Employee was observing a miner 3 slushing. Nut on the dydag holding the idler block & cable unthreaded itself, hitting employee above the right knee. He was sent for x-rays, which were negative. He returned to work, missing no days. His knee continued to bother him. MRI was done on Sept 09. ACL tear. Surgery scheduled for 11/11/09, which was his first day missed.