Comparison of six degree-of-freedom pure moment and non-uniform, physiologically-derived spinal loading at quasi-static and dynamic rates using a six-axis joint motion simulator

Current in vitro testing methods often fail to replicate the complex physiological loads present in the lumbar spine. The goal of this study was to develop and evaluate a novel approach to experimentally simulate non-uniform, physiologically-derived, six degree-of-freedom (6DOF) spinal loading in comparison to pure moment testing using a six-axis joint motion simulator (AMTI VIVO) at quasi-static and dynamic rates. Physiologically-derived 6DOF loading waveforms were developed from the Orthoload dataset, averaging three movement types (flexion-extension, lateral bending and axial rotation) across the reported subjects. Segmental range of motion (ROM) was measured during the simulated movements to compare the effects of loading rate (quasi-static, 0.5Nm/s vs. dynamic, 5Nm/s) and waveform type (pure moment vs. physiologically-derived 6DOF force control). Eight fresh-frozen cadaveric lumbar spine segments (four L2/L3, four L4/L5) from four donors (69 ± 4.7 years; 3 female, 1 male) were used. ROM was significantly greater under pure moment loading than physiologically-derived 6DOF loading protocols at both quasi-static and dynamic rates. Dynamic loading led to reduced ROM in pure moment and physiologically-derived tests compared to quasi-static rates. The findings from this study highlight a new potential approach to apply non-uniform 6DOF spinal loading waveforms using the VIVO joint motion simulator. Further, novel use of this system enabled dynamic ROM from 6DOF load control waveforms at physiologic loading rates (5Nm/s). Ultimately, the development and comparison of the different spinal loading conditions conducted in this study provides further advocacy for more comprehensive in vitro testing to understand lumbar spinal biomechanics.