Evaluation of a Follower Load with an Intact Rib Cage

Abstract

The overall goal of this research was to better characterize the motion of the thoracic spine by inclusion of the rib cage and a novel loading method, as well as by evaluation of all commonly used motion and stiffness parameters. Past researchers have reported on the importance of the rib cage in maintaining mechanical stability in the thoracic spine. However, because of inconsistencies in test machines and experimental design, the rib cage is often removed when testing the thoracic spine. Applying pure compressive loads to simulate muscle forces and body weight has proven difficult because of the natural curvature of the spine. Development of a follower load by previous researchers has improved upon this issue, allowing more physiologically representative loads to be used by applying the loads along the natural curvature of the spine. Most spine testing does not involve the thoracic spine, and of that research, even less involves the thoracic spine with an intact rib cage or compressive loads similar to that of thoracic musculature. Quantification of the motion of the thoracic spine with the rib cage and a follower load is important in order to provide the research and clinical spine communities with more relevant data that includes essential elements needed to obtain better characterization of the motion. An in vitro biomechanical study of human cadaveric thoracic specimens with rib cage intact in lateral bending, flexion/extension, and axial rotation under varying compressive follower preloads was performed. The hypotheses tested for all modes of bending were (i) range of motion, elastic zone, and neutral zone will be reduced with a follower load, and (ii) neutral and elastic zone stiffnesses will be increased with a follower load. Eight human cadaveric thoracic spine specimen (T1-T12) with intact rib cages were subjected to 5 Nm pure moments in lateral bending, flexion/extension, and axial rotation under follower loads of 0 to 600 N. Range of motion, elastic and neutral zones, and elastic and neutral zone stiffness values were calculated for functional spinal units and segments within the entire thoracic section. Significance at various levels and for certain parameters varied, but overall, combined segmental range of motion decreased with follower load for every mode of bending. Based upon this experimentation, it is seen that application of a follower load with an intact rib cage does alter the motion and stiffness of the human cadaveric thoracic spine. Future researchers should consider including both of these aspects to better represent the physiologic implications of human motion and improve clinically relevant biomechanical thoracic spine testing. Recommendations for future testing in this area involve further characterization of the thoracic spine, including, but not limited to, the effect of the follower load on the thoracic spine without an intact rib cage, evaluation of the contribution of the free-floating ribs, and changes in intradiscal pressure with application of a follower load.