Screw-bone interface loads from rod bending: Development of an instrumented spinal fusion simulator.

Abstract

During spinal fusion procedures, spinal rods are bent to match the current and desired curvature of a spinal segment. Following bending, the rods are attached to the heads of pedicle screws that have been driven into the vertebral pedicles. A mismatch between the curvature of the spine and the curvature of the rod can lead to increased loads at the screw-bone interfaces. These increased loads have been associated with screw loosening and pull-out. We have developed a novel and inexpensive spinal fusion simulation device to explore how variables such as surgical techniques, tools, and hardware designs affect the dynamic and residual loads on the screw-bone interfaces. In addition, our device can act as a training tool providing real time and post-trial feedback. Our design consists of a series of load sensing vertebral bone surrogates with modular heights and intervertebral spacing. The device and its included software are capable of synchronized video capture, real-time feedback of screw-bone interface loads, and the generation of interactive reports. We showed that the stiffness of the device is comparable to reported in vitro values. Additionally, we showed an acceptable alignment between the recorded and predicted loads on several test contours and demonstrated data collection on a simulated L1-S2 contour. The ability to record synchronized video and create interactive reports allows for unique contextual analysis. In addition to being used as a research tool, interactive reports and real-time feedback allow the device to provide quantitative insights into spinal fusion surgical training.