Multiple car wrecks did it for me. After that, being so screwed up from the wrecks, I had a couple of bad falls as well. Talk about a mess and felt like both knee caps broke when I hit the dash, then the blacktop parking lot holding 50 pounds of books. But believe me, I saved those damn books. LOL!!! My knees, on the other hand, took the damage that the books should have gotten and there ya go. SMH. LOL