Thanks for your question regarding your 4 year old Collie boy Bristol who seems to be acutely lame on one of his front legs. This is definitely of quite a concern and if he isn't putting ANY weight on the leg at all (at rest or at a gentle walk), then there is a greater concern of a potential fracture.
In a hind limb we can commonly see non-weight bearing lameness associated with a number of other ailments other than a fracture (including patella and crutiate ligament issues for example), however a fracture is of a greater likelihood in a dog who is non weight bearing in the forelimb. It could well be that Bristol has just badly sprained one of the joints in his affected forelimb, or that he has a paw wound that you haven't yet found, however I would definitely encourage you to get him seen this evening if you can. If nothing else - we know that he is very painful, so as a bare minimum Bristol needs some pain relief from your local ER Vet this evening. A full physical exam will likely isolate the area of pain, however we generally need xrays to confirm or deny (as well as to classify) any limb fracture. Your Vet may have a pretty good idea that the limb is fractured just by gently feeling it, however I would definitely encourage at least a couple of xrays to confirm or deny the suspicion of a break.
There isn't a lot else you can do for Bristol from home and if he is non weight bearing, then he really should be seen this evening. The main reason for this being that he needs some appropriate Veterinary pain medication.
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