**9. Discussion**

Patella luxation in small animals is a condition in which early treatment has not been highlighted. The suggested course has been to wait for the manifestation of clinical signs [5, 6, 8, 21], which results in lameness. However, dislocation and subluxation are already characteristic clinical signs of anatomical abnormalities, which certainly will cause articular changes. Subluxation [6] is a clinical challenge, especially in obese and heavy dogs like English Bulldogs, making it difficult to diagnose the cause of progressive lameness [6].

In the literature, there are few descriptions of lesions occurring in the stifle joints of dogs with patellar luxation. Cartilage erosion on the patellar articular surface was reported by Remedios et al. [30]; however, they did not provide information about the extent of the erosion or the weight, age or degree of luxation. Daems et al. [31] reported cartilage erosions mainly in heavier dogs and with grade IV patellar luxation. Destruction of cartilage on the articular surface of the patella in both medial and lateral luxation and marked synovial reaction at the origin of the long digital extensor tendon in lateral luxation were reported by L'Eplattenier and Montavon [3]; Pérez et al. [14] reported erosion in the medial trochlear ridge and shallow trochlear groove.

The results of this study indicate a high frequency of patellar and extra-patellar lesions, especially in grade II and III luxations. According to the literature, although these animals exhibit intermittent and persistent lameness, respectively [1, 5, 32], they continue to use the affected limb for ambulation. These facts account for the larger number of lesions found among animals with grade II or III luxation. The use of the affected limb leads to joint wear. This is in contrast to animals with grade IV luxation that do not bear their weight on the affected limb, but drag or carry it while the weight is transferred to the front limbs.

Roy et al. [19] did not find a significant association between the degree of luxation and the progression of radiological articular changes, which suggests that some of the alterations that are visible during surgery might not be detectable in radiological exams.

Lesions such as flattened patellar surface and shallow trochlear groove were also observed in

**Figure 8.** Photography of the stifle joint of a cat to surgery for patellar luxation. Notice the erosion of the edge of the medial condyle (A, blue arrow), flattened patellar surface (B, blue arrow) and shallow trochlear groove (C, blue arrow).

Patella luxation in small animals is a condition in which early treatment has not been highlighted. The suggested course has been to wait for the manifestation of clinical signs [5, 6, 8, 21], which results in lameness. However, dislocation and subluxation are already characteristic clinical signs of anatomical abnormalities, which certainly will cause articular changes. Subluxation [6] is a clinical challenge, especially in obese and heavy dogs like English Bulldogs,

In the literature, there are few descriptions of lesions occurring in the stifle joints of dogs with patellar luxation. Cartilage erosion on the patellar articular surface was reported by Remedios et al. [30]; however, they did not provide information about the extent of the erosion or the weight, age or degree of luxation. Daems et al. [31] reported cartilage erosions mainly in heavier dogs and with grade IV patellar luxation. Destruction of cartilage on the articular surface of the patella in both medial and lateral luxation and marked synovial reaction at the origin of the long digital extensor tendon in lateral luxation were reported by L'Eplattenier and Montavon [3]; Pérez et al. [14] reported erosion in the medial trochlear ridge and shallow

The results of this study indicate a high frequency of patellar and extra-patellar lesions, especially in grade II and III luxations. According to the literature, although these animals exhibit intermittent and persistent lameness, respectively [1, 5, 32], they continue to use the affected limb for ambulation. These facts account for the larger number of lesions found among animals with grade II or III luxation. The use of the affected limb leads to joint wear. This is in contrast to animals with grade IV luxation that do not bear their weight on the affected limb,

making it difficult to diagnose the cause of progressive lameness [6].

but drag or carry it while the weight is transferred to the front limbs.

cats with patellar luxation grades III and IV (**Figure 8**).

172 Canine Medicine - Recent Topics and Advanced Research

**9. Discussion**

trochlear groove.

Because of the anatomy and biomechanics, the friction between the articular surfaces of the patella and femoral trochlea will cause erosion of the medial femoral condyle and lateral patellar surface in medial luxation, and of the lateral femoral condyle and medial patellar surface in lateral luxation [8]. Subluxation also causes wear of the articular surface of the patella and flattening of the condylar edge that favours dislocation [6].

The quadriceps mechanism is responsible extension, and a healthy patellofemoral joint is essential for implementation of this function. The quadriceps, patella, trochlea, patellar tendon and tibial tuberosity must be aligned. Abnormal alignment of the extensor mechanism interferes with limb flexion and extension [8] and compromises its function.

