Yes we get the odd case of joint ill outdoors, although I can never be certain it isn't in lambs that were indoors briefly - although we do spray them if they come in.
Alamycin seems to work best for us; vet says use what works for you!
However, before getting out the a/bs, I'd be doing two things. 1. Check for scald - red/inflamed/weeping skin between the toes. Blue spray will sort that. 2. Assuming no obvious lumps or heat in any joints, observe for 48 hours. Most often it's an ouchie from landing badly, stepping on a thorn, or whatever, and goes away. (But if you know you
do have joint ill in the flock then yes I'd be jabbing anything limping asap too, as best treated early.)
In older lambs, conditions resembling joint ill can be due to infections borne by ticks, say our vet. We had a bad year for that a few years back - terribly lame lambs that, even though they rarely stood up didn't lose any condition. :/ Alamycin was the best for that; we said if it happened again we'd use an anti-tick med - but so far