The patella (kneecap) normally sits in a little groove at the end of the thigh bone. Sometimes it can slip out of this groove, and this is known as patellar luxation. The best treatment for dogs with this condition depends on how severe their signs are. Some dogs manage well with conservative management, while others will need surgery.
What does the kneecap do and how does it dislocate?
The kneecap is held within the tendon of the quadriceps muscle (the big muscle at the front of the thigh). This tendon attaches onto the front of the shinbone, so that when the muscle contracts, this pulls the bone forwards and extends the knee. The patella acts as a pivot to help the knee joint extend.
Normally, the patella sits in a groove in the thigh bone. In some dogs, this groove is too shallow, and this means the kneecap can slip out. This is shown in the following animation:
Animation showing a comparison between a normal groove and a shallow groove in the knee:
When the kneecap slips out, this is known as 'patellar luxation'. This is more common in smaller breeds with a 'bow-legged' stance, as this stance tends to pull the kneecap sideways out of the groove as the dog grows up, meaning the groove does not develop properly. Predisposed breeds include Chihuahuas, Pomeranians, Miniature Poodles, Yorkshire Terriers and Boston Terriers. However, large breeds and sometimes cats can be affected too.
While patellar luxation normally occurs because the groove is too shallow, it can also happen secondary to injury.
What are the signs of patellar luxation?
The signs of patellar luxation vary depending on how severe the condition is. Patellar luxation is graded from 1 to 4:
- Grade 1: The kneecap usually sits within the groove, but your vet can displace it with gentle pressure
- Grade 2: The kneecap usually sits within the groove, but it spontaneously pops out when your dog is exercising
- Grade 3: The kneecap usually sits outside the groove, but your vet can replace it
- Grade 4: The kneecap usually sits outside the groove, and cannot be replaced
The difference between Grade 2 and 3 luxation is shown in the following animations:
Animation showing Grade 2 patellar luxation:
Animation showing Grade 3 patella luxation:
Animals with Grade 1 disease may show no signs, and those with Grade 2 disease may show a 'skipping' lameness where they hold the leg up for a few strides when running. Those with Grade 3 or 4 disease may limp all the time or appear stiff on their back legs as their knees do not extend normally.
Signs often begin at a young age, though the condition can also develop in adult dogs. If the patella continues to dislocate in and out of the groove for some time, this can cause arthritis. The condition also makes dogs more likely to develop disease of the cruciate ligaments in the knee.
What will my vet do?
Your vet will be able to assess whether your dog has a luxating patella simply by examining them. However, they may well advise other tests such as X-rays of the knee to see if there are any other problems that have developed alongside or as a result of the condition. Sometimes, advanced imaging such as CT or MRI scans may also be advised.
Your vet will then be able to advise what is best to do for your pet. In general, surgery is advised for dogs with severe patellar luxation (Grades 3 and 4) and sometimes for dogs with Grade 2 luxation, depending on whether the dog is showing significant signs. Dogs with Grade 1 luxation will not require surgery.
Non-surgical management includes weight control, a controlled exercise regime and physiotherapy or anti-inflammatory painkillers when necessary. There are various surgical options available and two of them are shown in the animations below.
In a 'tibial tuberosity transposition', your vet will shift the part of the shinbone (tibia) where the quadriceps muscle attaches. This means the kneecap is better aligned with the joint so it is less likely to slip out of line.
Animation showing a tibial tuberosity transposition to correct patellar luxation:
Another option is to deepen the groove where the patella sits so it is less likely to slip out. In this procedure, your vet will cut out a 'wedge' of the middle of the groove, remove some bone to make a recess, and then replace the wedge so it sits a bit deeper.
Animation showing how deepening of the groove can correct patella luxation:
Following surgery, it is important to follow your vet's recommendations regarding aftercare. Typically, dogs will need to be strictly rested for at least 6 weeks, and then gradually return to normal exercise. Physiotherapy and hydrotherapy may be recommended to aid recovery. In general, the prognosis is good after surgery provided there are no particular complications.
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