Development and Validation of a Synthetic Lumbar Facet Joint and its Load Bearing Characteristics

Abstract

There is a substantial need in the spine research community for an anatomical and mechanically correct synthetic model of the human lumbar spine. This model could be used in nearly every aspect of spinal research including the design of implants and the research of new treatments for low back pain. An initial model has already been developed, and the present work focused on the development and validation of the synthetic facet joint in the model. This work was broken down into two primary goals. The first goal was to see if any improvements could be made to the current synthetic facet joint. The second goal was to investigate the load-bearing characteristics of the joint and to see how it compares to human facet joint behavior. The development of the synthetic facet joint focused primarily on improvements that could be made to the joints non-linear behavior and allow it to be used with currently accepted methods of facet load measurement. Material selection for facet interfaces was directed by considerations of commercial availability and manufacturability. A test protocol was developed which targeted the facet joints ability to provide a transition from neutral to extension stiffness while the specimen underwent axial rotation. The study compared different interfaces against each other and across multiple test beds. It was found that no single interface showed any significant improvements over the current joint. The investigation into the load-bearing characteristics of the synthetic facet joint utilized pressure sensitive sensors that were previously identified as the most widely accepted and used method of facet load measurement. A test procedure was designed to show the load-bearing characteristics of the synthetic facet and human facets at fixed applied moment intervals. A secondary procedure was also included to investigate changes in specimen behavior that were induced by the used of the pressure sensors which required an incision to be made in the capsular ligament of each facet joint. It was found that the synthetic facet joint exhibits similar behavior to the human facet joint however it is not statistically similar. It was also found that the impact of cutting the capsular ligaments and inserting the sensor was not significant. Future work on the synthetic facet joint should focus on the effect which facet geometry has on the overall performance of the facet joint and on improved methods of facet load measurement.