A COMPARISON OF IMPEDANCE AND ELECTROMYOGRAM MEASUREMENTS IN DETECTING THE PRESENCE OF PEDICLE WALL BREAKTHROUGH

Study Design. A prospective comparison of impedance measurements, elec trically elicited electromyograms, and mechanically elicited electromy ograms to detect pedicle well breakthrough. Objective. To determine wh ether impedance measurements are as Sensitive as electromyogram measur ements in evaluating pedicle well breakthrough. Summary of Background Data. In a previous animal study, impedance values in pedicle screw pl acement were tested, to determine a baseline value for an intact pedic le. If pedicle wall breakthrough occurred, it was thought that the imp edance values should be significantly lower. Methods. Impedance measur ements, electrically elicited electromyograms and mechanically elicite d electromyograms were recorded in 20 patients undergoing surgery for spinal degeneration, using previously de-scribed standard protocol. An alysis of variance statistics were used to evaluate the data. Results. Impedance values for the pedicle holes varied from 500 ohms to 31,000 ohms. There was no correlation between these values and those of the two pedicles in which breaches were detected on visual inspection. Ele ctrically elicited electromyograms detected the breakthroughs in both pedicles, mechanically elicited electromyograms detected one of the br eakthroughs. Conclusions. Electrically elicited electromyograms were m ore sensitive in detecting pedicle wall breakthrough than were impedan ce measurements. This may be because of the inability to ascribe absol ute impedance values to human pedicle bone caused by the wide variabil ity in bone quality.