Study design: This study consisted of a laboratory investigation of triggered electromyographic stimulation of pedicle screws placed in a pig spine, with a correlative prospective clinical series of lumbosacral pedicle screws stimulated in a similar fashion.
Objectives: To determine the threshold of stimulus intensity necessary to confirm accuracy of lumbar pedicle screw placement via a triggered electromyographic peripheral response.
Summary of background data: Documentation of lumbar pedicle screw placement is imperative to perform proper spinal instrumentation and to avoid perioperative complications. Previous electrophysiologic techniques using stimulation of a pedicle opening or pedicle screw with peripheral recording of electromyographic activity from the lower extremity muscles have been used to identify varying threshold values that indicate a break in the bony pedicle wall.
Methods: Six adult pigs had 107 pedicle screws placed bilaterally into the pedicles of the lumbar spine. These screws were stimulated with an ascending stimulus intensity until a peripheral triggered electromyographic response was recorded. Pedicle screws were placed in the pig either entirely in the pedicle (Group A), medial to the pedicle without direct contact to the nerve root and dura (Group B), or purposely medial to the pedicle with direct contact to the nerve root and dura (Group C). A correlative clinical series of 233 pedicle screws placed in 54 patients had a similar intraoperative neurophysiologic technique.
Results: In the 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). Ninety-three percent of the clinical Group A screws had threshold stimuli 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 pedicle wall defect; < 4.0 mA--strong likelihood of pedicle wall defect with potential for nerve root and dura contact.