WHAT IS PATELLAR LUXATION?
The patella, or kneecap, is part of the stifle joint (knee). In patellar luxation (dislocation), the kneecap luxates, or pops out of place, either in a medial or lateral position. Bilateral involvement is most common, but unilateral is not uncommon. Animals can be affected by the time they are eight weeks of age. The most notable finding is a knock-knee (genu valgum) stance. The patella is usually reducible, and laxity of the medial collateral ligament may be evident. The medial retinacular tissues of the stifle joint are often thickened, and the foot can be seen to twist laterally as weight is placed on the limb.
Patellar Luxation Categories:
Medial Luxation - knee cap rides on the inner aspect of the knee known to be heritable or strongly suspected
Lateral Luxation - knee cap rides on the outer aspect of the knee known to be heritable or strongly suspected
Luxation Resulting from Trauma
MEDIAL LUXATION - Knee Cap Rides on the Inner Aspect of the Knee
Although the luxation may not be present at birth, the anatomical deformities that cause these luxations are present at that time and are responsible for subsequent recurrent patellar luxation. Patellar luxation should be considered an inherited disease.
Clinical Signs- Three Classes of Patients are Identifiable
1) Neonates and older puppies often show clinical signs of abnormal hind-leg carriage and function from the time they start walking; these present grades 3 and 4 generally.
2) Young to mature animals with grade 2 to 3 luxations usually have exhibited abnormal or intermittently abnormal gaits all their lives, but are presented when the problem symptomatically worsens.
3) Older animals with grade 1 and 2 luxations may exhibit sudden signs of lameness because of further breakdown of soft tissues as result of minor trauma or because of worsening of degenerative joint disease pain.
Signs vary dramatically with the degree of luxation. In grades 1 and 2, lameness is evident only when the patella is in the luxated position. The leg is carried with the stifle joint flexed, but may be touched to the ground every third or fourth step at fast gaits. Grade 3 and 4 animals exhibit a crouching, bowlegged stance (genu varum) with the feet turned inward and with most of the weight transferred to the front legs.
Permanent luxation renders the quadriceps ineffective in extending the stifle. Extension of the stifle will allow reduction of the luxation in grades 1 and 2. Pain is present in some cases, especially when chondromalacia of the patella and femoral condyle is present. Most animals, however, seem to show little irritation upon palpation.
LATERAL LUXATION - Knee Cap Rides on the Outer Aspect of the Knee
Lateral luxation in small breeds is most often seen late in the animal’s life, from 5 to 8 years of age. The heritability is unknown. Skeletal abnormalities are relatively minor in this syndrome, which seems to represent
a breakdown in soft tissue in response to, as yet, obscure skeletal derangement. Thus, most lateral luxations are grades 1 and 2, and the bony changes are similar but mirrored to those described for medial luxation. The dog has more functional disability with lateral luxation than with medial luxation.
In mature animals, signs may develop rapidly and may be associated with minor trauma or strenuous activity. A knock-knee or genu valgum stance, sometimes
described as seal-like, is characteristic. Sudden bilateral luxation may render the animal unable to stand and so simulate neurological disease. Physical examination is as described for medial luxation.
PATELLAR LUXATION GRADES
A method of classifying the degree of luxation and bony deformity is useful for diagnosis, and can be applied to either medial or lateral luxations by reversing the medial-lateral directional references. The position of the patella can most easily be palpated starting at the tibial tubercle and working proximally along the patellar ligament to the patella.
Grade 1: The patella easily luxates manually at full extension of the stifle joint, but returns to the trochlea when released. No crepitation is apparent. The medial, or very occasionally, lateral deviation of the tibial crest (with lateral luxation of the patella) is only minimal, and there is a very slight rotation of the tibia. Flexion and extension of the stifle are in a straight line with no abduction of the hock.
Grade 2: There is frequent patellar luxation, which, in some cases, becomes more or less permanent. The limb is sometimes carried, although weight bearing routinely occurs with the stifle remaining slightly flexed. As much as 30 degrees of medial tibial torsion and a slight medial deviation of the tibial crest may exist. When the patella is resting medially the hock is slightly abducted. If the condition is bilateral, more weight is shifted onto the forelimbs. Many dogs with this grade live with the condition reasonably well for many years, but the constant luxation of the patella over the medial trochlear ridge of the trochlea causes erosion of the articulating surface of the patella and also the proximal area of the medial lip. This results in crepitation becoming apparent when the patella is luxated manually.
Grade 3: The patella is permanently luxated with torsion of the tibia and deviation of the tibial crest of between 30 degrees and 50 degrees from the cranial/caudal plane. Although the luxation is not intermittent, many animals use the limb with the stifle held in a semi-flexed position. The trochlea is very shallow or even flattened.
Grade 4: The tibia is medially twisted and the tibial crest may show further deviation medially with the result that it lies 50 degrees to 90 degrees from the cranial/caudal plane. The patella is permanently luxated. The patella lies just above the medial condyle and space can be palpated between the patellar ligament and the distal end of the femur. The trochlea is absent or even convex. The limb is carried, or the animal moves in a crouched position, with the limb partly flexed.