Please see Dr. Lee's answer above which is quite correct. Expect that an anesthetized orthopedic examination with xray imaging of knees, hips and possibly lumbar spine will be suggested to better clarify the problem. Muscle tone can mask orthopedic disease and complete relaxation (a s when anesthetized) is required for thorough examination and for proper positioning for xray imaging. Good luck and Happy New Year!
Some breeds are more predisposed to a "luxating patella," which is when the kneecap can move in and out of the knee joint. Rarely, smaller breed dogs can get an "ACL" injury too, but this is less common and typically a large breed disease (obese Labrador).
There are surgeries that can be performed for LP, and if it's severe (the knee cap is locked out the majority of the time), it should be surgically corrected by "deepening" the knee joint so the knee cap sits in there better. Otherwise, chronic arthritis can occur. When in doubt, take your dog to a vet for an exam and x-rays.
I as well have a Westie and am wondering what about his back leg? Is he going up and down the stairs ok? Or is he sort of "skipping" when he walks, runs, climbs stairs? If this is the case his knee could be slipping out of place. My friends Westie has the same problem by favoring her back left leg and that's what the vet told him. There's really nothing that can be done about it as long as he's not in any pain. Please let me know if this may help you a little at least.