Patellar luxation (2024)

Congenital luxation of the patella represents one of the most common orthopedicconditions in small animal practice. Medial luxations accountfor 75% to 80% of cases in all breeds. The majority ofpatients are small breed dogs including miniature and toy poodles, Yorkshireterriers, Pomeranians, Chihuahuas, Boston terriers, Pekingese, andcavalier King Charles spaniels(1–4). An increasing number of young, large breed dogs are being presentedwith medial patellar luxation, particularly the Akita, Labrador, goldenretriever, malamute, boxer, and husky(1,4). Lateral luxation is less frequent but is most common in giant breeds, especiallythe St. Bernard(3,4). A few cases may show the ability to luxate both medially and laterally(2). Patellar luxation is seen infrequently in cats, especially in the DevonRex and Abyssinian breeds, and has been associated in this specieswith congenital hypoplasia of the medial femoral condyle, a shallow trochleargroove, and hip dysplasia(2).

The overwhelming majority of patellar luxation are congenital and certainlyhereditary, although a mode of inheritance has not been described(4,5). Occasionally, traumatic cases do occur when a blow is sustained to theretinacular structures, particularly on the lateral side of the stiflejoint(4,5). Females have been reported to be 1.5 times more likely to be affectedthan males(4). Published estimates of the incidence of bilateral patellar luxationrange from 20% to 52%(1,4). A survey of 59 surgical cases in our practice between April 1998 andSeptember 2005 revealed that 74.5% of canine cases were in smallbreeds; 4 cases were in cats; and 58.5% of small breed caninecases were female. Bilateral luxation occurred in 46% of thesmall dogs but only 36% of the large dogs.

Patients with patellar luxation can be broadly classified into 3 categories:

1. Puppies and young adult dogs with an intermittent “skipping” gait. Theowners describe the dog as pulling up the leg for severalsteps before returning it to the ground and then resuming normal ambulationwith no sign of lameness. The nonweight-bearing phase correspondswith luxation or subluxation of the patella and the gait returnsto normal when the luxation spontaneously reduces. As such, this representsa mechanical lameness, in that the patellar luxation impedes normalfunction of the stifle rather than producing significant pain. Theskipping gait may be widely intermittent or may be almost continuous, dependingon the severity of the luxation. Severely bilaterally affecteddogs may have major impairment of their ability to extend the stifle, thuspresenting in a crouching, “pigeon-toed” stancewith limited locomotory capacity(1,4–6).

2. Middle-aged or older dogs with a more constant hind limb lameness. Carefulquestioning of the owner is essential with these patients to determinethe course of the lameness. If the luxating patella is responsiblefor the lameness, there will be a long-term history of intermittentlameness, including the characteristic “skipping gait.” Theowner may comment that the dog has had a problem with the leg fromtime to time for as long as they can remember, but the symptoms seemto be getting worse. This is often due to eburnation of cartilage onthe medial trochlear ridge and the underside of the patella(1,2,4). Osteoarthritic changes in the joint may also contribute to lamenessin these patients, but this may be of somewhat lesser importance, sincethe degenerative changes seem to be slower in developing, are of a moreminor nature, and are less clinically debilitating than the changesseen in rupture of the cranial cruciate ligament, for example(3,7). If questioning of the owner reveals an acute lameness or an acute worseningof an intermittent lameness, the patellar luxation is frequentlya “red herring.” More often, rupture of the cranialcruciate ligament or other causes of acute lameness will be present asa more significant clinical entity(1,2,5,8).

   Medial patellar luxation and rupture of the cranial cruciate ligament areintertwined in several ways. It is estimated that at least 15% to 20% ofdogs with patellar luxation will eventually rupturetheir cranial cruciate ligament(1). This may be due to a combination of 3 factors: First, dogs with significantpatellar luxation usually have internal rotation of the tibia, whichputs stress on the cranial cruciate ligament. Second, the quadricepsmusculature-patella-patellar tendon mechanism normally providescranial stability to the stifle joint. In the dog with patellar luxation, agood portion of this mechanism is deviated medially, thus offeringless resistance to forces that would tend to subluxate the proximaltibia cranially. Third, cartilage erosion and degenerative joint diseasemay create an environment in the stifle that promotes degenerationof the cranial cruciate ligament(1,5).

   See Also

   Luxating Patella in Dogs: A Complete Guide

3. Asymptomatic dogs. A significant number of dogs with patellar luxationmay show few or no clinical signs(1). Fewer large breed dogs with patellar luxation would fall into this category. Ourpractice data show a mean age at surgery in large dogs of 1.6 y (range 8 moto 3.5 y), while the corresponding mean age in smalldogs is 3.4 y (range 7 mo to 9 y). The younger age and narrower agerange in large breeds suggest that larger dogs are less likely to be asymptomaticthan smaller dogs.

A grading system based on the findings at physical examination has beendeveloped for the categorization of patellar luxation. A Grade 0 patellaluxation is normal and the patella will not luxate during the physicalexamination. A Grade 1 patellar luxation is one in which the patellawill luxate when digital pressure is applied, usually with the stiflein extension, but will immediately return to its normal position whenthe pressure is removed. A Grade 2 patellar luxation is one in whichthe patella will readily luxate with digital pressure and tends toremain luxated. However, it can be returned to the trochlear groove andwill remain in place most of the time. A Grade 3 patellar luxation isone where the patella is in the luxated position most of the time, althoughit can be returned temporarily to the trochlear groove with digitalpressure. A Grade 4 patellar luxation is one where the patella isin the luxated position at all times and cannot be returned to the trochleargroove(1,4–6). While the grading system is useful in communicating the degree of patellarluxation, the anatomical abnormalities that might be present toproduce the degree of luxation, which patients may require surgery atsome point, and in suggesting the prognosis for surgical patients, thereis a danger in reading too much into the classification system. Forexample, one cannot base recommendations for surgical repair solelyon the grade of luxation present, because the correlation between thegrade of luxation and the clinical signs is not strong. Many Grade 1 smalldogs will never encounter lameness problems; however, others, especiallymany large breed dogs, will be clinically affected. On the otherhand, nearly all Grade 3 and 4 dogs will show signs of lameness anddisability. However, these signs are not always severe and, perhaps moreimportantly, some owners may not view the problem as significant inthe context of the limited physical demands placed on their dogs, especiallyin the case of the small breeds(5).

The next article in this series will deal with the pathogenesis of patellarluxation and how the pathogenetic theories lead towards surgicalcorrection of the problem.

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