There are a number of different things that could be causing this problem in your Westie, and although Legg Perthes disease is on the list, it is way WAY down on the list. This is because he really is too old to be considered a strong risk of getting this problem.
Legg-Perthes disease is also called Aseptic Necrosis of the Femoral head. It is a problem that mostly affects small and toy breeds of dogs between 3 and 13 months of age. Usually, it is diagnosed between 5 and 8 months.
What happens is that the blood supply to the head of the femur in the hip joint seems to be cut off (we aren't sure why) and thus the head of the femur starts to break down.
Here is more about it:
There are, however, a few things that I would consider in a Westie dog this age. If your dog came to see me, the things that I would be checking him for are as follows:
1. He could have a patellar luxation. This is a problem in some smaller dogs in which the groove that the patella (kneecap) sits in isn't deep enough. With this problem, the patella can pop out of the groove and suddenly the dog cannot use the leg. You can see this often if you watch Jack Russels running - you will see them hold up a hind leg for a few beats and then put it down again. This is a classic symptom of a patellar luxation, and then spontaneous return to the normal position. BUT sometimes it doesn't pop back in. In these cases, surgery is usually recommended to deepen the groove in which the patella sits. Has your Westie boy ever had this "skipping" type behaviour on either hind leg in the past? If yes, then I would be very suspicious that this is the problem.
Here is more: http://www.petplace.com/dogs/medial-patella-luxation-in-dogs/page1.aspx
2. He may have stretched or ruptured his cranial cruciate ligament. This is something humans get as well, but in humans it is called Anterior Cruciate Ligament.
The cranial cruciate ligament is one of the structures that stabilizes the knee joint. It is prone to problems, especially in overweight dogs, though it can occur even in slim dogs. If the dog is in motion, and the lower leg is held still (by going down a hole, getting caught on a string, etc) while the upper leg keeps moving forward, there will be damage to this cranial cruciate ligament.
The symptoms seen are usually a sudden onset of hind-end lameness, with toe touching seen at standing and walking, but the dog often carries the leg when running.
Here is more information about this problem:
3. Your boy may have a fracture (broken bone) of one of the tiny bones in the foot. I have seen this many times with a dog getting their toes jammed under the door of a crate, and then twisting somehow and causing a fracture, usually in the foot area. Symptoms are a sudden onset of non weight bearing lameness. X-rays would help to diagnose this.
Here is more: http://www.petplace.com/dogs/fracture-of-the-metatarsus-and-metacarpus-in-dogs/page1.aspx
So, in summary, there are a number of different orthopedic conditions that can all give these same symptoms you are describing in your Westie.
In order to diagnose this, a veterinarian would have to perform a thorough orthopedic examination, and a neurological exam. The vet might well suggest an x-ray. For this part, it might be necessary to give your dog a pain killer to allow proper positioning for the x-ray without hurting him.
I do think your dog needs to see his vet to have this looked at. Something is going on, and the sooner it is diagnosed and treated the better. He may just need some anti-inflammatories until he feels better!
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