CLAMPING STIFFNESS AND ITS INFLUENCE ON LOAD DISTRIBUTION BETWEEN PAIRED INTERNAL SPINAL FIXATION DEVICES

The load distribution between two internal spinal fixation devices dep ends, besides other factors, on their stiffness. The stiffness ranges were determined experimentally for the clamps of the AO internal fixat or with lateral nut and with posterior nut as well as for the clamps o f the SOCON fixator. The stiffness of eight devices each differed by a factor of 3.1 for the clamp with lateral nut, by a factor of 1.5 for the clamp with posterior nut, and by a factor of 1.4 for the clamp of the SOCON fixator. For the AO clamp with lateral nut, the influence of the nut-tightening torque on the stiffness was determined. Using inst rumented internal spinal fixation devices mounted to plastic vertebrae and simulating a corpectomy, the load distribution between the implan ts was measured for different tightening torques. It could be shown th at, for the AO internal fixator whose clamps have a lateral nut, a nut -tightening torque >5 Nm has only a negligible influence on load-shari ng between the implants. Tooth damage occurs when the teeth of the cla mp body and clamping jaw of the clamp with lateral nut do not gear tog ether exactly, which leads to changes in the clamping stiffness and lo ad-sharing between the two implants.