A novel design for application of pure moments in-vitro: application to the kinematic analysis of the cervical spine

The simple and cost-effective cable-pulley apparatus proposed by Crawford et al. (1995) has been long and widely used to assess the kinematics and biomechanics of the spine in-vitro. A major limitation of that fixed-ring system relies on the manual readjustment of the cable guides, which is required to maintain parallelism of the tension-cables during spine rotations. While several solutions have since been suggested to improve this loading approach, their implementation may be challenging for research groups with limited resources.In this study we propose an upgrade to the traditional fixed-ring design which aims to improve its usability while retaining the simplicity. The main novelty of this setup is the coupling of a through-hole along the diameter of the pulley with a rod fixated to the top of the spine. The coupling allows relative translation of the pulley along the rod's axis, thus permitting readjustment of the tension-cables' orientation with minimal manual intervention. The effectiveness of the system was demonstrated by measuring intervertebral kinematics in three 7-vertebra cervical spines.During sagittal and frontal-plane rotations, tension-cables parallelism could be easily and quickly restored by pushing the pulley along the rod's axis. The measured intervertebral rotations were consistent with data from the literature. The interspecimen standard deviation of the total range of motion ranged between 2.1° and 9.6° across all loading configurations.The simple and light mobile-pulley fixture presented in this study has shown to be easy to use and its application may represent a viable alternative to the original fixed-ring design.