The invention relates to a device for fixing screws in osteoporotic bones, formed by biocompatible fixing elements and an instrument for placing same. The instrument consists of two parts, a male part (A) and a female part (B), which can perform a relative linear sliding movement. The caudal end of each part has a pivoting jaw (Pa, Pb) forming an inverted U therebetween. The biocompatible element is placed in the jaw of the male part, which element will be traversed by a screw inserted through interchangeable guides (g) placed in the other jaw, acting as a locking and fixing nut for said screw. Both parts have a handle (m) to allow moving the jaws closer together. The opening of the device, restricted by a closure (C), is achieved by the action of a compression spring (R), upon releasing, by the action of the thumb, the catch inserted in the upper face, cut in a saw-toothed manner, of part A.
