Correlation of strain on instrumentation to simulated posterolateral lumbar fusion in a sheep model

Determining the stability and integrity of posterolateral lumbar spinal fusions continues to be one of the leading challenges facing surgeons today. Radiographs have long been the gold standard for evaluating spinal fusion, but they often give delayed or inaccurate results. It is the goal of this research to develop a new method for determining the strength and stability of a posterolateral lumbar spinal fusion using a sensor based on two strain gauges attached to a spinal rod. It was hypothesized that the spinal implants, in particular the plates or rods, would respond to this change in strain as the stiffness of the fusion increased. To investigate this hypothesis, an in vitro sheep model of the lumbar spine was developed and bony fusion was simulated with polymethylmethacrylate (PMMA), also known as bone cement. Eight sheep spines were prepared for use in a test fixture that applied a physiological moment of 5 Nm in flexion. One of the spinal rods was instrumented with two strain gauges in a Wheatstone half bridge, and all of the results were ported directly to a data collection system on a dynamic fatigue test machine. For each spine, the magnitude of the strain was plotted versus amount of simulated fusion. To evaluate the effect of simulated fusion, a moving average of the slope of three sequential strain values was used. The results showed there is a strong correlation between strain and spinal fusion and that a computer algorithm could be developed that would be more accurate than current techniques in predicting when a spinal fusion is mature enough for a patient to resume normal activities.