Establishing Appropriate Test Protocols for Biomimetic Facet Joint Implants as a Basis for Test Rig Development

Chronic low back pain affects a significant proportion of the global population, costs health services large amounts of money, and often treatment is ineffective in the long term. For some patients, this pain originates from nerve compression as a result of de¬generative changes associated with facet joint osteoarthritis (FJOA). Non-surgical treat¬ments for FJOA are commonly not effective in the long term and surgical treatments like total joint replacement are very invasive, increasing the likelihood of complications and prohibiting future joint treatment options. Particularly for the young, there exists a therapeutic gap. Minimally invasive intra-facet implants, which sit within the facet joints, are a promising solution, offering the long term relief of a mechanical solution with reduced complication rates, but the limited number of existing solutions of this kind are not widely in clinical use. The lack of a standard way of testing these novel devices impedes their development. Current testing platforms focus on assessing the integrity of an implant under mechanical loading and lack the fidelity to assess the performance and efficacy of novel intra-facet implants. Completely synthetic lumbar spine sections are emerging, but their focus is on simulating the gross mechanics of the spine (i.e. range of motion and stiffness) and do not simulate the complex tribological environment of the joint which is significant because unfavourable transient tribological states may be a key initiator and driver of osteoarthritis. For rigorous testing of intra-facet implants, a testing platform which appropriately simulates the gross biomechanics of the spine as well as the tribological environment of the facet joints is paramount. This is particularly true for future generations of minimally invasive implants. This research focusses on the development of tests which recreate the low-friction movement that exits in the joints, before being scaled up to an appropriate geometrical model. The friction coefficient of a biomedical tribological system is one of the critical properties determining its performance, with high static friction having previously been related to articular cartilage damage ex vivo. To this end, a test method for measuring the friction coefficient between two reciprocating lubricated polymer sheets was devised in this study. A silicone lubricant of viscosity 350cP, was placed between the surfaces and start-stop dwell time experiments were run with dwell periods of 15, 40, 70, 140, and 310 seconds under a 0.5 MPa contact pressure, 2.7 mm/s relative sliding speed, and lubricant temperature of 28°C. Steady-state friction coefficients of 0.03 or less were achieved, along with predictable stiction friction profiles. The biggest barrier to further progress in the development of a synthetic joint is the lack of a method for measuring the friction coefficient between artificial cartilage surfaces in situ. The author proposes some incremental experiments as a way to gain further insight until such a method is devised.