I agree with Judith in the amount of information needed to help is missing. Where is your pain? Do you have any pins and needles or numbness? Why did you have the lumbar fusion in the first place? What was effected by your lumbar spine? Generally back surgery occurs because of numbness or weakness in the legs. If your leg is giving way the number of times you mention, it is possible that the swelling is due to this rather than to damage in the knee.