One of my cats unexpectedly started limping about 2 weeks ago. She was fine one day, then the next day she was heavily limping on her left hind leg and within another day or so she would not even attempt to step on it, simply jumping on one hind leg keeping the other close to her belly. If it's important, she is around 12-13 years old (she's a rescue, so nobody really knows for sure).
As it was Christmas, it was impossible to get to the vet on the day, so I took her to a vet as soon as I could a couple of days later. They did an x-ray and afterwards the vet told me that from the x-ray it's most likely that she had a torn ligament in her knee. I took the cat home along with 2 weeks' worth of anti-inflammatory medication. They said a surgery could be considered, but only after the cat would lose some weight. Otherwise, just give her a lot of peace and quiet, make sure she doesn't need to jump/run/etc - and it would heal on its own over a period of a couple of months or maybe a bit more.
A few days later I got a phone call from the vet that they were reviewing the x-rays again with another vet and that other vet commented that there may be a bit of bone loss in her leg, which means that she's got a bone tumor and her leg would need to be amputated to spare her constant pain. The vet emphasised that such a tumor is usually benign.
Now, this doesn't seem to add up for me. If it were bone loss caused by a bone tumor, it would be gradual; instead, the limping came on abruptly: everything was perfectly fine one day and she was not stepping on the leg the next day. Also, she doesn't seem at all to be in pain. Her behaviour hasn't changed other than less movement. She is still happily eating and drinking.
I'm taking her to the vet again tomorrow - they want to do a biopsy of the tissue. Yet, I'm now suspicious about the whole thing. Obviously, if there's no other way to save the cat, we'll go for the amputation, but I don't want to do it just because "whatever". The vet stated it almost in passing, the same way I would state that I need to buy some cat food.
My question is: how likely is this really to be a bone loss due to tumor vs. what else it can be? Even if this is a tumor, is there any way to avoid amputating the leg?
In regards to her weight, for her size, the ideal weight is about 5 kg. She is currently 6.7 kg. She was abused early in her life, then taken away from her owner by RSPCA and I adopted her from RSPCA. When I brought her home, she was under 3 kg. Despite trying to control her diet, she quickly gained weight (she'd go outside and catch mice or birds and eat them) and got to 8.5 kg within a year. We've managed to lose about 2 kg since then. Overall, she's lived with me for about 10 years or so.