Thank you so much, everyone, the TDK white light has worked its magic again! While my parents and I don't want to count our chickens too soon, the news on Sam is much better than expected.
He does have a fracture, poor lamb. My parents got him as a rescue cat from the SPCA when he was already two or three years old, so we don't know his history. But the x-rays showed an old -- and bad -- fracture to his leg, which he must have got as a young cat and which had never properly healed. Somehow he's broken it again and -- because it didn't heal properly the first time -- has also snapped his cruciate ligament. So he does need an operation, poor lad, but it's not as major as we feared, and it seems that he won't lose the leg, which we also feared. So we are counting our blessings tonight and feeling much more optimistic.
Again, thank you all so much for your kind words and thoughts. Sam is a cheerful cat and a fighter so I am sure he can come through this, especially if you keep a kind thought in the back of your mind for him. Bless you all for your kindness!