EE WAS WALKING DOWN STEPS FROM HIS WORK STATION. HIS LEFT FOOT CAUGHT ON FOURTH STEP FROM BOTTOM OF STAIRS. HE WAS UNABLE TO HOLD HIMSELF WITH THE HANDRAIL CONSEQUENTLY ALL HIS WEIGHT WAS APPL IED TO THE ANKLE WITH THE FOOT POINTED STRAIGHT BECAUSE HIS BOOT HEEL WAS CAUGHT ON STAIR TREAD. HE FELT A ""POP"" IN THE JOINT AND SAT DOWN AT THE BOTTOM OF THE STAIRS.