Force transfer between the plate and the bone: relative importance of the bending stiffness of the screws and the friction between plate and bone

Stability in plate fixation of fractures relates to the motionless fastening between plate and bone. When the plate is affixed to the bone, high shear force may appear between plate and bone, particularly near the end screws, which can lead to motion under weight-bearing. Two mechanisms might be involved in the prevention of motion: the bending stiffness of the screws or the friction between the screw and the plate. Experiments performed in vivo and in vitro show that with conventional plates, motion is prevented by friction and depends upon the axial force of the screw, pressing the plate onto the bone. The torque applied to the screws is crucial. Motion appears under smooth plates under relatively low physiological loads. With newly developed internal fixators, the motion is prevented by the structural stiffness of the plate-screw system.