Percutaneous Endoscopic Transforaminal Discectomy with Foraminoplasty in the Treatment of L4-L5 and L5-S1 Disc Herniation: A Minimum 1-year Follow-up Study.

The anatomic and biomechanical aspects of the L5-S1 level present unique operative challenges compared with the L4-L5 level. We aim to explore the clinical outcomes and radiographic measurements of L4-L5 patients compared with L5-S1 patients after undergoing Percutaneous Endoscopic Transforaminal Discectomy with Foraminoplasty (PETDF). Methods 84 Preoperative, perioperative, demographic data, and outcomes compared between with


Background
Lumbar disc herniation (LDH), a common degenerative spinal disease 1 , is characterized by displacement of intervertebral disk material beyond the normal margins of the disk space. Approximately 95% of reported LDH cases are L4-L5 or L5-S1 levels 2 .
Percutaneous Endoscopic Transforaminal Discectomy (PETD) is the conventional treatment approach for lumbar disc herniation. PETD is more effective compared with traditional open lumbar discectomy. Its advantages include less paravertebral muscle injury, preservation of bony structures, and rapid recovery.
Foraminoplasty spinal surgery expands the neuro foramen ensuring convenient and effective operation of herniation 3 . Extensive foraminoplasty is required for patients who suffer from hypertrophy of the facet joints, for decompression of exiting nerve roots 4 . However, foraminoplasty may result in injury of the facet joint, generating postoperative lower back pain, and increasing lumbar instability and high recurrence rate post-operation [4][5][6][7] .
Previous studies report con icting ndings on clinical outcomes and complications for L4-L5 patients undergoing full-endoscopic lumbar discectomy compared with patients with L5-S1 level 8,9 . Biomechanical studies on discectomy procedures of lumbar spine report differences in load resistances at each lumbar segment 10 . However, studies have not explored the clinical outcomes and radiographic measurements of L4-L5 patients compared with L5-S1 patients after undergoing Percutaneous Endoscopic Transforaminal Discectomy with Foraminoplasty (PETDF) surgery. Therefore, we carried out a retrospective study, comprising 84 patients who underwent PETDF and compared differences in clinical outcomes and radiographic measurements between L4-L5 group and L5-S1 group after at least one-year follow-up.

Patients
This study was approved by our institutional review board. A total of 84 patients who underwent PETDF for LDH treatment between January 2017 and June 2020 were included in this study. The inclusion criteria were: (1) single-level (L4-L5 or L5-S1) lumbar disc herniation, (2) radiating pain in the unilateral lower limb, (3) Magnetic Resonance Imaging (MRI) indicating herniated disc at L4-L5 or L5-S1, in agreement with clinical symptoms and signs and (4) failure of ≥ 6 weeks of conservative treatment. guidance. An 18-gauge spinal needle was inserted into the target disc level under uoroscopic guidance. The needle tip was placed at the posterior edge of the target disc space on the lateral view whereas the needle tip was placed at the medial pedicular line on the anterio-posterior view. A guidewire was inserted into the spinal needle then the spinal needle was removed. A dilating obturator was passed over the guidewire. A 7.5 mm trephine was then used to cut off the anterior part of the superior articular process to facilitate insertion of the working cannula. Further, an endoscope was introduced through the cannula. The herniated disc was resected using endoscopic forceps and a bipolar radiofrequency coagulator.

Outcome measurements
Radiographs were assessed preoperatively and at the nal follow-up. Radiologic study parameters included intervertebral space height (ISH) ( Figure.1), intervertebral space angle (ISA), foraminal area (FA) (The Foraminal Area was measured on T2-weighted MRI at the most stenotic sagittal plane) and lumbar lordosis.
MacNab criteria were used to evaluate surgical effectiveness. Visual analog scale (VAS) and Oswestry disability index (ODI) were used to assess low-back pain and leg pain preoperatively and postoperatively during regular follow-ups at 1 week, 1 month and at least 1 year.

Demographic characteristics of patients
A total of 84 patients (L4-L5 group, 40 patients; L5-S1 group, 44 patients) were enrolled in this study. Preoperative demographics were not signi cantly different (P > 0.05) between the two groups ( Table 1).

Clinical outcomes
The mean follow-up period was 13.46 ± 1.94 months (range: 12-21 months). Postoperative pain scores using VAS and ODI showed a signi cant improvement in leg and lower back pain in both groups compared with preoperative scores (P < 0.05, Table 2 and Table 3). Preoperative and Postoperative VAS and ODI scores were not signi cantly different for the two groups (P > 0.05, Table 1).
ISA increased signi cantly in two groups (P < 0.05, Table 2 and Table 3) The mean preoperative to postoperative change in ISH and ISA were not signi cant different (P > 0.05, Table 1) for the two groups

Lumbar lordosis
Preoperative and postoperative lumbar lordosis were not signi cant different (P > 0.05, Table 1) for the two groups. No signi cant improvement on lumbar lordosis was observed for both groups (P > 0.05, Table 2 and Table 3).

