(LUXATION) OF THE PATELLA
Note for Pet Owners:
This information is provided by
Provet for educational purposes only.
You should seek the advice of
your veterinarian if your pet is ill as only he or she can correctly advise
on the diagnosis and recommend the treatment that is most appropriate for
Topics on this Page:
Dislocations (also called luxations) of the patella (the "knee cap") are common in dogs and cats.
They cause lameness, hopping and sometimes severe disability. If the problem is
severe and it is left untreated it can result in the development of degenerative joint disease (osteoarthritis) in
the joint and also developmental abnormalities in adjacent structures.
The relationships of the patella - which usually sits at the front of the stifle
(knee) joint - with other local bones (the femur and tibia) and soft tissues,
and even with other parts of the limb (eg the pelvis) is complex.
Misalignment of the patella can be caused by deformity of these other
structures, and conversely abnormal positioning of the patella can lead to the
development of deformity (eg bowing of the bones) in the other structures. This
is because the structures of the knee are all dependent on each other, and
biomechanical forces exerted on them will alter their shape and form.
precise cause of patellar dislocations has not yet been identified,
however there are three main reasons why patellar dislocation occurs :
- Congenital, that is present at birth - common, and usually bilateral.
- Developmental - common and usually bilateral
In these forms of the disease the trochlear groove in which the patella
sits may be very shallow, or there may be abnormal conformation (bowing) of
the long bones of the limb so that the patella is pulled out of it's normal
position by biomechanical forces. Sometimes patellar dislocation can occur at
the same time as other anatomical deformities such as the chest deformity
pectus excavatum, and in cats taurine-deficient diets fed during pregnancy can
result in both pectus excavatum and luxated patellae in the kittens.
- Following trauma to the stifle - uncommon and usually unilateral. Injury
causes tearing/stretching of the ligaments (cruciate and collateral) and
other soft tissues around the joint resulting in instability which can
allow the patella to slip out of the trochlear groove in which it sits.
When an animal is examined and found to have a dislocated patella and
abnormally curved long bones it is impossible to know which abnormality was
Dislocation of the patella occurs in:
- Large dogs - most often it dislocates laterally - to the outside of the
- Small dogs - most often dislocates medially - to the inside of the joint
Some breeds of dog are commonly affected as a result of their natural
- Large dogs - Great Danes and other large breeds susceptible to develop
curvature (eg genu valgum) of the long bones of the hind leg.
- Small dogs - Staffordshire Bull terriers and Jack Russell terriers
in which natural conformational curvature of their limbs is likely to be a
predisposing cause, and small Toy breeds eg miniature Poodles in which the
patella groove is often poorly formed and shallow.
The signs vary depending upon the severity of the dislocation
and Toy breeds of dog with congenital dislocation the dog may be seen to hop
for several steps during walking/running because the leg locks in a flexed
position. In these cases the patella can be easily lifted out of, and
replaced back into the trochlear groove on the femur. The soft tissues
around the joint are very slack and there is little or no discomfort
associated with this form of the disease, unless secondary degenerative joint
disease (osteoarthritis) forms.
In severe congenital dislocations, and
following trauma the patella may be permanently out of position and it can not
be placed back into position easily. Normal extension of the joint is
impossible and the leg is held permanently flexed. Although the animal may
attempt to use the limb it has an abnormal gait.
Putnam has described
a classification scheme for the different types of patella dislocation :
- Grade 1 - The patella can be dislocated manually, but returns to its
normal position in the trochlear groove when it is released
- Grade 2 - The patella can be dislocated manually or during flexion of
the stifle joint - but the patella stays where it is until it is replaced
- Grade 3 - The patella remains dislocated most of the time. It can
be replaced manually, but dislocates as soon as it is released
- Grade 4 - The patella is dislocated all the time and can not be
replaced manually...even if the leg is held extended.
ComplicationsIf untreated, dislocation of the patella can lead to :
- Degenerative joint disease (osteoarthritis) in the joint
- Developmental remodelling and deformity in adjacent structures - causing
curvature of the long bones, and soft tissue changes
Diagnosis is made by physical examination of the stifle and radiography
There are several different forms of treatment depending upon the severity of
the condition :
- No treatment may be required for mild cases especially in
small and toy breeds..Grades 1 & 2.
- Pain killers can be given when necessary
- Surgery to deepen a shallow trochlear groove - called trochleoplasty
- Surgery to stabilise the joint - eg repair of ruptured ligaments, or
tightening of the soft tissues around the joint
- Surgery to change the relationship of the patella to the adjacent bone -
eg transplantation of the tibial crest
The prognosis depends upon the age at which the dislocation occurs, and how
severe it is.
The prognosis is generally good providing the patella can be
returned to it's normal position and normal stifle joint movement can be
Putnam (1968) Patellar Luxation in the Dog.
Masters Thesis University of Guelph.
Updated October 2013