Loads on a spinal implant measured in vivo during whole-body vibration.

After spinal surgery, patients often want to know whether driving a car or using public transportation can be dangerous for their spine. In order to answer this question, a clinically proven vertebral body replacement (VBR) has been modified. Six load sensors and a telemetry unit were integrated into the inductively powered implant. The modified implant allows the measurement of six load components. Telemeterized devices were implanted in five patients; four of them agreed to exposure themselves to whole-body vibration. During the measurements, the patients sat on a driver seat fixed to a hexapod. They were exposed to random single-axis vibrations in X, Y, and Z directions as well as in multi-axis XYZ directions with frequencies between 0.3 and 30 Hz. Three intensity levels (unweighted root mean square values of 0.25, 0.5 and 1.0 m/s(2)) were applied. Three postures were studied: sitting freely, using a vertical backrest, and a backrest declined by an angle of 25 degrees . The patients held their hands on their thighs. As expected, the maximum force on the VBR increased with increasing intensity and the number of axes. For the highest intensity level and multi-axis vibration, the maximum forces increased by 89% compared to sitting relaxed. Leaning at the backrest as well as lower intensity levels markedly decreased the implant loads. Driving a car or using public transportation systems-when the patient leans towards the backrest-leads to lower implant loads than walking, and can therefore be allowed already shortly after surgery.