The influence of facet joints on intervertebral disc pressures under complex loading

Abstract

Purpose: The purpose of the study was to determine the influence of the loading history on changes in the recorded pressure in the intervertebral disc (IVD) and the influence of changes in the support conditions as a result of the removal of facet joints on changes in pressure. Materials and Methods: The tests were carried out on motion segments of the thoracic spine for the physiological system (with articular processes preserved) and the pathological system (with articular processes removed). Our analysis assessed changes in intradiscal pressure for three consecutive loadings, i.e., cyclic compression loading, compression loading constant in time, and unloading. The tests were conducted while maintaining full hydration of the IVD. Results: During cyclic loading, we observed a progressive decrease in intradiscal pressure. Suppression of the pressure decrease is clearly visible in the system with the preserved complete articular triad, which should be regarded as characteristic of a properly functioning system. Constant compression loading caused a progressive decrease of intradiscal pressure in all tested systems. In general, greater decreases were recorded in the systems with articular processes removed. A decrease in intradiscal pressure leads to a simultaneous change in the stiffness of the analyzed systems. The process of relaxation was unstable and showed an upward trend throughout the unloading period. Conclusion: The obtained results clearly indicate that facet joints play an important role in the transmission of loads by the spinal column and that those structures play a protective role in the overload conditions of the IVD.