Vet thinks Hoshi has a trick knee

Last Friday Hoshi let out a whimper jumping down the basement steps. He always flies down them skipping the last few altogether. He also fell flat on his face going up the steps. I took him to the vet and I think she said she could move his left knee cap out of place (luxating patella) which isn’t normal. She gave him some pain pills and suggested he rest for a few days. He’s better now but I’ve suspected he has some kind of problem by the way he walks.