The objectives for the mechanical evaluation of spinal instrumentation have changed

The objectives for the mechanical evaluation of spinal implants have changed because many modern devices are designed to modify the mechanics of the disc rather than to simply fix the segment. This means that a biomechanical objective must be decided, a priori, for a particular device. It is then relatively straightforward to design a biomechanical evaluation protocol that can either test whether this objective is fulfilled, or optimise the device in the context of the objective. Because 'soft stabilisation' systems are soft, their performance is affected by the magnitude of the loading sustained by the bridged segment. This means that is vital to reproduce a realistic loading regime for the biomechanical evaluation, if its results are to be relevant to a clinical problem. Similarly, the condition of the segment in terms of disc degeneration, facet joint condition, etc. affect the mechanical performance of the segment and must be relevant to the performance objectives set for the device. Loading protocols for testing short and long segments are discussed. Since the aim of many spinal devices is to modify the loading of the intervertebral disc, it is important to quantify their effect in terms of how both the internal loads and deformations are changed. A number of different technologies for quantifying both loads and deformations in intact discs are described and discussed.