Luxatting patellas. Very common in small dogs, or dogs who have bad muscular skeletal issues. My 30kg dog had this when she was 10 months old, she had surgery on 1 knee to correct it (as LBJ mentioned it is caused by the knee cap being too shallow so the patella slips out), the second one also had issues but as she got older it self corrected to a point and it no longer happens. I actually wish we didnt do surgery so early on the first knee as it may have self corrected but at the time I didnt know any better. In the end my dogs knee issues were caused by other issues such as hip displasia, the vet missed that fact.
Ell – Many people don’t notice luxatting patellas because it is on and off, if the patella pops out the dog is non weight bearing because it physically can’t put the leg on the ground, once the patella pops back into place the dog continues to walk as if nothing happened, but dont be fooled the patella popping in and out is very painful.
If you can find a canine rehab specialist or osteo/chiro vet in your area I would recommend you make an appointment to see one of them, they will be able to set you up with exercises to specifically strengthen the muscles and ligaments around the knee joint to help stabilise it and reduce the likelihood that in future surgery may be needed and will also help to reduce the onset of arthritis.