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DISLOCATION (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 your pet.

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Description
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.

Cause
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.

The 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 present first.

Breed Occurrence
Dislocation of the patella occurs in:

  • Large dogs - most often it dislocates laterally - to the outside of the joint
  • Small dogs - most often dislocates medially - to the inside of the joint
  • Cats 

Some breeds of dog are commonly affected as a result of their natural conformation :

  • 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.

Signs

The signs vary depending upon the severity of the dislocation

In small 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 manually
  • 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.

 

Complications
If 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
Diagnosis is made by physical examination of the stifle and radiography

Treatment
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

Prognosis
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 achieved.

References

Putnam (1968) Patellar Luxation in the Dog. Masters Thesis University of Guelph.

 

Updated October 2013