Well, because I couldn't spell it, I initialed it but Su helped me out(thanks)He tore one knee racing to the door between the frig and a chair that was pulled out instead of in (where it belongs Dad!)and skidded to a stop and the other by favoring it. A week later it was properly diagnosed so during that time he must have been in agony (no signs of it just couldn't get up well)My vet rushed over one night with a stretcher (me hysterical at 11pm leaving frantic phone messages)The next day made energency appt w/ortheopedic surgeon. Three months in solidary then the other leg 3 months later. He was the best patient, I on the other hand was a mess, took off work in order to help him outside and comfort him during both surgeries. God I love this dog! He is a Golden Retreiver/Yellow Lab mix DOB: Dec 2 1991. Oh..never answered your ?..I think it's the ligament behind the knee that enables it to bend and bear weight-lots of human sports players get it constantly. By the way, he lost 2lbs! 98 now lots of walking to keep limber (arthritis in hips, front left paw) masssages (doggie heaven) and NO people food (99.9% of the time) Just shut me up! Cody! Cody! Cody!! Hugs and kisses to you all for your comments!