The options in vets surgery have really been extended by the advent of pet insurance and surgery that would have never been undertaken in the past due to financial reasons are now being pursued. That doesn't always mean it is the right thing for the animal and animals that are in pain and with affected quality of life should be humanely destroyed / put to sleep.
Where did the diagnosis of arthritis come from? In humans this is a degenerative condition and even the early onset / autoimmune / unusual forms are not present at birth. Don't know much about sheep though.
I suggest you speak to your vet about what is possible, they will be able to find out if they do not already know. Expect it to be expensive (i.e. thousands not hundreds) if it does exist. Ask careful questions about experience, prognosis, complications. Remember that this is something that you should do for the sheep, not for yourself and if there is no reasonable expectation of a good, pain free quality of life then it is not the right thing to do.
I know I may sound harsh but for many vets it is a business and it is also much easier to "have a go" at something as it's a bit different or sounds like fun than it is with people! My cat has had a perineal urethrosomy (
Don't ask) but I went ahead as it is an established procedure with known and accepted complications and expectation of a full and normal life after.