(All small breeds can have this problem.)
Patellar Luxation in the Boston Terrier
By: Christiane J. Scarpino
Luxated patella, also known as ‘slipped stifle’ or ‘slipped kneecap’ is a common orthopedic
problem in small dogs. We’ve all seen that Boston that holds up a rear leg, skips occasionally when it moves, or holds
a rear leg up for a few seconds, kicks it back and returns it to normal position. These are all common signs of a luxated
patella. The BTCA Health Survey in 2000 suggested that 9.6% of Boston Terriers are affected with patellar problems.
Clinical symptoms of luxated patella may occur as early as weaning or may go undetected until later in life. Signs to look
for include intermittent rear leg lameness, shifting form one leg to the other, inability to fully extend (straighten) the
stifle joint, carrying the hind leg up, kicking the rear leg back, hopping or skipping action. Dogs with luxated patella may
also exhibit these features: high in the rear with more weight on the front, poor drive from the rear, an over muscled front,
dropping the hindquarters or holding the rear legs farther out from their body as they walk.
with a little review of anatomy. The stifle is the joint in the rear leg that is the anatomical equivalent to the human knee.
The three major components of the stifle joint are the femur (thigh bone), the patella (kneecap) and the tibia (calf bone)
There is a ligament that helps hold the patella in place. In a normal stifle, the femur and tibia are lined up, so that the
patella rests in a groove called the trochlea. The patella glides up and down when the knee joint is bent back and forth.
(flexion and extension) The patella is attached to the tibia by the patellar tendon.
Luxation or dislocation of
the patella occurs when these structures are not in proper alignment. The tibia rotates inward which allows the patella to
slip out of its groove and ride on the inner surface of the femur. Sometimes the luxation occurs due to soft tissue problems,
and other times, due to the trochlea not being deep enough for the patella to stay in place. Medial luxation, (movement of
the patella towards the inside of the leg) is much more common than lateral luxation (movement of the patella towards the
outside of the leg) 80% of all luxations are medial. Bilateral patellar luxation occurs approximately 25% of affected dogs.
When the patella luxates, it usually cannot return to its normal position until the quadriceps muscle (thigh muscle)
relaxes and increases in length. This explains why the affected dog may be forced to hold its leg up for a few minutes. There
is pain and the dog may yip when the knee cap slides across the bony ridges of the femur. Once the patella is out of place,
the animals feels no pain or discomfort.
Diagnosis may be made by palpation of
the stifle joint and manual luxation of the patella.
The palpation is done with the dog awake, and consists of manipulating
the patella while pulling the leg upward and backward. X-rays may be used to determine the degree of rotation if a problem
is palpated. A general veterinary practitioner knowledgeable in orthopedics or a veterinary orthopedist is qualified to perform
a patella evaluation as outlined by the Orthopedic Foundation for Animals (OFA). OFA recommends screening of patella at six
to eight weeks of age, and again at 4 – 6 months. After that point, patella should be checked at 12 months and every
two years after for OFA Certification.
Four grades are used to classify the severity of luxation.
1 is the mildest form. The patella is easily luxated at full extension of the stifle, but returns to the trochlea
when released. Flexion and extension of the stifle (bending and straightening of the knee joint) is in a straight line. Deviation
of the tibia is minimal.
In a Grade 2 luxated patella, the limb is sometimes carried and the stifle
is fixed at rest. There is as much as 30 degrees of medial tibial torsion. When the patella is resting, the hock is slightly
abducted (turned out) If the condition is bilateral, more weight is thrown on the forelimbs. Arthritic changes and reduced
range of motion may occur over time.
In a Grade 3, the patella is permanently luxated with torsion
of the tibia and deviation of the tibial crest between 30 and 50 degrees. Many dogs keep the stifle in a semi flexed position.
The trochlea is very shallow or even flattened, so that the patella is unable to stay in the groove.
4 luxation is characterized by the tibia being medially twisted and the tibial crest may be deviated 50 to 90 degrees.
The patella is permanently luxated. The trochlea is absent or even convex. The limb is carried or the dog moves in a crouched
position with the limb partly flexed.
What is the best form of management of this problem? Medical therapies have
little corrective value. Surgery is the treatment of choice. Surgery consists of soft tissue reconstruction and/or bone reconstruction.
Frequently, the trochlea is deepened to better contain the knee cap, or the knee cap itself may be tied down laterally (on
the outside) to prevent it from deviating. The success rate of surgery is high, and there is a good prognosis if joint degeneration
has not progressed too far.
Most researchers believe that luxated patella is inherited, but the exact mode of
inheritance is unknown. It is thought to be controlled by more than one gene or ‘polygenic.’ Dogs that
exhibit luxated patella should not be used for breeding.
Due to the polygenic nature of inheritance, it
is recommended that breeders monitor all of the puppies in a litter as a measure of the genes selected. Normal appearing dogs
from litters in which 25% or more of the puppies exhibit luxated patella should not be used in a breeding program. Breeders
can decrease patellar problems by selecting breeding stock from litters in which all the puppies are free from patellar
The good news is that dogs whose patella are surgically corrected live long and happy lives, with no
restriction in activity. The bad news is that we lack objective data in our breed to determine the true incidence and to study
inheritance of the problem. We can increase the information available to researchers about luxated patella by having our pugs
evaluated, and registering evaluation results with OFA
Prister, W.A.: Sex, Size, and Breed as Risk Factors in Canine Patellar Luxation. J. Am. Vet Med Assoc. 160:740;
Miller, ME Anatomy of the Dog, WB Saunders, Co., Philadelphia, PA, 1964
Orthopedic Foundation for
Foster, 4, and Smith, M. Luxating Patella in the Dog. www.PetEducation.com