[The influence of the countersink geometry on the cortical screw force (author's transl)].

In vitro examinations on deformation and relaxation were carried out on cortical countersink of different geometry under a compression load of up to 2000 N through a screw head. Conical countersink showed plastic deformations of an average of 1300 N whereas with spherical countersink they did not occur at the same load. After reaching the load of 2000 N and keeping the deformation constant we saw a significantly stronger reduction of force with the conical countersink then with the spherical type. As far as fracture fixation is concerned a smaller postoperative reduction of screw force can be expected if applying a cortical screw with a spherical head in a spherical countersink.