TRIGGERED ELECTROMYOGRAPHIC THRESHOLD FOR ACCURACY OF PEDICLE SCREW PLACEMENT - AN ANIMAL-MODEL AND CLINICAL CORRELATION

Study Design. This study consisted of a laboratory investigation of tr iggered electromyographic stimulation of pedicle screws placed in a pi g spine, with a correlative prospective clinical series of lumbosacral pedicle screws stimulated in a similar fashion. Objectives. To determ ine the threshold of stimulus intensity necessary to confirm accuracy of lumbar pedicle screw placement via a triggered electromyographic pe ripheral response. Summary of Background Data. Documentation of lumbar pedicle screw placement is imperative to per; form proper spinal inst rumentation and to avoid perioperative complications. Previous electro physiologic techniques using stimulation of a pedicle opening or pedic le screw with peripheral recording of electromyographic activity from the lower extremity muscles have been used to identify varying thresho ld values that indicate a break in the bony pedicle wall. Methods. Six adult pigs had 107 pedicle screws placed bilaterally into the pedicle s of the lumbar spine. These screws were stimulated with an ascending stimulus intensity until a peripheral triggered electromyographic resp onse was recorded. Pedicle screws were placed in the pig either entire ly in the pedicle (Group A), medial to the pedicle without direct cont act to the nerve root and dura (Group B), or purposely medial to the p edicle with direct contact to the nerve root and dura (Group C). A cor relative clinical series of 233 pedicle screws placed in 54 patients h ad a similar intraoperative neurophysiologic technique. Results. In th e animal model, the mean threshold differences were: Group A screws 21 .9 mA, Group B screws 8.5 mA, and Group C screws 4.2 mA (P < 0.05). Ni nety-three percent of the clinical Group A screws had threshold stimul i less than 8.0 mA, whereas Groups B and C screws had a mean threshold of 3.3 mA. Conclusions. Triggered electromyographic stimulation is a valuable aid to determine appropriate placement of pedicle screws. We recommend the following interpretation of threshold stimulus intensity : > 8 mA-screw entirely in the pedicle; 4.0-8.0 mA-potential for pedic le wall defect; < 4.0 mA-strong likelihood of pedicle wall defect with potential for nerve loot and dura contact.