Exam, Screening Tests, and Imaging

The diagnosis of patellar luxation is essentially based on palpation of an unstable kneecap on orthopedic examination. Additional tests are usually required to diagnose conditions often associated with patellar luxation and help the surgeon recommend the most appropriate treatment for your pet.

These may include:

  • Palpation of the knee under sedation to assess damage to ligaments
  • Radiographs of the pelvis, knee and occasionally tibias to evaluate the shape of the bones in the rear leg and rule out hip dysplasia
  • 3-dimensional computed tomography to provide an image of the skeleton of the entire rear legs. This advanced imaging technique helps the surgeon plan surgery in cases where the shape of the femur or tibia needs to be corrected.

  • Because there is great individual variation in the pathologic deformities seen, a graded classification of medial patellar luxation (Putnam 1968) has been formulated as a basis for recommending which type of surgical repair is most appropriate for each individual.

    Surgical treatment is typically considered in grades 2 and over. While the grading system is useful in communicating the degree of patellar luxation, the anatomical abnormalities that might be present to produce the degree of luxation, which patients may require surgery at some point, and in suggesting the prognosis for surgical patients, there is a danger in reading too much into the classification system. For example, one cannot base recommendations for surgical repair solely on the grade of luxation present, because the correlation between the grade of luxation and the clinical signs is not strong.

    Many Grade 1 small dogs will never encounter lameness problems; however, others, especially many large breed dogs, will be clinically affected. On the other hand, nearly all Grade 3 and 4 dogs will show signs of lameness and disability. However, these signs are not always severe and, perhaps more importantly, some owners may not view the problem as significant in the context of the limited physical demands placed on their dogs, especially in the case of the small breeds.


    The anatomic alignment of the stifle is normal with the patella luxating only when pushed out of the socket.


    The patella is returned by manual pressure.


    The patella is permanently dislocated but can be reduced manually with the limb extended.


    The patella is permanently dislocated and cannot be manually reduced. In dogs with grade IV patellar luxation more aggressive surgery is often required. This involves straightening of the femur and/or tibia with complete bone cuts (a.k.a. osteotomies) and stabilizing the bones with bone plates. Lateral patellar luxation, or LPL, is less common than MPL and occurs when the kneecap occasionally rides on the outside of its normal groove. It, too, can be congenital or acquired, with the congenital form again being more common. While it can occur in any dog, it is more common in large and giant breeds. LPL is frequently accompanied by malformation of the femur and/or tibia. The disease can produce marked lameness and progress to crippling arthritis. Because of the accompanying bony malformations, extensive surgery may be required to correct this problem.