete their plan. This is especially true if they don’t plan to survive. If your character is willing to die, they may be willing to take their hand off with an axe, in order to slip out of the cuffs, and then attack the person who put them there.
So, no, probably not. But, if you’ve got a character who legitimately planned for this, there are subtler ways out.