EMPLOYEE REPORTED: HELPING COWORKER INSTALL ROOF BOLTS ALONG THE LONG WALL FACE, SLIPPED IN THE WET ENVIRONMENT AND BUMPED HIS LEFT KNEE AGAINST THE LONG WALL PAN LINE. CONTINUED TO EXPERIENCE PAIN AND SWELLING TO THE KNEE. DETERMINED AT A LATER DATE, TORN CARTILAGE IN THE KNEE. STARTED LOSING WORK DAYS 7/23/02, BECAME AN MSHA REPORTABLE ON THIS DATE.