My knees are not set well within the socket. I learned this after reading for a couple hours with one leg tucked back under me and when I went to stand not having a leg to stand on. It had dislocated itself from the position. I had minor knee surgery for that one but learned that I have to sit with my legs straight down or straight up but never bent under me again (or crossed). The knee dislocated six more times before I had MAJOR surgery. I always check to make sure its in and locked before I stand now. The older I get the easier my body falls apart. Take care of yours!