Patellar luxation (medial, lateral and upward) and instability of the femoropatellar joint are the major causes of lameness in the hind limbs of dogs. Among these disorders, medial patellar luxation (MPL) is considered congenital and also reported to be hereditary. An epidemiological survey covering all canine breeds has shown that MPL accounts for 7580% of the cases of patellar luxation, that it is bilateral in20-25% of the cases. It is also reported that it may occur 1.5 times as frequent in females as in males.
Our research group has recently published a report, suggesting that "medial luxation and laxity of the patella at the femoral trochlea occur because of displacement of the quadriceps femoris muscle group towards the medial side of the thigh due to tension in the vastus medialis at the fetal stage, and sustained tension of the muscle group resulting in deformation of the epiphyseal growth plate in the hind limb skeleton and hypoplasia of the joint structure". This has been recently drawing much attention as a major hypothesis on the etiology of MPL. Subsequently, the conventional methods of diagnosis and treatment of MPL have been reviewed and various studies are presently underway for finding more useful methods of diagnosis and treatment.
Clinical symptoms of MPL are varied, ranging from "mild instability of the patella bone in the trochlear groove" to "complete luxation", representing different grades of severity of the conditions. For convenience, our research group has classified MPL into two types, habitual and permanent. The characteristics of each of these categories are explained below.
Mostly occurs in the late stage of growth or after the dog has become an adult. As for the clinical symptoms, the skeletal morphology is near normal and the gait can be varied, ranging from "occasional carrying of the affected limb during walking but returning to normal gait immediately" to "frequent intermittent lameness or carrying of affected limb". Normal stifle function can be restored through appropriate corrective surgery in most cases of habitual MPL with such clinical symptoms.
In permanent MPL, luxation is already present at the time of birth. The clinical symptoms include sigmoid skeletal deformation of the distal femur and the proximal tibia. When such puppies are laid on their backs and held, they cross their hind legs at the various points, such as toe ends, the tarsi, and the stifles. In contrast, normal puppies alternately flex and extend the hind limbs, pointing their soles upwards. The more severe the case, the more proximal point the puppies cross their hind limbs. In very severe cases, puppies bend the stifle in a posture like that of a Zen Buddhism priest sitting in meditation. In addition to this, the extension of the joint becomes extremely limited in such severe cases. Most of these cases show the gait like the unsteady walking of a small child, with significant bowleg of the hind limbs or bent knees from the age of about 4 weeks, when the puppy starts walking on four legs. The conditions of the puppies, which have permanent MPL showing such clinical symptoms, only get worse with growth, and the maximum development of the symptoms occur during the early stage of growth (4-6 months after birth). Whatever treatment given after this stage is, it would not restore normal skeletal structure, bone growth and motor functions.
Until now, there was no effective therapy for permanent MPL. When our research group gave passive flexion and extension exercise of the stifle joint to newborn puppies, not more than30 days old, for strengthening the muscles, expanding the range of motion of the stifle joint, and promoting skeletal growth, all cases showed remarkable improvement in motor functions of the hind limbs, and even surgical intervention became unnecessary in many cases. Moreover, corrective surgery on patients who had not shown sufficient improvement with passive exercise of the stifle joints, undertaken at least 45 days after birth, when general anesthesia became possible, resulted in the skeletal structures showing the signs of normal skeletal morphogenesis that had earlier shown inhibited growth or hypoplasia because of patellar luxation. Furthermore, it was confirmed that the motor functions, such as walking, also became normal.
The importance of passive flexion and extension exercise of the stifle as the first medical intervention in effective treatment of severe MPL (permanent MPL) became clear from the above results. This is because, the passive exercise can strengthen the muscles of a hind limb whose development has lagged behind from the fetal stage, expand the range of motion of the affected stifle joint, and significantly improve the therapeutic effect of corrective surgery. Permanent MPL can be diagnosed soon after birth. Then, normal skeletal morphogenesis, bone growth and motor functions can be restored through corrective surgery by the age of 3 month. In other words, collective surgery should not be performed on the adult dogs with permanent MPL due to the reason that the appropriate period for the treatment has already passed.
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