Biomechanical evaluation of spinal fixation devices: I. A conceptual framework.

In the field of spinal fixation devices, there is a profusion of new instrumentations. Often, the biomechanical evaluation is done in a nonstandardized manner, which makes it difficult to compare the results of one researcher with those of another, for the same device or for different devices. There is a need for a conceptual framework under which guidelines may be suggested for the evaluation of these devices in some uniform and comprehensive manner. There are three basic biomechanical tests: strength, fatigue, and stability. The strength test evaluates the failure load of the device, determines its weak points, and is helpful in the initial development of the device. The fatigue test provides a measure of longevity of the device, either alone or as part of the spinal construct, by testing the device to failure using cyclically varying loads. In contrast, the stability test measures the capability of the device to provide multi-directional stability to the injured spine. There is no failure of the device, and the results of this test are clinically important, as they characterize the potential for early fracture healing and early fusion. A conceptual framework for the evaluation of multi-direction stability of spinal fixation devices and guidelines for designing the necessary experiments are described.