EE was performing preventive maint.@#276 belt Dr. #13 mainline, tripped on a joy pin anchor that was hidden by accumulation of muck in drive enclosure. EE was given 1st aide & returned to work. EE continued to work while under Dr. care w/unrestricted work activity. EE had a MRI & has seen a Ortho Specialist. EE had Surgery on 1/23/2012 to repair cartilage damage to his L. knee.