A Multi-Laboratory Study Towards Standardizing Spine Testing: Evaluating Reproducibility and Temporal Changes in Lumbar Spine Surrogates

BACKGROUND: Composite spine surrogates have been designed to address the challenges of biomechanical testing using cadavers; however, their biofidelity and reproducibility among laboratories remain unknown. This study evaluated inter-laboratory reproducibility and temporal variability of a composite lumbar spine surrogate under standardized pure moment testing conditions. METHODS: Nine lumbar (L2-L5) spine surrogates were tested in seven laboratories with expertise in spinal biomechanics. Each specimen underwent pure moment testing in flexion-extension, lateral bending, and axial rotation (±7.5 Nm), using each lab's respective test setup and procedure. The spines were divided into two groups and shipped in opposite testing sequences to control for order effects. Qualitative condition was recorded following each round. Differences in range of motion (ROM) among specimens and laboratories were assessed. Additionally, changes in ROM were evaluated as a function of testing sequence. RESULTS: Consistent and progressive damage to the facet joints was noted, but only minimal degradation was observed to the other structural components. Specimen ROM increased significantly over time, in all three rotations, flexion/extension, lateral bending, and axial rotation, after repeated loading (p < 0.05). Independent of time, inter-specimen variability was high, with almost all specimens differing from each other in flexion and axial rotation (p < 0.05). CONCLUSIONS: Conceptually, composite spine models offer several advantages over cadaveric specimens. However, this study identified limitations in durability, performance, and reproducibility with the aim of advising future surrogate modeling research and advancing efforts towards standardized spine testing protocols.