Biomechanical evaluation of a full-length (T12-S) synthetic lumbar spine model

Abstract

Cadaveric tissue-based testing is the standard method for biomechanically evaluating spinal instrumentation. Synthetic lumbar spine models have been developed and are commercially available as a cost saving alternative to human tissue. Validation studies have been done on single lumbar motion segment units (MSU) and partial synthetic lumbar models. A fulllength synthetic lumbar spine model (T12 to sacrum) has become available that can serve as an alternative to tissue, but the biomechanical structural properties are not available. Additional kinematic parameters (like center of rotation (COR) and instantaneous axes of rotation (IAR)) also do not exist. The objectives of this study were to determine the flexion/ extension stiffness properties of a full-length synthetic lumbar spine model and to define the segmental and global kinematic parameters. The synthetic lumbar model was mounted in a robotic testing platform and tested using pure moment loads and 20N axial compression between 25 flexion and 10 extension. Global position of each vertebra was measured and used to calculate the global COR and global and local IAR values of each MSU. Rotational stiffness, global COR, and local and global IAR parameters were calculated every 2 change in 0.5 increments. MSU rotational responses were normalized to the total rotation and expressed as a percent contribution (%Rot). All spinal levels were involved in contributing to the overall motion and the %Rot was within 2 standard deviations of similar rotational data sets from in vivo studies and in vitro tests on cadaveric spines. The CORs of each MSU were located near the mid-point of their respective IAR path. Global IAR paths were approximately mid-distance between the respective spinal level and the fixed sacral body. The biomechanical properties of the full-length synthetic lumbar model were comparable to in vivo and in vitro studies supporting its use in other testing applications.