2000 Volvo Award Winner in Biomechanical Studies - Monitoring in vivo implant loads with a telemeterized internal spinal fixation device

Study Design. Implant loads were measured in 10 patients using telemeterize d internal spinal fixation devices. Objective. To determine the postoperative temporal course of implant loads. Summary of Background Data. Little information exists regarding the tempor al course of loads on internal spinal fixation devices. Methods. The telemeterized internal spinal fixator allows the measurement o f three force components and three moments acting in the fixator. Implant l oads were determined in up to 20 measuring sessions for different activitie s, including walking, standing, sitting; lying in the supine position, and lifting an extended leg while in the supine position. Results. Implant loads often increased shortly after anterior interbody fus ion was performed. Several patients retained the same high level even after fusion had taken place. This explains the reason why screw breakage someti mes occurs more than half a year after implantation. The time of fusion cou ld not be pinpointed from the loading curves. Conclusions. The results show that fixators may be highly loaded even after fusion has occurred. A flexion bending moment acts on the implant even wit h the body in a relaxed lying position. This means that already shortly aft er the anterior procedure, the shape of the spine is not neutral and unload ed, but slightly deformed, which loads the fixators. Pedicle screw breakage more than half a year after insertion does not prove that anterior interbo dy fusion has not occurred.