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Nakamura et al. Mini-invasive Surg 2019;3:29 I http://dx.doi.org/10.20517/2574-1225.2019.28 Page 3 of 5

Figure 1. Train wave of free-run electromyogram. This wave was observed in TF-PELD at L4-L5 case when the sleeve was manipulated
hand-down. Same waves were also observed in IL-PELD at L5-S1. TF-PELD: transforaminal percutaneous endoscopic lumbar discectomy;
IL: interlaminar

lateral edge of the nerve root. Then, after the sleeve was rotated and thecal sac was retracted, the herniated
disc was removed.

Statistical analysis
Descriptive analysis of group characteristics was performed using JMP version 11.2 software for Macintosh
(SAS Institute Inc., Cary, NC, USA). The independent two-sample t-test and Wilcoxon test were used to
compare the clinical outcomes. A probability value of P < 0.05 was considered to be statistically significant.

RESULTS
There were 49 patients (43 men, 6 women) with a mean age of 52.9 (range, 17-88) years. The mean follow-
up period was 10 (range, 6-14) months. The affected level was L2-L3 in one patient (TF-PELD), L3-L4 in
9 (TF-PELD), L4-L5 in 17 [TF-PELD, 14; interlaminar (IL)-PELD, 3], and L5-S1 in 22 (TF-PELD, 3; IL-
PELD, 19) patients.

The mean operative time was 63 ± 29 (range, 30-143) min and intraoperative blood loss was negligible in
all cases. The mean hospital stay was 3.2 ± 1.5 (range, 1-5) days. The mean numerical rating scale score for
the affected leg improved significantly from 7.7 to 1.1 at follow-up, and the mean Oswestry disability index
had improved from 62.3 to 20.5. Two patients experienced recurrence of the herniated nucleus pulposus.

In all cases, single waves were observed but were not considered to be clinically significant. A true-
positive was observed in one case during TF-PELD at L4-L5 [Figure 1]. When the sleeve was manipulated
downward by hand, an alarm sounded. Careful performance of the procedure will prevent ringing of the
alarm, which indicates a postoperative motor deficit. However, this patient complained of dysesthesia for
3 weeks postoperatively. The numbness gradually improved with the use of pregabalin and disappeared by
the final follow-up.

False-positives were observed in 2 patients following IL-PELD at L5-S1. No alarm sound was observed
when the nerve root was retracted by rotating the sleeve. Train waves appeared with the alarm several
seconds after the herniated disc material was removed. At that time, no surgical maneuver was performed.
In these 2 cases, no neurological deficit was observed after surgery. Including these 2 cases, no alarm
sounded during IL-PELD when the nerve root and dura were retracted.

A false-negative was observed in one patient following IL-PELD at L5-S1, but no abnormal wave was
observed. However, this patient complained of severe numbness for 6 months postoperatively.