[Torque load and types of fractures of threaded holes in cortical bone by lag screw. Mechanic and histological studies (author's transl)].

For the better understanding of fracture fixation by means of screws the torque load and types of fractures from threaded screw holes in cortical bone were studied. The experiments were performed on sheep tibia and femora in vitro and in vivo. We applied the 4.5 mm cortical bone screw (AISF) as lag screw. The screw was threaded in until total destruction of the thread occurred. The process of torque load was registered as a torque-angle of twist curve with 4 typical parts. Histologically in most cases shear fractures with different geometry occurred. In some cases we found conical geometric, and in others cylindrical geometric fragments in the threaded screw holes. The value of the torque at which destruction of a threaded hole occurs is a linear function of the thickness of the cortical bone. The gradient of this linear function is 6.6 kpcm/mm. In practical case, the insertional torque may be estimated by multiplying the cortical bone thickness in mm (if known) by a factor 4.