An expansible and retractable mechanism of the type disclosed in U.S. Pat. No. 5,924,175 and/or a medical grasper of the type disclosed in U.S. Pat. No. 5,906,622 in which (1) the fixed flexure elements are self-biased to move into the expanded condition thereof. A handpiece for the medical grasper is constructed and arranged so that after a stone has been grasped, a further digital pull on the movable handpiece structure effects the application of a limiting resiliently yielding force to the gripping action.