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 telemeterized internal spinal fixation devices. Objective. To determine the postoperative temporal course of implant loads. Summary of Background Data. Little information exists regarding the temporal course of loads on internal spinal fixation devices. Methods. The telemeterized internal spinal fixator allows the measurement of three force components and three moments acting in the fixator. Implant loads were determined in up to 20 measuring sessions for different activities, 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 fusion was performed. Several patients retained the same high level even after fusion had taken place. This explains the reason why screw breakage sometimes occurs more than half a year after implantation. The time of fusion could 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 with the body in a relaxed lying position. This means that already shortly after the anterior procedure, the shape of the spine is not neutral and unloaded, but slightly deformed, which loads the fixators. Pedicle screw breakage more than half a year after insertion does not prove that anterior interbody fusion has not occurred.