Objective
The purpose of this study is to evaluate the searching stimulus intensity at each pedicle and to identify the most vulnerable roots in transpedicular screw fixation of lumbosacral spine.

Method
Thirty-two patients with unstable lumbosacral vertebra were treated with intrapedicular screw fixation. Small holes were made by an air drill on the pedicle from L2 to S1 for screw fixation. Constant current stimulation pulses(0.2 msec duration) were delivered through a ball-tipped nasopharyngeal probe used to palpate the walls of each pedicle, and observation was made of electromyogram(EMG) evoked lower extremity muscles. The probes were placed in each pedicular wall manually, and evaluated for searching stimulus intensity, the current necessary to evoked EMG.

Results
The cases that the searching stimulus intensity was above 5 mA were 99 cases(97%), above 10 mA were 61 cases(59.8%) and below 5 mA were 3 cases(2.9%). The vastus medialis muscle is most sensitive in L2(100%) and L3(78.9%), tibialis anterior is in L4(81.8%), Peroneus longus is in L5(50.5%) and gastrocnemius is in S1(87.5%).

Conclusion
We can regard the searching stimulus intensity as 5 mA and the root located at Infero-medial side of pedicle is most vulnerable in transpedicular screw fixation. Furthermore, stimulus-evoked EMG monitoring during transpedicular spine instrumentation is helpful to avoid neural tissue injury.