EE WAS REPAIRING AN AIR LEAK ON A DRILL THAT WASON TOP OF A TRAILER. FINISHING, HE EXITED THE DRILL ONTO THE TRAILER. HE THOUGHT HE WAS ON THE GROUND AND STEPPED OFF OF THE TRAILER ONTO THE GR OUND. HE LANDED ON HIS LEFT LEG THEN FELL TO THEGROUND. MEDICAL TREATMENT SOUGHT ON 12/14 AND A FOLLOW UP ON 12/19 PLACED EMPLOYEE ON LIGHT DUTY.