A
dog from one of the smaller breeds runs across the yard
chasing a tossed ball. In mid-stride, he yelps in pain and
pulls his left hind leg off of the ground. After a second,
he continues limping on in a three-legged fashion. After ten
minutes, the rear leg drops back down to the ground and he
uses it normally. This episode occurs maybe once a week. It
never really seems to bother him that much – a yelp of pain,
a short period of lameness, and in a few minutes he is back
to his old self. Typically, he is a small or toy breed such
as a Lhasa Apso, Pekingese, Pomeranian, Poodle, or Boston
Bull.
A luxating patella may affect some animals much more
severely. They may hold the leg up for several days and show
considerable discomfort. Dogs who have a luxating patella on
both hind legs may change their entire posture, by dropping
their hindquarters and holding the rear legs farther out
from the body as they walk. Those most severely affected may
not even use their rear legs, walking by balancing
themselves on their front legs like a circus act, or holding
their hindquarters completely off the ground.
Normal knee anatomy
The
patella is the bone we know as the knee cap. A groove in the
end of the femur allows the patella to glide up and down
when the knee joint is bent back and forth. In doing so, the
patella guides the action of the quadriceps muscle in the
lower leg. The patella also protects the knee joint.
Looking at the lower front portion of the femur (the
thigh bone) in a normal dog, you will notice two bony ridges
that form a fairly deep groove in which the patella is
supposed to slide up and down. These structures limit the
patella’s movement to one restricted place, and in doing so,
control the activity of the quadriceps muscle.
The entire system is constantly lubricated by joint
fluid. It works so that there is total freedom of motion
between the structures.
What occurs when the patella is
luxated?
In some dogs, because of malformation or trauma, the
ridges forming the patellar groove are not prominent, and a
too-shallow groove is created. In a dog with shallow
grooves, the patella will luxate (jump out of the groove)
sideways, especially toward the inside. This causes the leg
to 'lock up' with the foot held off the ground.
When the patella luxates from the groove of the femur, it
usually cannot return to its normal position until the
quadriceps muscle relaxes and increases in length. This
explains why the affected dog may be forced to hold his leg
up for a few minutes or so after the initial incident. While
the muscles are contracted and the patella is luxated from
its correct position, the joint is held in the flexed or
bent position. The yelp is from the pain caused by the knee
cap sliding across the bony ridges of the femur. Once out of
position, the animal feels no discomfort and continues his
activity.
Which dogs are at risk of having a
luxated patella?
Smaller
breeds of dogs, especially Miniature and Toy Poodles, have
the highest incidence of patella luxation. Genetics can play
a role.
In certain breeds that have extremely short legs such as
the Basset Hound or Dachshund, patellar luxation is thought
to be secondary to the abnormal shape of the femur and
tibia. The curvatures of the bones in these breeds work in
conjunction with the forces of the quadriceps muscles to
displace the patella to the inside. Please do not
misunderstand – not all members of these breeds are affected
with patellar luxation, only a small portion.
What are the symptoms?
Most dogs are middle-aged, with a history of intermittent
(on-again-off-again) lameness in the affected rear leg(s).
An affected dog commonly stops and cries out in pain as he
is running. The affected leg will be extended rearward, and
for a while, the dog is unable to flex it back into the
normal position.
What are the risks?
Uncorrected, the patellar ridges will wear, the groove
will become even shallower and the dog will become
progressively more lame. Arthritis will prematurely affect
the joint, causing a permanently swollen knee with poor
mobility. Therefore, a good evaluation needs to be done by
your veterinarian early in the condition to prevent
long-term arthritic crippling.
Treatment for luxating patellas
As would be expected, medical therapy has little
corrective ability in this disorder and surgery is therefore
required and is the treatment of choice. A surgical
treatment is not necessary in every individual with this
condition.
Surgery can alter both the affected structures and the
movement of the patella. The groove at the base of the femur
may be surgically deepened to better contain the knee cap.
The knee cap itself may be 'tied down' laterally (on the
outside) to prevent it from deviating medially (toward the
inside). The bony protuberance at the site of the attachment
of the quadriceps tendon on the tibia, may be cut off and
then re-attached in a more lateral position. All of these
procedures work well and the type performed depends on the
individual case and the clinician. The animal should respond
quickly after surgery and is usually completely recovered
within thirty days, using his legs in normal fashion.
Breeding considerations
Because of the strong genetic relationships, we really
feel that animals with this disorder should not be used for
breeding. They can still be excellent pets - and those that
do require surgery will usually lead perfectly normal lives
without any restrictions on activity.
© 2006 Foster & Smith, Inc.
Reprinted as a courtesy and with permission from PetEducation.com
(http://www.PetEducation.com)
On-line store at
http://www.DrsFosterSmith.com
Free pet supply catalog: 1-800-323-4208
The Chihuahua Club of America Thanks Drs. Foster &
Smith for use of their articles.
