**2. Anatomy and etiopathogenesis**

with functional disability (**Figure 1A,B**).In puppies, the surgical correction must be performed between 1 and 3 months of age, and not later than 3 months [2]. At this age, the skeletal deformities can be reversed after alignment of the limb. Patellar luxation may occur at birth, during growth or at a later stage. The pathophysiology of congenital luxation remains a topic of discussion [3–5]; a consequence of complex skeletal abnormalities that alter the limb alignment is considered [5]. The condition may be unilateral or bilateral and can be asymptomatic, and most cases are medial [3, 5]. Occasionally, the luxation can occurin both directions in the same joint [3], which is a surgical challenge. It is possible to find medial patellar luxation in one joint and lateral luxation in another joint in the same dog. Some cases exhibit patellar subluxation. Although much less frequently than in dogs, patellar dislocation also occurs in

**Figure 1.** (A) Abnormal position associated with bilateral grade IV medial patellar luxation in puppies 60 and (B) 90 days old; (C) ventrodorsal radiograph, showing bilateral medial luxation of the patella, and angular deformities in a 90-day-old poodle; (D) ventrodorsal radiograph, showing bilateral lateral luxation of the patella in a 9-month-old dog

The incidence of severe articular lesions found during routine surgeries in small-, medium- or large-breed dogs presenting with patellar luxation is high. In some cases, the patella injury is so serious that correction is not possible or prudent. In these instances, a better treatment option may be prosthetic replacement. Patellectomy is suggested as a treatment alternative in some severe cases [5, 6], but the removal of the patella does not correct the skeletal changes [6], calling

According to the literature [5], surgery might not be necessary in cases without clinical manifestations or when lameness is mild; however, even under such circumstances, the joint

Patellar luxation is a degenerative condition, and surgical treatment should be performed as early as possible, while clinical signs are mild or even before they appear. The aims of this

breed Sharpei; (E) abnormal stance associated with bilateral grade IV lateral patellar.

this therapeutic option into question.

damage is irreversible.

cats [3].

160 Canine Medicine - Recent Topics and Advanced Research

The patella is an oval-shaped sesamoid bone that is located in the quadriceps tendon [7, 8] and connected to the fabella by a thin band of loose connective tissue known as the medial and lateral patellofemoral ligament [3, 7, 8]. The tendon located between the distal aspect of the patella and the tuberosity of the tibia is known as the patellar ligament [3]. The patella acts as a lever arm favouring the extension of the quadriceps [3], and the patellofemoral joint increases the mechanical efficiency of the quadriceps mechanism [8]. The correct alignment of the quadriceps femoris, patella, trochlea, patellar ligament and tibial tuberosity prevents patellar luxation or subluxation [8]. An adequate supply of articular cartilage depends on a normal joint between femoral trochlea and patella. Patellar luxation causes articular cartilage degeneration [3].

The pathogenesis of developmental patellar luxation remains speculative and controversial [3, 5, 9]. Based on the thesis by Putnam [10], abnormal femoral and neck angles of inclination and anteversion are proposed to influence development and eventual malformation of the pelvic limb. However, this has not been proven [11].

The III and IV degrees of luxation are associated with severe angular and rotational deformities of the femur and tibia [8], which are more pronounced in medial luxation; the limb has an Sshaped conformation (**Figure 1B, C**). Some cases are difficult to treat, with poorer prognosis. Corrective osteotomy must be considered in these cases to restore the normal alignment of the quadriceps complex [5, 6]. It is sometimes necessary to also remove up to 1.0 cm of bone extension to return the patella to the femoral trochlea. In these cases, patella tibial tuberosity alignment is achieved but paw rotation remains. Therefore, early intervention should be considered in cases of patellar luxation independent of the existence of clinical signs.

Clinical evidence shows that intermittent patellar luxation, contrary to the logical assumption, produces varying degrees of injury to the articular surfaces of the patella and the femoral condyle, causing degeneration of articular cartilage. This may also result in a flattening of the femoral condyle and consequently facilitate dislocation [8]. The position of the patella relative to the femoral trochlea, more proximal, is considered by some authors [8, 13, 14] to be an important factor in canine patellar luxation.

Although seldom discussed in the literature, subluxation also occurs and is observed in clinical practice. The affected dogs are typically adults presenting with pain and lameness due to the wear on the patella [8]. This condition is difficult to diagnose, especially in heavy English Bulldogs; a careful physical examination is necessary. Other findings, including cruciate ligament rupture and even the presence of intra-articular loose bodies and flap formation, can be seen in cases of chronic patellar dislocation.

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 is performed before 60 days of age [2].

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