The following article was first published in the Havanese Hotline, 2001 and permission to post here on the Havanese Resource Center granted by the author, Dr. Joanne Baldwin, DVM.
Luxating patellas are defined as kneecaps which are displaced from
their normal anatomic location in the femoral trochlea (groove in the
front, bottom end of the upper hindleg bone. The displacement can be
either to the inside (medial) or outside (lateral). It is a
condition predominantly found in toy and miniature breeds of dog,
more common in females, and is usually medial. There are four
grades of patellar luxation ranging from mild to severe (I-IV). In
the mildest form (I), the patella can be manually luxated but returns
to its correct place when pressure is released. In Grade II, the
patella can be manually luxated or may spontaneously luxated when the
joint is flexed. In Grade III, the patella remains luxated most of
the time but can be manually restored to its proper place if the
joint is in extension (the leg straightened.) In Grade IV the
patella is permanently luxated and cannot be manually reduced.
Luxating patellas can be congenital or traumatic. Congenital luxation
would be evident on the younger pup so it is important to have all
pups checked at 8-9 weeks by a competant vet to be sure the knees are
normal. It would also be prudent to have the knees checked again
after 4 months. The inheritance of patellar luxation is not
clear...Proposed are recessive, polygenic (influenced by several
genes) and/or multifocal (arising from many foci, ie. locations).
The more severe grades of congenital luxation involve rotational
deformities of the hindleg bones which contribute to the likelihood
of luxation. Simply put, what keeps the patella in place in a
normal dog is a groove in the end of the femur AND the patellar
ligament attaching the tibia (lower leg bone) in a "straight" line.
If the groove is shallow, there is less to hold the patella in
place. If the bones are rotated in a way that causes the ligament to
attach to the inside of the leg then there is a tendency for the
patella to be pulled out of the groove to the inside of the leg.
Trauma can cause the patella to luxate but there is generally a
predisposing anatomical deformity which contributes to the problem.
If a patella is traumatically luxated there should be an obvious
period of acute lameness and pain involved. Without a complete
history it may be impossible to tell whether a luxation is congenital
or traumatic.
It is recommended not to breed any dog which is affected by luxating
patellas. In addition, any sire/dam combination which produces pups
affected pups should not be repeated.
Most cases of Grade I and some cases of Grade II patellar luxations
can be treated conservatively. Activity may be restricted depending
on severity of symptoms. It is important to keep weight under
control to reduce the stress on the joints. Most Grade II and all
Grades III & IV should be considered candidates for surgical
reduction and repair. Surgery can range from fairly simple deepening
of the trochlear groove with imbrication (tightening) of the soft
tissues around the joint all the range to alteration of the
longitudinal axis of the long bones by corrective osteotomies (wedges
are removed from the bone and the bone is realigned.
There are several intermediate possibilities for surgical
correction. It is not uncommon for luxations to recur following
surgery, especially of the physical forces causing deviation have not
been completely corrected.
Dogs with patellar luxation run a much greater risk for anterior
cruciate ligament rupture than normal dogs. This is due to the added
stress on the ligament due to instability and twisting of the
joint. It is likely beneficial to treat affected dogs with
chondroprotective drugs such as glucoamine, chondroitin sulfate
and/or glycosaminoglycans. These help increase joint fluid and may
assist in restoration of cartilage. Most dogs which have successful
surgery will function well enough not to show clinical lameness or
dysfunction but nearly all will show evidence of degenerative joint
disease on radiograph.
Joanne Baldwin, D.V.M.
