This article is from the Canine Medical Information FAQ, by Cindy Tittle Moore with numerous contributions by others.
Thanks to Edwin Barkdoll for this summary, and to Jeff Parke for
There are many types and degrees of patellar luxation. The patella
(kneecap) can luxate (dislocate) medially (towards the body midline)
or laterally (away from the midline) and can be traumatic or
congenital in origin. Small or toy breeds are much more likely to have
this problem than larger breeds and they tend towards medial
luxations; larger breeds tend to have lateral luxations if they
develop this problem.
A system has been devised for grading patellar luxations: Type I -
luxation seen only with leg in extension and when pressure is applied
to the patella directly say during a physical exam by the vet,
luxation resolves spontaneously when pressure is removed. Type II -
patella is usually in normal position, but luxates with pressure or
during flexion of the limb. The patella does not spontaneously return
to normal but can be returned to normal manually or by the dog itself.
Type III - patella is luxated most of the time but can be temporarily
returned to normal position manually. Type IV - patella is always
luxated and cannot be returned to normal position manually.
Surgical correction is not usually considered necessary unless the dog
shows symptoms - pain, gait abnormalities - but you should talk with
your vet about your options and get a second opinion if necessary.
Regarding surgical success, apparently about 50% of surgically treated
cases demonstrate _recurrent_ patellar luxation after 1-7 years
although the severity of the patellar luxation at followup was reduced
and about 90% (!) showed no signs of lameness. For the curious, the
(incomplete) reference for these data is Willauer and Vasseur (1987)
in _Veterinary Surgery_.