Introduction: Electromyographic recording during stimulation of cervical, thoracic, and lumbar screws has become common practice over the past decade. Its function is determining which screws may be close to nerve roots or spinal cord. Activity seen at lower thresholds indicates screws are in close proximity to neurologic tissue. There fails to be an accepted threshold for when screws should be repositioned/removed in order to prevent postoperative injury to the patient.
There is little data regarding the significance of screws at various potentials. Screws with low thresholds may cause no signs and/or symptoms in the post-operative period.
Further research is necessary to define significance of thresholds obtained during intraoperative monitoring. Clinical outcome should drive the definition of a “safe” threshold for which screws may be left in place.
Methods: The demographics, levels fused/instrumented, type of instrumentation, clinical outcome, postoperative radiographic data (if present), length of follow up, and stimulation thresholds of each screw and pilot hole recorded intra-operatively were collected in prospective fashion.
Data was analyzed using widely available free statistics software. The help of a statistician was employed when deemed necessary by the principal investigator. Data interpretation was based on resultant statistical analysis.
Results: Average pilot hole stimulation thresholds at C3, C4, C5, C6, C7 levels were 15.9, 13.5, 12.8, 12.8, and 13.0 respectively. Average lateral mass screw stimulation thresholds at C3, C4, C5, C6, and C7 were 16.8, 14.4, 14.4, 14.3, and 15.0 respectively. No patients had evidence of nerve root injury on post operative follow up.
Conclusions: Stimulation of pilot holes prior to screw placement provides reliable estimation of final screw stimulation thresholds. Our data indicates that screws typically stimulate at higher thresholds than the pilot hole. Additionally, pilot holes stimulating greater than 7 mAmp do not appear to result in nerve root injury post operatively.
Patient Care: This research may prevent inadvertent injury to nerve roots and/or spinal cord from poorly placed screws.
Learning Objectives: Stimulation of lateral mass pilot holes prior to screw placement may help prevent poor screw placement.
References: (1) Shi YB, Binette M, Martin WH, Pearson JM, Hart RA. Electrical stimulation for intraoperative evaluation of thoracic pedicle screw placement. Spine (Phila Pa 1976). 2003 Mar 15;28(6): 595-601.
(2) Raynor BL, Lenke LG, Kim Y, Hanson DS, Wilson-Holden TJ, Bridwell KH, Padberg AM. Can triggered electromyograph thresholds predict safe thoracic pedicle screw placement? Spine (Phila Pa 1976). 2002 Sep 15;27(18):2030-5.
(3) Raynor BL, Lenke LG, Bridwell KH, Taylor BA, Padberg AM. Correlation between low triggered electromyographic thresholds and lumbar pedicle screw malposition: analysis of 4857 screws.
Spine (Phila Pa 1976). 2007 Nov 15;32(24):2673-8.
(4) Parker SL, Amin AG, Farber SH, McGirt MJ, Sciubba DM, et al. Ability of electromyographic monitoring to determine the presence of malpositioned pedicle screws in the lumbosacral spine: analysis of 2450 consecutively placed screws. J Neurosurg Spine. 2011 Aug;15(2):130-5.
(5) Djurasovic M, Dimar JR 2nd, Glassman SD, Edmonds HL, Carreon LY. A prospective analysis of intraoperative electromyographic monitoring of posterior cervical screw fixation. J Spinal Disord Tech. 2005 Dec;18(6):515-8.
(6) Glassman SD, Dimar JR, Puno RM, Johnson JR, Shields CB, Linden RD. A prospective analysis of intraoperative electromyographic monitoring of pedicle screw placement with computed tomographic scan confirmation. Spine (Phila Pa 1976). 1995 Jun 15;20(12):1375-9.
(7) Rodriguez-Olaverri JC, Zimick NC, Merola A, De Blas G, Burgos J, et al. Using triggered electromyographic threshold in the intercostal muscles to evaluate the accuracy of upper thoracic pedicle screw placement (T3-T6). Spine (Phila Pa 1976). 2008 Apr 1;33(7):E194-7.
(8) Isley MR, Zhang XF, Balzer JR, Leppanen RE. Current trends in pedicle screw stimulation techniques: lumbosacral, thoracic, and cervical levels. Neurodiagn J. 2012 Jun;52(2):100-75.
(9) Kim YJ, Lenke LG, Bridwell KH, Cho YS, Riew KD. Free hand pedicle screw placement in the thoracic spine: is it safe? Spine (Phila Pa 1976). 2004 Feb 1;29(3):333-42; discussion 342.