What is it and how does it occur?
Patella luxation is otherwise known as knee dislocation and is a common condition typically affecting young small and toy-breed dogs. The patella in each knee lies in a trochlear groove and when it luxates it moves out of this groove causing pain and instability. Usually luxations are related to congenital changes, that is, the animal is born with a shallow groove or the quadriceps tendon attaches to the tibia bone unusually so that the patella slips out when the quadriceps are flexed. Trauma can also cause luxation and obese dogs are more prone to luxation.
Initially pain may only be apparent when the patella is not sitting in the groove however over time the groove becomes worn and flattened so the patella slips in and out easily and arthritis develops within the joint.
What are the signs?
Most dogs will show intermittent lameness. Often the lameness is described as suddenly lifting the back leg up at an odd angle for a few steps, perhaps flicking the leg a few times before weight bearing on it again. If arthritis has developed in the joint the lameness and pain will be more constant and severe. Some patients will have luxating patellas that never cause discomfort and are only noted on veterinary physical examination.
We tend to grade patellar luxation based on severity:
Grade 1: The patella luxates with manual pressure and returns spontaneously.
Grade 2: The patella luxates with flexion and extension of the joint but returns to the trochlear groove spontaneously. Some lameness may be present.
Grade 3: The patella luxates with flexion and extension of the joint but can be reduced manually. Considerable lameness exists.
Grade 4: The patella is permanently luxated out (usually towards the middle of the body). The limb or limbs are unable to extend causing severe lameness.
How is it diagnosed?
As mentioned above patella luxations, particularly those of low Grade may be noted on a routine physical exam. During the physical examination, the vet might simply feel the knees and be able to manually push the patella out of place. For those patients presenting lame a more thorough exam of the joint may be performed which, if the patient is sore may need to be completed under sedation. X-rays (under general anaesthetic) may be recommended to further examine the joint and rule out other causes of lameness or secondary arthritis.
Treatment in mild cases may include exercise restriction, non-steroidal anti-inflammatories (NSAIDs), “Zydax” pentosan polysulphate injections (usually one a week for 4 weeks, followed by repeat courses every 3 to 6 months), nutraceuticals & supplements (such as “Glyde”) and weight reduction in obese dogs.
If lameness and pain persists and becomes more prominent, surgery should be considered. Grade 3 and 4 patellar luxation typically require surgical correction. There are many different approaches to surgery, which may depend on the patient, grade and the surgeon’s preference. Sometimes a general vet can perform these procedures whilst more complicated cases may be referred to a specialist veterinary surgeon.
Surgery usually involves deepening the trochlear groove so the patella sits more firmly in place, moving the quadriceps muscle to tighten its hold on the patella and soft tissue manipulation to reduce tension on the joint. If both knees are affected, they will likely be operated on separately to ensure the patient has one “good” leg to stand on. Post-operatively patients require a 4-8 week period of strict confinement and following this prognosis is good and reluxation unlikely (although in some more severe cases still possible). Complications of surgery may include wound breakdown, implant failure, infection or inadequate effect so luxation still occurs however all of these are rare.