I may also get a loan (if I do not get enough inheritance to cover the costs) however, 682 is right, and I wouldn't take my own advice if I was anyone else
after all, you can always do the surgery later on, and it's a kind of crazy think to put yourself in financial stress over.
In my opinion, even if you want to get a loan and don't want to wait, save up first to make sure you'll have enough money over the year or so of limb lengthening to pay back the first year's payment, because during this time you will almost certainly not be working. (At least enough money for the first year). Even better, save up more, as it's possible if there are a lot of complications you will not be able to get back to work in time to continue paying back your loan.
Also, pay attention to how much you will need to pay back every year, and compare that to the bare minimum you might be earning in the year after limb lengthening surgery. Because I want to continue my education, and if I do so I will get a living grant, that means I will have roughly UK minimum wage to live on (that's if I don't also get a part time job, which I probably will if paying back a loan, but I also have to consider the small possibility I can't get a job for example). In the worst case scenario that I can't continue my education and all that is available for me is a minimum wage job (or nothing!) I can still get together the money to pay it back. For me, paying back the sort of loan I may go for on this amount of money is perfectly manageable and reflects my normal spending habits, but of course it really depends on the individual.
I don't know how much that would be, but I imagine a 21k loan would cost you roughly 3k euros a year for 8 years in back pay. That might be a lot of debt to live with but it all depends on your circumstances.
I'd also say, don't sacrifice education and so on in order to do the surgery if possible.