EE was making a screen replacement on screen #2. EE had placed new screen in place & was exiting the upper level of screen #2. EE placed EE's feet onto spring cover to place screen rail & bolts. After installation EE began dismount from spring cover onto catwalk approx. 2 ft. Rt foot slipped off, lost balance & Rt foot wedged Btwn deck & screen. Body twisted causing strain to knee.