My cat has VERY weak hind legs, but is otherwise normal. 2 dif. vets could not tell me what is wrong.?

I have been to 2 different vets, the only options are a CT scan & spinal tap. She is playful(well, as much as she can be w/o the use of her back legs), eats, drinks, & purrs. Does anyone have any idea what this could be?
Update: I have been to 2 different vets, the only options are a CT scan & spinal tap. She is playful(well, as much as she can be w/o the use of her back legs), eats, drinks, & purrs. Does anyone have any idea what this could be? Okay, I forgot to mention that she is a 7 mo. old persian that is indoors. The vets know... show more I have been to 2 different vets, the only options are a CT scan & spinal tap. She is playful(well, as much as she can be w/o the use of her back legs), eats, drinks, & purrs. Does anyone have any idea what this could be?

Okay, I forgot to mention that she is a 7 mo. old persian that is indoors. The vets know she has arthritis in the knees, but do not think that is the reason for the weak hind legs. I don't mind the CT scan, but the spinal tap sounds rather painful, even if she is drugged.
Update 2: Let's see....my cat weighs 6 pounds. She has a luxating patella(floating knee cap) in both hind legs, but the vet does not think this is the problem. I have had knee problems in the past and I know it is extremely painful. I was using Meow Mix for a while, but have changed to Nutro Natural Choice. I really... show more Let's see....my cat weighs 6 pounds. She has a luxating patella(floating knee cap) in both hind legs, but the vet does not think this is the problem. I have had knee problems in the past and I know it is extremely painful. I was using Meow Mix for a while, but have changed to Nutro Natural Choice. I really appreciate everyone's answer!
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