As a bit of good hope, neither animal has had to have repeat surgery on the knees that were operated on.
As to the (expletive deleted) that said to "tell the breeder you want a new pup" I hope then meant an additional one and not for you to trade in your current one. Animals are loving creatures not bond with us and love us, not a car that you trade away when it doesn't work right -- and I am not accusing you of such callousness or you would not be concerned enough to write. Just tell those people they should maybe be glad their folks didn't take them back to the hospital to trade in, when they had something wrong.
As to getting money back from the breeder -- I'm no breeder, as I think everyone knows by now -- but you dog is 13 months old. And nothing in life is guaranteed. More than likely your Maltese just jump wrong and had a little freaky landing, like many people who sprain an ankle or twist a knee skiing -- since it is winter and I live in Denver, CO and have done that.
So that's my advice from one that's been there. It was an accident and