EE was moving a tailpiece on a unit. As EE placed material on the tailpiece, a section of the rib broke off & hit EE from behind knocking EE to the mine floor. EE was taken to the hospital to be examined and later released to return to work. EE continued to work until an MRI revealed that EE had a torn ACL & Meniscus. EE started missing work on 04/24/18, waiting for surgery.