This information is to help you understand the small dogs and
toy breeds ,This can happen to any breed but is more prominent in toy
breeds .Patella luxation, or knee dislocation, can range in severity from a
patella that can be luxated (dislocated) only in extreme extension and then
snaps readily into place, to a patella that is permanently luxated medially
(toward the center of the dog’s body). One of the causes of patella luxation
is an abnormal shape of the distal end of the femur, resulting in a shallow
patellar
groove. The other major cause is a displacement of the quadriceps tendon's
attachment to the tibia, so that the patella is displaced medially when the
quadriceps muscle is flexed.
The tendency to luxate to the medial side causes a transient lameness, at
least until the patella returns to its normal position, if it is able to. The
extent of patella luxation increases with time as the femur's trochlear
groove becomes flatter and flatter, as increased bowing of the leg takes
place, and as the structures of the stifle joint weaken. Continued deformity
of the joint results in degenerative joint disease, pain, and decreased
mobility.
This condition can be genetic, but not all cases are.
Some breeds have a higher incidence than others. Breeds known to have
this condition include: Affenpinscher, Brussels griffon, Chihuahua, English
toy
spaniel, Greyhound, Japanese spaniel, Maltese, Manchester terrier,
Miniature pinscher, Papillon, Pekingese, Pomeranian, Poodle, Pug, Shih tzu,
Silky terrier, and Yorkshire terrier.
Patellar luxation is a common condition. Four grades of dislocation are
recognized:
1) The patella luxates with manual pressure and returns spontaneously.
2) The patella luxates with flexion and extension of the joint, but returns to
the trochlear groove spontaneously. Some lameness may be present.
3) The patella luxates with flexion and extension of the joint, but can be
reduced manually. Considerable lameness exists.
4) The patella is permanently luxated to the medial side. The limb or limbs
are unable to extend and the animal walks balancing its weight on the
forelimbs.
Treatment in mild cases may include exercise restriction, aspirin, non-
steroidal anti-inflammatories (NSAIDs), polysulfated glycosaminoglycans,
glucosamine/chondroitin sulfate (Cosequin), and weight reduction in obese
dogs. The addition of surgery to the treatment regimen is usually indicated
in severe cases, and may benefit mild cases also.

Safety
Be sure to teach your small breed puppy  to walk safely with
a Harness and leash, but use good judgment about when to
leave the dog on the ground. Let the dog see that you can
be trusted for protection. Use a chest harness to attach the
leash to the dog, not a collar that would put pressure on the
tiny throat. If you can’t find a small enough dog harness, a
cat harness will work.
While training is important in large dogs so that we can
control them, training is necessary for small dogs so that we
can protect them. Don’t neglect training your small breed
puppy just because you can exert control by picking up the
dog. There are many situations for which that won’t work,
such as when the dog is 20 feet away and barking at a big
dog. And when a dog can understand your directions, the
world feels a great deal less scary and more orderly to your
canine friend.
Do not allow your  puppy  to antagonize bigger dogs! Many
tiny dogs have been killed this way. Check for safety as to
the other dogs in the area before putting your puppy into
any situation. When there is a big size difference, even play
can injure or kill a tiny dog. Pick playmates for your dog who
are not radically larger.
People (especially children) tend to charge right up to small
dogs and to put hands on them without hesitation. Don’t
permit people to do this to your  puppy  Protect the dog’s
space and get people to slow down—you can pick up your
dog to achieve this. You can also set the dog up on a
surface to be safe from people’s feet and feel less
vulnerable, but make sure the dog can’t fall from there.