All DPLF–PELD procedures employed by the author were essentially a classic THESSYS technique popularized by Hoogland [9]. The procedure of inserting needle, guide wire, obturator and trephine protection tube was performed as we previously reported [12]. A trephine protection tube (6.5-, 7.5-, or 8.5-mm-diameter) was introduced over the obturator until it was situated in the proper position. The depth-limited trephine designed by us (6.5-, 7.5-, or 8.5-mm-diameter, selected based on pathologic conditions) was used to perform two-time foraminoplasty, which was facilitated by changing the trajectory of the trephine, to aim for different compressive portions. The details of the two foraminoplasty procedures are shown in Table 2 and Fig. 3.

Details of the two foraminoplasty procedure

a, b Schematic diagram of the inclination of the trephine trajectory in lateral and AP (anterior-posterior) views in the first foraminoplasty. c, d Schematic diagram of the inclination of the trephine trajectory in lateral and AP views in the second foraminoplasty. The cross point of the white solid line is described in the text. e schematic diagram of the two-time percutaneous lumbar foraminoplasty procedure. f schematic diagram of the difference between classical TESSYS technique and double percutaneous lumbar foraminoplasty and PELD (DPLF–PELD) technique

In the first foraminoplasty, the scale of the resection could be slightly adjusted, based on different pathologic conditions. After the first foraminoplasty, a radiofrequency probe was endoscopically used (working tube with an elevator tip, ID 7.2 mm, OD 8.0 mm, and L178 mm; spinal endoscope, 30° direction of view, WC 3.75 mm, OD 6.3 mm, and WL 181 mm) to control bleeding and to adequately expose bony structures by resecting any adherent soft tissue. The margin of exposure should run from the upper-ventral surface of the SAP to the lower-ventral surface of the SAP and upper surface of the pedicle. Next, a 1.5-mm Kirschner wire was inserted into the aiming site. After removing the spinal endoscope, the second foraminoplasty was then performed, after positioning the trephine protection tube over the Kirschner wire and adjusting its tip to embrace the ventral-basal aspect of the SAP. In some severe stenosis cases, to prevent injury to the nerve roots, we only inserted the trephine into the three quarters of the SAP, thus breaking the involved SAP, instead of performing a complete resection by the trephine. For the two foraminoplasty procedures, the trephine needed to be underdraught, thus aiming to resect more of the SAP. The order of the two steps can be adjusted according to different situations.

In the following step, the trephine protection tube was replaced with the working tube with an elevator tip. High-speed drilling was then used to resect the remaining hypertrophied SAP or IAP as needed. The working tube was adjusted to completely remove decompressive factors: the hypertrophied ligamentum flavum, facet joints, and anterior herniated disk. To reduce the recurrence rate of lumbar disk herniation (LDH), we did not perform discectomy (only decompress dorsal compressive factors) for patients whose annulus was not damaged. The compressed nerve root was decompressed and explored from the distal end to near-end, especially at the attachment point of the annulus. The surgeon could see and mobilize both the traversing nerve root and the exiting nerve root under endoscopic visualization. Free movement of the dural sac and nerve root could be a sign of complete decompression. Epidural bleeding was controlled with a radiofrequency probe under saline irrigation.

For each operation duration, times of intraoperative C-arm fluoroscopy use and any complications were recorded. Every patient was asked to wear lumbar protection devices for 2-4 weeks after the operation, and to take muscle function exercise the initial 2 weeks following the surgery.