Correlation Between Low Triggered EMG Thresholds & Lumbar Pedicle Screw Malposition: Analysis of 4587 Screws

Summary: 4857 pedicle screws were placed in 1032 patients from L2 to S1 using lower extremity myotome triggered EMG's for directional confirmation. To reexamine established normative ranges, thresholds values and screw placement were compared. Medial malposition rarely (0.31%) occurred with threshold values of greater than 8.0 mA. Medial malposition occurred in 17.4% of screws between 4 and 8 mA, in 54.2% of screws below 4 mA, and in 100% of screws below 2.8 mA.

Objectives: This was a retrospective analysis of 1032 spinal surgical procedures with lumbar pedicle screw placement at a single institution. Based on previously established normative values, triggered electromyographic stimulation (TrgEMG) was reexamined to evaluate its efficacy in determining screw malposition.

Methods: TrgEMG threshold values for 4857 pedicle screws placed from L2 to S1 from 1996 to 2005 were analyzed. An ascending method of constant current stimulation was applied to each pedicle screw to obtain a compound muscle action potential (CMAP) from lower extremity myotomes. Previously determined threshold value normative data from a published clinical series of 233 screws were as follows: 0-4 mA, high likelihood of pedicle wall breach; 4-8 mA, possible pedicle wall breach; >8 mA, no pedicle wall defect.

Results: 7.74% (376/4857) of all screws tested had threshold values <8 .0 mA. 19.1% (72/376) of these were <4.0 mA: 54% (39/72) repositioned (26) or removed (13) while the remaining 33 screws left in place following repalpation. 80.9% (304/376) had thresholds between 4 & 8 17.4% (53) (38) (15). Nine 2.8 mA less and either confirmation a medial wall breach. 74.5% (280/376) all with <8.0 verified as correctly placed by repalpation/radiography therefore place.

Conclusions: The probability of a medial breach pedicle screw detected by triggered EMG stimulation increases with decreasing triggered EMG thresholds: 0.31% for >8.0 mA, 17.4% for 4.0 - 8.0 mA, 54.2 % for <4 .0 mA, and 100% for <2.8 mA. TrgEMG is an adjunct technique should always be used in conjunction with palpation radiography to optimize safe pedicle screw placement.