In-vitro biomechanical testing has been critical in the design and evaluation of spinal surgical instrumentation, however determination of realistic physiologic loading levels has proven difficult outside of the in-vivo setting. Unconstrained pure moment testing combined with the hybrid testing method is currently the gold standard test protocol for evaluation of motion preservation technology and adjacent level effects. Pure moment testing is well suited for making relative comparisons between treatments, but is currently not based on or representative of in-vivo spine motion, bringing the clinical relevance into question.

The human cervical spine supports substantial compressive load in-vivo arising from muscle forces and the weight of the head. However, traditional in-vitro testing methods rarely include compressive loads, especially in investigations of multi-segment cervical spine constructs. Therefore, a systematic comparison of standard pure moment testing without compressive loading versus published and novel compressive loading techniques (follower load, axial load, and combined load) was performed. To achieve a pure moment test, a robot/UFS testing system was programmed with hybrid control, which combined load and displacement control to overcome the limitations of either control methodology alone. A follower load system was developed with actively controlled linear actuators and integrated into the robot/UFS testing system’s control algorithm. Thorough investigation of the integrated system ensured that the pure moment assumption was upheld and enabled characterization of the kinetics resulting from the application of follower load. In contrast, axial load was applied perpendicular to superior most vertebral body using the robot end-effector;​ it did not maintain the pure moment assumption resulting in alterations of the segmental motion patterns.

The pure moment testing protocol without compression or follower load was not able to replicate the typical in-vivo segmental motion patterns throughout the entire motion path. Axial load or a combination of axial and follower load was necessary to mimic the in-vivo segmental contributions at the extremes of the extension-flexion motion path. It is hypothesized that dynamically altering the compressive loading throughout the motion path is necessary to mimic the segmental contribution patterns exhibited in-vivo—a novel concept that will be explored in future investigations.