Shear force measurements on low- and high-stiffness posterior fusion devices

Low-stiffness posterior fusion devices for the lumbar spine have been developed to treat degenerative spinal conditions. However, the demands on an implant vary between a stable motion segment and one which exhibits a significant degree of sagittal plane instability. Shear motion in the antero-posterior direction is a relevant mode of instability for clinical conditions such as degenerative lumbar spondylolisthesis. Shear load-sharing between the implant and spine in conditions of antero-posterior instability has not been studied, nor have there been comparisons between traditional rigid implants and novel low-stiffness implants. The objective of this study was to develop a method to measure in vitro shear forces on three clinically relevant fusion implants when they are applied to an unstable model of degenerative spondylolisthesis in a human cadaver spine. Uniaxial strain gauges were affixed to the surface of the implants and a spine-segment-specific calibration method was used to calibrate the strain output to an applied shear force. The accuracy of the force measurements was within 3.4 N for all implant types and the repeatability was within 5.4 N. The force measurement technique was sufficiently accurate and reliable to conclude that it is suitable for use in in vitro experiments to measure implant shear force.