Basic science of spinal instrumentation.

A wide variety of spinal implants are available to the clinician for the surgical treatment of spinal disorders. Most of the implants are associated with fusion--they are designed either to promote fusion, or, in the case of the newer devices such as artificial discs and disc implants, to offset the perceived disadvantages of fusion. The contributions of biomechanics to the improvement of spinal implant design and clinical implementation are detailed. Benchtop tests of device components and assemblies, in vitro studies of spinal constructs using osteoligamentous spinal segments, and analytical (finite element) and animal models are reviewed. Through these studies, the quantification of parameters such as stresses and strains within the spinal structures and within the fixation devices has permitted a better understanding of the relationship between the clinical observations after surgery and the mechanical factors. However, despite improvements in fusion techniques that have reduced the pseudarthrosis rate, there is still room for improvement. New concepts such as the biological enhancement of spinal fusion and alternatives to fusion such as the artificial disc are currently the subjects of intense, multidisciplinary study, but await the ultimate test of clinical trial with long term followup.