Current in vitro testing methodologies remain limited in the ability to explore spinal dynamics. The gold standard of flexibility testing has traditionally focused on evaluating MSU rotational ranges of motion only. While such data may be applied towards evaluation of the Instantaneous Axis of Rotation (IAR), many systems lack the needed sensitivity. The result is that there is currently no consensus on the location of the IAR. Further, very limited data or insight can be gathered as to the precise kinematic or dynamic state of the MSU, or the influence of surgically implanted motion restoration devices. For example, total disc arthroplasty devices are typically rigid mechanical devices that impose an IAR or IAR range. How might this imposed IAR affect MSU mechanics? How might variations in surgical placement of an implant be scientifically quantified? More recently an emerging group of compliant motion restoration devices are being developed that require new methods of evaluation. How well does a compliant device restore the native mechanics of the disc or MSU? To address and understand these increasingly important issues, novel, more advanced biomechanical testing protocols need to be developed.

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