A luxating patella is a “trick knee” or a “floating knee” and in which the kneecap moves out of it’s normal spot. Unfortunately, this can be somewhat common in Boston Terriers and must monitored between the ages of four to six months.
The issue can be cause by trauma to the dog’s leg but most commonly it is caused due to a developmental congenital defect. The condition can also be caused by genetics or obesity.
Diagnosis is made through palpation of the knee to see if it slips inside the joint more than it normally should. Luxating Patella cannot be present without the knee being loose, but a loose knee is not necessarily slipping out of the joint. Diagnosis can only be made from physical examination and manual manipulation, so please see your vet if you fear your dog may have this condition.
There are four grades of luxating patella:
Grade I – the patella can be manually luxated but is reduced (returns to the normal position) when released;
Grade II – the patella can be manually luxated or it can spontaneously luxate with flexion of the stifle joint. The patella remains luxated until it is manually reduced or when the animal extends the joint and derotates the tibia in the opposite direction of luxation;
Grade III – the patella remains luxated most of the time but can be manually reduced with the stifle joint in extension. Flexion and extension of the stifle results in reluxation of the patella;
Grade IV – the patella is permanently luxated and cannot be manually repositioned. There may be up to 90° of rotation of the proximal tibial plateau. The femoral trochlear groove is shallow or absent, and there is displacement of the quadriceps muscle group in the direction of luxation.
Unfortunately, Grades II, III and IV require surgery to correct, if the animal has difficulty walking.
[Information provided by PetMD]