Correcting deformity using pedicle screw fixation in patients with adolescent idiopathic scoliosis is currently widely accepted. Although the use of thoracic pedicle screws may result in better deformity correction and fixation, the safe placement of these screws is considered technically challenging because of the small size of the thoracic pedicle and the potential risk to the great vessels, spinal cord and spinal nerve roots [
1-
3]. The rates of penetrating the pedicle cortex, depending on the employed techniques, have ranged from 15.9% to 54.7% in laboratory studies with cadavers [
4,
5]. Clinically, bony landmarks and intraoperative C-arm X-rays are commonly used to determine the correct placement, yet these methods are frequently inaccurate [
1,
4,
6,
7]. Screw violation of the thoracic pedicle cortex has been reported at a rate of over 8% [
8]. Incorrect placement rates as high as 42% have been reported with manual screw insertion. Although several methods are used for increasing the accuracy of pedicle screw placement, fluoroscopic guidance has been shown to be insufficient to reliably predict the exit of screws [
2,
7,
9]. Computed tomographic (CT) scans are significantly more sensitive and accurate for evaluating pedicle screw placement than are plain radiograph [
7], but performing CT scans is feasible only after surgery in most medical centers. Computer-aided stereotaxic guidance systems have been used in the operating room to assist in the correct placement of the pedicle screws, and this may help lower the pedicle wall perforation rate [
10], but these systems are costly and they require much space for the equipment and also more time, which make the general application of this technique difficult. After the use of electrical stimulation of pedicle screws for the evaluation of their placement was developed by Calancie et al. [
11], several recent animal studies have investigated the evoked electromyography (EMG) responses to assess pedicle screws that were placed in levels above the lumbar spine. Danesh-Clough et al. [
12] used an ovine model for examining lower thoracic pedicle screws. They recorded the evoked potential from the transversus abdominis, intercostal and iliopsoas muscles. The study concluded that an evoked EMG threshold of < 10 V was an effective tool for detecting pedicle screw misplacement. The sensitivity and specificity were 94% and 90%, respectively. Lewis et al. [
13] demonstrated the reliability of triggered EMG stimulation for confirming the thoracic pedicle screw placement in a porcine model. They chose the intercostals muscles. However, no absolute threshold or range of threshold values was correlated with medially misdirected screws. So, the rectus abdominis muscles were alternately chosen as a recording site because of their innervation from the T6-T12 thoracic nerve roots. Raynor et al. [
14] recently reported that triggered EMG thresholds of < 6.0 mA, coupled with the values of a 60-65% decrease from the mean of all the other thresholds during thoracic pedicle screw placement, should alert the surgeon to suspect a breach of the medial pedicle wall. There are only a few studies that have evaluated the reliability of intraoperative triggered EMG for predicting safe pedicle screw placement in adolescent idiopathic scoliosis patients. We evaluated the threshold of intraoperative triggered EMG and the accuracy of pedicle screw placement using postoperative CT scanning for assessing the position of pedicle screws.