WMD is a deficiency in selenium/vitamin E.
Speak to your vet, and they will be able to advise on supplementing your ewes and/or other lambs born if it is likely that you have deficiencies (certain areas of the country have deficiencies due to ground being short, so grass, hay, silage etc will all be lacking).
Joint ill is a bacterial infection infecting the whole lamb, and settles out in the joints as blood supply is slower to take it away. Usually the bacteria get in through the navel, so try to make sure the pens are as clean as possible, lambs get plenty iodine or spray to help dry up and disinfect the navels. Sometimes the antibiotic injection/oral dose that lambs often get at birth to help prevent scour/watery mouth will help to keep the navel/joint ill bacteria from multiplying in the blood.
Hope you see some improvement in your little one soon.
Suzanne