"Luxating Patella" is a fancy term for a loose kneecap. It is very common for puppies to have loose kneecaps at an early age.
Grade 1 is when the kneecap can be forced off the base manually, but won’t come off the base on its own.
Grade 2 luxations occur when the cap will move more easily, but still snaps back into place when released. There might be occasional lameness in Grade 2 (e.g. if you see your pup carry a rear leg for two or three steps, but then continue on as normal), but the cap returns to normal on its own.
Grade 3 is when the kneecap can easily be pushed off the base and often must be manually returned to its normal position. This phase often causes some loss of function: puppies will have more frequent “skipping” episodes, might not want to jump up onto things, and may have frequent pain.
Grade 4 is when the cap falls out of position on its own and will not stay in place. In this case, the legs are painful enough that the pup tries to avoid using them. The leg cannot be fully straightened manually and there is evidence of chronic pain or disability, including poor or no jumping ability.
Severe Grade 4 means that the dog won’t use his legs at all or walks with a very stiff gait.
With proper precautions and care, Grades 1 and 2 rarely cause serious problems and most often firm up over the first year of a puppy’s life. Preventing the puppy from jumping off furniture or decks is important at this stage, as too is a proper diet that will ensure the puppy’s appropriate weight.
Most vets recommend surgically repairing dogs in Grade 3+ without question and sometimes advocate fixing Grade 2 cases. If you notice symptoms or have questions, head to the vet!