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  • Pilot hole stimulation thresholds accurately predict screw stimulation thresholds for cervical lateral mass screws

    Final Number:

    Brandon C. Gabel MD, BS, BA; Erik Curtis MD, MS; Joseph D. Ciacci MD

    Study Design:

    Subject Category:

    Meeting: Congress of Neurological Surgeons 2014 Annual Meeting

    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.

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