EE using 16' extension ladder(extended out two rungs-10'overall)to access a cover plate on mill feed pipe. EE was on 2 or 3rd rung of ladder (approx 2'-3' high) Rung lock wasn't fully engaged, ladder slipped, EE fell to ground. Sent employee to phys. for evaluation-was placed on restricted duty-no ladder climbing, heavy gripping with index finger. will re-evaluate 3-23