What age is she BB ? (and how much do you want to spend keeping her going ?)
If it is mycoplasma you can treat with lincomycin or tiamulin injections for 4 days which should clear it up, or you can get fluid drained from the joint to test for it. (Obviously if there's no swelling at the hock joint it's not going to be possible to test it - but that seems to be a symptom of mycoplasma, so if there's no swelling that might be a good thing ?) According to the Pig site, sows normally develop an immunity which is also passed on to their offspring, but if she's not immune she might not be passing the immunity on to her offspring either.
I think (if it was me) I'd be trying to test for it first and take it from there - it could be something else altogether

If there's a good breed example in her latest litter, it might be worth keeping her instead - a cold, damp winter could see the sow off her feet and if she's in-pig it might be even worse

All that's assuming it
is mycoplasma arthritus - only one way to check for sure

I've not had any personal experience of it, sorry can't be of more help.