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18-08-2011 | Surgery | Article

EMG monitoring ineffective for detecting intra-operative pedicle screw breach


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MedWire News: US study findings suggest electromyographic (EMG) monitoring is ineffective for determining intra-operative pedicle screw breaches, but is a valuable tool for determining the accuracy of screw placement when used together with intra-operative radiography and postoperative computed tomography (CT) scanning.

"Pedicle screws provide efficient stabilization along all three columns of the spine, but they can be technically demanding to place, with malposition rates ranging from 5% to 10%," say Scott Parker (Johns Hopkins University, Baltimore, Maryland) and co-authors.

Parker and team evaluated the utility of intra-operative EMG monitoring during placement of 2450 consecutive lumbar pedicle screws in 418 patients between 2002 and 2009. All screws were inserted using a free-hand technique and anatomical landmarks, stimulated at 10.0 mA, and evaluated with CT scanning within 48 hours after surgery.

Screws that exhibited a negative response at 10 mA were assumed to be accurately positioned without medial pedicle breach (defined as having greater than 25% of the screw diameter extending outside of the pedicle). When a positive response occurred at 10 mA, the authors undertook further investigation by determining the EMG threshold of stimulation, with a threshold of less than 7 mA representing a potential medial cortical breach.

The sensitivity and specificity of intra-operative EMG monitoring for detecting the presence of a medial screw breach was evaluated based on a combination of EMG stimulation results and CT scanning to reveal true-positives, false-positives, true-negatives, or false-negatives.

In total, 115 (4.7%) pedicle screws showed positive stimulation during intra-operative EMG monitoring. At stimulation thresholds less than 5.0, 5.0-8.0, and over 8.0 mA, the specificity of a positive response was 99.9%, 97.9%, and 95.9%, respectively. The sensitivity of a positive response at these thresholds was low, at 43.4%, 69.6%, and 69.6%, respectively.

At a threshold less than 5.0 mA, 91% of screws with a positive EMG response were confirmed as medial breaches. However, at thresholds of 5.0-8.0 mA or greater than 8.0 mA, a positive EMG response was associated with 89% and 100% false-positives, respectively.

"EMG monitoring is a poor screening tool for detecting medial pedicle screw breaches. Its most useful application is as a warning tool for likely screw malpositioning in the presence of positive stimulation at thresholds of 5.0 mA or less," conclude the researchers in the Journal of Neurosurgery: Spine.

In an accompanying editorial, Christopher Shaffrey (University of Virginia Health System, Charlottesville, Virginia, USA) commented: "The decision whether to use monitoring should be determined by the complexity of the surgery, the surgeon's and institution's experience with monitoring, and the likelihood that the results of the monitoring will actually modify the surgical procedure to reduce risk."

By Ingrid Grasmo

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