[JoeyP]

Here is some info for you
I'd talk to your Vet about it
Generally a evaluation of 3 or 4 does warrant surgery
but I am not sure if you should do this pre-emptively or wait until some symptoms appear

Some toy breeds, including the Yorkshire Terrier, suffer from luxating patella, in which the kneecap actually moves from its normal position. A groove in the top of the femur bone allows the kneecap to slide up and down naturally when the leg moves. Nevertheless, with luxating patella, the ridges do not develop normally, allowing the kneecap to move out of the groove. The dog will generally try to walk with the leg held off the ground when this happens.

This can be a genetic defect and should be one of the conditions you talk about with a breeder when you are looking at puppies. A reputable breeder will not only be aware of the possibility of luxating patella in toy breeds, but will also take care to use breeding parents known to be free of this condition. Surgery by a veterinary specialist may be necessary to correct this condition. With successful surgery, the pet can lead a normal, even active, life.