Abstract: In one embodiment, the structure comprises, in addition to a body which is usually cylindrical or tubular, a plurality of tines which extend radially from the body. The tines, which are usually flat in shape and tapered, are positioned, relative to the body, with a keying angle other than zero and hence in a generally helical arrangement. The principal axes of the tines in question are preferably also inclined to the longitudinal axis of the body, giving rise to a generally anchor-like shape.