Foraminal area (FA)
Preoperative and postoperative FA were not signi cant different (P > 0.05, Table 1) for the two groups FA of the operating side increased signi cantly in all patients after PETDF (P < 0.05, Table 2 and Table 3).
FA of the contralateral side decreased signi cantly in all patients after PETDF (P < 0.05, Table 2 and   Table 3). least 95% of LDH cases occur at the L4-L5 or L5-S1 segment. Therefore, the differences in clinical outcomes and radiographic measurements due to different levels of disc herniation in patients after undergoing PETDF should be explored.

Discussion
The clinical outcomes of PETDF recorded in this study were similar to ndings reported in previous studies [12][13][14] . Analysis showed signi cant improvements in pain score and functional status of all patients during the nal follow-up. These ndings imply that PETDF is effective in relieving both low back and leg pain in patients with LDH.
Interestingly, contralateral foraminal area (CFA) signi cantly decreased in all patients at the nal followup. A few studies have explored degeneration of contralateral intervertebral foramen and lateral recess after PELDF. Lumbar facet joints are the conventional methods for guiding and stabilizing the spinal motion segment, especially in maintaining lumbar stability in forward exion [15][16][17] . A previous study using nucleotomy model 18 reports that facet joints supported signi cantly greater load after nucleotomy. Supported load (15.8%) was almost doble the applied external force (8.6% of). Notably, low quantity of nucleus removal (range: 0.7-1.7g) increased the forces transmitted over the facet joints. Previous studies report that destruction of lumbar facet joints results in increase in rotation, loss of strength, and decrease in stiffness of spinal motion segment 16,19,20 . Moreover, facetectomy results in facet joint diseases and lower back pain, especially on the contralateral side 4 . Therefore, discectomy and foraminoplasty performed during PETDF may result in increased contralateral facet joint loading in order to maintain spinal mechanical stability in a variety of directions and loading scenarios. This increase in contralateral facet joint loading ultimately results in decreased CFA. 21,22 . However, several studies on PETD report reduction of ISH post-operation 5,23 . Reduction of ISH is attributed to loss of the nucleus-pulposus volume; loss of nucleus-pulposus constituent due to use of bipolar radiofrequency, including loss of mucopolysaccharide protein complex, chondroitin sulfate and high amounts of water; defects in annulus brosus during surgical discectomy; and intervertebral disc degeneration 5 . In our study, ISH decreased in the two groups. Eun, Sang Soo et al 23 report that the average postoperative disc-height ratio in patients (73.7% patients were at L4-L5 spinal level, 10.5% patients were at L5-S1 spinal level) who underwent PELD was 81.54% of the original disc height at 10 years prospective follow-up. These ndings were consistent to ndings of our study. In addition, Qiao P 5 reports reduction of ISH post-operation in 64 patients with L4/5 spinal level who underwent PETDF. These ndings show that higher reductions of ISH are correlated with higher increase in intervertebral space angle (ISA). Furthermore, pain is correlated with decrease in lumbar ISA 24 , whereas ISA increases with decrease in pain after operation. 5 Although, ISA increased in the two groups, ISA was not correlated with decrease in ISH. This nding can be attributed to the small sample size.

Maintenance of Intervertebral space height (ISH) is a good prognostic index after PETD
This study has several limitations. First, there may be errors associated with measuring the FA on MRI.
We measured the FA in the sagittal T2 images at the most stenotic sagittal plane. However, these sagittal images may be heterogeneous due to differences in the cutting angle of the MRI resulting from individual anatomic variations and technical problems. A 3.0 mm slice of sagittal T2-weighted MR image is also thicker than an ideal slice. Second, this study only used FA measurement. Therefore, our results may be limited regarding measurement of the nerve root pressure or morphologic changes. Finally, another limitation of this study is its retrospective nature.

Conclusion
PETDF is an effective and safe treatment approach for lumbar disc herniation in both L4/L5 and L5/S1 level. However, it may increase lumbar ISA (intervertebral space angle) and result in low CFA (contralateral foraminal area) and ISH (intervertebral space height). L4-L5 and L5-S1 level patients showed no difference in clinical outcomes and radiographic measurements despite biomechanical and anatomic differences between these levels.   Figure 1 Intervertebral space height is the height between inferior endplate of cranial vertebra to superior end plate of caudal vertebra. Both anterior and posterior heights are measured to calculate average height. The lumbar lordosis angle. The lumbar lordosis angle is the angle between the vertical line of the upper edge of L1 and the vertical line of the upper endplate of S1.