[Biomechanical studies on plate prebending and pretensioning in compression osteosynthesis (author's transl)].

By tensioning plates which are contoured exactly to the bone shaft a torque is produced leading to gapping of the cortex opposite to the plate. The gapping depends on the amount of tension force and plate stiffness. In osteosyntheses performed on transversely osteotomized tibial and femoral shafts with plates adapted to the bone surface the gapping angles of tibia osteotomies with narrow plates are twice as large as those of femur osteotomies with broad plates. In order to eliminate the torque the plates must be prebent before application. Three phases have been defined to describe the tension process of prebent plates. During phase I the fragments are in contact opposite the plates only. By further tensioning the plates are straightened and the fragments come in full contact, thus beginning phase II. When the center of compression is situated in the middle of the diaphysis cross-section a uniform compression distribution is achieved. Phase III begins when the force along the plates distracts the cortex opposite the implant. The boundaries of the phases and the prebending angles which guarantee uniform interfragmental compression distribution in dependence of the tension forces are of paramount importance. Therefore the relationship between the forces necessary to bend back the prebent plates and the prebending angles were investigated. With the obtained values diagrams were constructed for compression osteosynthesis either with narrow round hole and DC plates applied to radius/ulna and tibia, or with broad round hole and DC plates applied to humerus and femur.