Intervertebral disc properties: challenges for biodevices.

Intervertebral disc biodevices that employ motion-preservation strategies (e.g., nucleus replacement, total disc replacement and posterior stabilization devices) are currently in use or in development. However, their long-term performance is unknown and only a small number of randomized controlled trials have been conducted. In this article, we discuss the following biodevices: interbody cages, nuclear pulposus replacements, total disc replacements and posterior dynamic stabilization devices, as well as future biological treatments. These biodevices restore some function to the motion segment; however, contrary to expectations, the risk of adjacent-level degeneration does not appear to have been reduced. The short-term challenge is to replicate the complex biomechanical function of the motion segment (e.g., biphasic, viscoelastic behavior and nonlinearity) to improve the quality of motion and minimize adjacent level problems, while ensuring biodevice longevity for the younger, more active patient. Biological strategies for regeneration and repair of disc tissue are being developed and these offer exciting opportunities (and challenges) for the longer term. Responsible introduction and rigorous assessment of these new technologies are required. In this article, we will describe the properties of the disc, explore biodevices currently in use for the surgical treatment of low back pain (with an emphasis on lumbar total disc replacement) and discuss future directions for biological treatments. Finally, we will assess the challenges ahead for the next generation of biodevices designed to replace the disc.