Is it possible to simulate physiologic loading conditions by applying pure moments?: A comparison of in vivo and in vitro load components in an internal fixator

Study Design. Loads acting in an internal fixator measured in vitro under the application of pure moments such as those commonly used for implant testing and basic research were compared with loads measured in 10 patients in vivo. Objectives. To investigate whether these recommended loading conditions are valid by comparing in vivo measurements and those obtained in an in vitro experiment. Summary of Background Data. Pure bending moments are often preferred as loading conditions for spinal in vitro testing, either for implant testing or basic research. The advantage of this loading pattern is that the bending moment is uniform along the multisegmental specimen. However, functional loading of the spine by muscles or external loads subjects the spine to a combination of forces and moments. Methods. In an in vivo experiment, loads acting on an internal spinal fixator in 10 patients were determined before and after anterior interbody fusion during flexion, extension, left and right lateral bending, and left and right axial twisting of the upper body with the patient standing. For comparison, an equivalent in vitro data set was created with 7 human lumbar specimens in which the same type of fixator was used. All specimens were tested under the application of pure bending moments in the three main motion planes in the intact state with fixator, after corpectomy, and with bone graft. Results. Consistent qualitative agreement between in vivo and in vitro measurements for the loads acting in the internal spinal fixator were found for axial rotation and lateral bending. For flexion and extension, reasonable agreement was found only for the intact spines with fixators. After corpectomy and after inserting a bone graft, the median values for axial force and bending moment in the sagittal plane in vitro did not agree with in vivo measurements. An axial preload in the in vitro experiment slightly increased the axial compression force and flexion bending moment in the fixators. Conclusions, The application of pure moments to intact lumbar spinal specimens in vitro produces forces and moments in implants comparable with loads observed in vivo. During basic research on intact specimens or implant testing involving a removed disc or corpectomy, muscle forces are necessary to simulate realistic conditions.