A surgical stapler for effecting simultaneous cutting and stapling of tissue includes a frame and a staple cartridge positionable in the frame. The stapler includes a knife blade mounted for movement distally of the staple cartridge, the knife blade having a retraction lock feature including a locking portion for abutment to a roof member of a spacer carried on the frame in a locked position, to prevent movement of the knife blade, unless the stapler is fitted with a fresh, unused staple cartridge. The locking portion is spring loaded by a leaf spring into the locked position. The knife blade includes a spring notch which coacts with the leaf spring to mechanically lift and pivotally thrust the knife blade to the locked position when the knife blade is retracted after use. The lift or thrust feature clears out tissue accumulation which enhances the ability of the knife to restore to its full elevated return position.