Misalignment of the quadriceps leads the kneecap to become dislocated and press and brush on the lateral or medial condyle surface during limb movement. Erosion is observed in dislocations when the quadriceps' extension function is maintained. In these cases, the pressure of the quadriceps extensor during movement, acting on an improper surface for receiving the force, causes wear injuries. The injury is more severe in animals weighing more than 15 kg. The greater frequency of lesions in grade II and III luxations can be explained by moderate functional changes that allow member extension, promoting compression between the patella and condyle edge. In grade IV luxation, there is a significant decreased range of extension in the joint associated with contracture of the soft tissues caudal to the joint [6, 8] (**Figure 2B, D**), which prevents the friction pressure between the condyle and patellar edge. Patellar luxation is also responsible for the resulting absence of a trochlear groove.

In this study, the frequency and severity of articular lesions were higher among the animals weighing at least 15 kg, as Daems et al. [31] observed. This finding might be attributed to the biomechanical instability resulting from the greater load to which the stifle joint is subjected. Biomechanical stability is considered to be essential for an appropriate supply of blood to the articular cartilage [3], because inadequate nutrition results in joint degeneration, which is observed in cases of patellar luxation.

The high frequency of patellar lesions among animals aged older than 24 months might be the result of disease duration. As some authors have observed [5, 31, 33, 34], patellar luxation mainly affects young animals still in the growing phase, and they are often not referred for treatment for several reasons, among which, the lack of symptoms or the presence of only mild clinical signs, stands out. Consequently, alterations of the patella resulting from chronic friction have already appeared by the time surgery is performed. These are sufficient reasons for early surgical intervention based on the cause—patellar luxation—and not in the presence of clinical signs as reported in the literature [5, 6, 8, 21].

The severity of the long digital extensor tendon injuries, also mentioned in the literature [6], varies according to the chronicity of the case (**Figure 5A, B**) and affects small and large dogs.

As mentioned in the literature [21], severe skeletal changes associated with unilateral patellar aplasia were observed in one cat (**Figure 7A, B**), whose two brothers also had grade III patella luxation and no tail. The litter was a result of crosses between brothers.

The re-luxation rate observed in this study (12.9%) is within the range reported by Arthurs and Langlay-Hobbs [35] and Wandgee et al. [35], although rates up to 50% have been reported [36]. Arthurs and Langley-Hobbs [35] report a greater frequency of major and patellar re-luxation complications in dogs weighing 20 kg or more compared to smaller dogs. In this study, two (6.6%) re-luxations were observed among the 30 dogs that weighed more than 20 kg.

Recurrence of patellar luxation is a common complication associated with surgery; among the factors that contribute to re-dislocation are the severity of the lesions, because grade III and IV luxations indicate poor shaft alignment, failure to align the tuberosity of the tibia with trochlea, and contracture of soft tissues caudal to the joint, affecting the post-operative joint range of motion [6, 8]. As with the loss of convexity of the patella, it loses its slot in the femoral trochlea. Daems et al. [31] postulate that one of the reasons for surgical failure in stable post-surgical patellae is the presence of cartilage erosion. Mostafa et al. [14] proposed that the proximodistal malalignment of the patella (patella alta and baja) influenced re-luxation. Patella alta is defined as the proximal displacement of the patella within the femoral trochlear groove. It has been speculated that patella alta may play a role in canine patellar luxation [12, 37]. The position of the patella in the trochlea, as Mostafa et al. [14] observed, might be the reason for one case of patellar re-luxation in our study. One dog weighing more than 20 kg with recurrent grade II bilateral medial luxation was evaluated. To assess the patellar position on the femoral trochlea, radiographic evaluation was performed according to the method described by Mostafa et al. [13]. After osteotomy of the tibial tuberosity, as described by Johnson et al. [37] and distally transposing 0.5 cm the tibial tubercle, the patella remained in the groove.

We found no instance of implant failure among animals weighing at least 15 kg because the implants used were compatible with the dogs' weights.

The treatment was challenging in both cats with severe deformities of the distal femur, tibia and tail. It was not possible to correct all deformities, but there was improvement in ambulation. Although they were able to move around the house, as related by Hubler et al. [38], the gait abnormalities were extremely severe.

Post-operative measures, such as activity restriction and physical therapy, contribute to the success of treatment and might prevent some post-operative complications and favour an early recovery of muscle mass and the functional performance of the limb.
