**3. Clinical signs**

The clinical signs of patellar luxation vary according to the degree of deformity, duration of the condition, unilateral or bilateral stifle involvement [8] and the age of onset. Degree III or IV luxation in puppies prevents ambulation (**Figure 1A, B**) and causes the characteristic changes of Genu varum or a "bow-legged" conformation in some cases of medial patellar luxation (**Figure 2A, B**). In lateral patellar luxation, the associated abnormal anatomic features are reversed. These dogs may have a knock-kneed appearance when affected bilaterally (**Figure 2C, D**).

**Figure 2.** Abnormal stance associated with: bilateral grade IV medial patellar luxation in a (A) 5- and (B) 3-year-old pinscher; lateral patellar luxation in a 6-month-old Fila Brasileiro (C) and (D) 3-year-old poodle; (E) unilateral grade III lateral luxation in a crossbreed 4-year-old dog.

In clinical practice, four classes of patients [5] are encountered: neonates and older puppies with grade III or IV luxation and inability to walk (**Figure 1A**), young or mature dogs with grade II to III luxation and mild clinical signs for a long period of time until presenting when the clinical condition worsens, older animals with grade I or II luxation and sudden claudication due to the cruciate cranial ligament rupture or degenerative joint disease pain and asymptomatic dogs.

Medial patellar dislocation is responsible for approximately 80% [5] of cases, with bilateral involvement in up to 50–65% [5, 14]. Lameness is one of the presenting clinical signs; however, it is subjective; lameness is seen in other conditions that affect the hind limb, which should be included in the differential diagnosis [6, 15]. Medial luxation is considered characteristic of small breeds, but it has been reported with increasing frequency in large and giant breeds [5].

Although many dogs present at 6–12 months of age, some animals with mild lameness present later, with rupture of the cranial cruciate ligament or pain caused by osteoarthritis [8]. Often, the patellar luxation is not considered to be an important joint disorder until it becomes severe. In puppies, the situation is even more critical. Post-operative studies show that it is possible to correct the limb deformities and achieve complete reversal of skeletal changes if the surgery

Lateral luxation also occurs in small dogs; the skeletal deformities are the reverse of those seen with medial luxation (**Figure 1D, E**), demonstrating an increased angle of anteversion, coxa valga, medial torsion of the distal femur, lateral condylar dysplasia, lateral rotation of the tibia and external rotation of the paw [16, 17]. The articular cartilage injuries in both types of luxation result in osteoarthritis [16, 18], which is usually mild-to-moderate and unrelated to the degree of luxation or lameness [19]. Lateral luxation of the patella may cause injury at the origin of

The clinical signs of patellar luxation vary according to the degree of deformity, duration of the condition, unilateral or bilateral stifle involvement [8] and the age of onset. Degree III or IV luxation in puppies prevents ambulation (**Figure 1A, B**) and causes the characteristic changes of Genu varum or a "bow-legged" conformation in some cases of medial patellar luxation (**Figure 2A, B**). In lateral patellar luxation, the associated abnormal anatomic features are reversed. These dogs may have a knock-kneed appearance when affected bilaterally

**Figure 2.** Abnormal stance associated with: bilateral grade IV medial patellar luxation in a (A) 5- and (B) 3-year-old pinscher; lateral patellar luxation in a 6-month-old Fila Brasileiro (C) and (D) 3-year-old poodle; (E) unilateral grade III

the long digital extensor tendon, leading to rupture of the tendon in severe cases.

is performed before 60 days of age [2].

162 Canine Medicine - Recent Topics and Advanced Research

**3. Clinical signs**

(**Figure 2C, D**).

lateral luxation in a crossbreed 4-year-old dog.

Clinical signs vary from animal to animal and may be intermittent or continuous. Associated joint damage and overweight may worsen the clinical signs. The physical examination should be performed carefully; gentle palpation without causing pain is the goal, considering the difficulty of identifying the patella in toy and miniature animals as well as in dogs with severe deformities. In general, dogs with lateral luxation have more ambulation problems than those with medial luxations [5].

The physical examination should consider aspects such as instability in both directions, crepitus, degree of tibial rotation, limb deformity, inability to reduce the patella, location of the reduced patella within the trochlea, inability to stand the limb to a normal standing angle and presence/absence of cruciate ligament rupture. This information is necessary for surgical planning [5].

Patellar dislocation is classified into four grades [20] to facilitate the diagnosis and plan the method of surgical repair:

Grade I. A dog with grade I patellar luxation rarely shows lameness and carries the limb occasionally. The patella can be manually luxated when the stifle is extended, but it returns to the trochlea when released. No crepitation is apparent. Internal rotation of the tibia and displacement of tibial tuberosity are minimal.

Grade II. Luxation occurs more frequently than in grade I. Lameness signs are usually intermittent and mild. The patella moves easily, especially when the foot is rotated, while the patella is pushed. The proximal tibial tuberosity may be rotated up to 30° with medial luxations and less with lateral luxations. Many grade II patients live reasonably well for many years, but the injuries from constant friction between the patella and femoral condyle result in crepitation and increasing discomfort [5].

Grade III. The patella is permanently luxated, with torsion of the tibia and deviation of the tibial crest between 30° and 60°. It can be reduced, but luxation recurs immediately. The trochlea is very shallow or even flattened. Although they have permanent luxation, many animals use the limb with the stifle held in a semi-flexed position.

Grade IV. The patella is permanently luxated, and it is not possible to manually reposition it in the trochlea. It lies just above the medial condyle (if a medial luxation) (**Figure 1C**). Angular and rotational deformity of the femur and tibia are generally marked, and the tibial crest is displaced 60°–90°. The trochlea is shallow, absent or even convex. The limb may be carried if unilateral, or the animal moves in a crouched position, with the limbs partially flexed or carried in toy animals (**Figure 2B**).
