Compared with the traditional posterior extension fixation and fusion surgery, PELD has many advantages in the treatment of symptomatic ASD after lumbar fusion. In terms of anesthesia, the posterior lumbar revision, extension, fixation and fusion surgery needs to be performed under general anesthesia, while the PELD can be performed only under local anesthesia, the surgical risk is significantly lower for elderly people with more basic diseases. On the other hand, the surgeon can judge whether the nerve root is damaged through the real-time feedback of the patient's sensory and motor function changes during the operation, which ensures the curative effect of the operation and avoids the occurrence of serious neurological complications . Therefore, performing surgery under local anesthesia is of great significance to ensure the efficacy of surgery and patient safety.
The PELD only needs to accommodate the working sleeve into the operating area, and the length of the incision is only 7-10mm, which is significantly smaller than traditional minimally invasive surgery and open surgery. Reducing the surgical incision can reduce the amount of intraoperative blood loss, and the amount of intraoperative blood loss in endoscopic surgery is obvious less than open surgery. A small incision and less bleeding can reduce the trauma associated with the surgical approach, shorten the recovery time and hospital stay, and enable the patient to return to work as soon as possible. In this study, the average operation time was (62.7 ± 28.1) minutes, the average blood loss was (10.0 ± 8.3) ml, and the average hospital stay was (4.5 ± 2.3) days, which are significantly shorter than traditional fixed fusion surgery. The following three reasons are mainly considered. First, the surgical incision of the traditional lumbar posterior revision extension fixation and fusion surgery overlaps the surgical incision of the first lumbar fusion surgery. The first operation has destroyed the normal anatomical structure of the paravertebral muscles, spinal bones and ligaments. Coupled with the formation of postoperative scar tissue, it creates difficulties for revision surgery. Second, the PELD requires small surgical incisions, small exposure area of the surgical area, simple intraoperative hemostasis, and the difficulty of establishing a surgical channel is lower than that of open spinal surgery. Third, the PELD only completes the adjacent segment nerve decompression, without internal fixation and fusion, while the traditional posterior lumbar surgery will destroy the normal paravertebral muscle tissue, spinal ligament and bony structure, the stability of the spine is decreased , so intraoperative extension and fixation and fusion are required, and the complexity of the operation is significantly higher than that of endoscopic surgery.
The PELD has less damage to the bony structure and ligament muscle tissue of the spine, avoiding extensive exposure of the posterior muscles and removal of lamina, facet joints and ligamentum flavum and other posterior ligament complex structures. The biomechanical stability of the adjacent segments of the spine is preserved as much as possible, which speeds up postoperative recovery, reduces the postoperative pain syndrome of the patient's lower back, and also reduces the risk of secondary degeneration of adjacent segments of the lumbar spine. At the same time, the technology has little interference to the dural sac and nerves in the spinal canal, and can reduce the incidence of spinal canal and nerve adhesions due to scar hyperplasia after surgery. There is no need to replace or extend the internal fixation, which avoids many complications such as nerve root and dural sac injury during the nail placement, and the loosening and rupture of the internal fixation after the operation.
The surgical approach of PELD can be divided into transforaminal approach and interlaminar approach. The transforaminal approach does not require a posterior incision, avoiding the influence of the initial operation to destroy the anatomical structure of the posterior approach. The operation only needs to complete the foraminoplasty, without massive destruction of the paravertebral muscles, ligaments and bony structures, and can fully preserve the biomechanical stability of the spine. The interlaminar approach is difficult to separate the scar tissue formed during the initial open surgery under the endoscope, which may easily lead to complications such as dural sac tear, nerve damage, and residual nucleus pulposus. Some patients have internal fixation nails and has a horizontal connection, which can affect the placement of the working channel of the interlaminar approach. All the patients in this study adopted the transforaminal approach, without the first surgical incision to reach the operating area, which can avoid the inconvenience caused by the original incision.
The traditional concept believes that spinal fusion surgery should be performed to reconstruct the stability of the spine in patients with instability, but the clinical significance of spinal instability remains to be clarified . At present, there is still controversy regarding the treatment of patients with unstable adjacent segments by PELD. Telfeian et al.  pointed out that degeneration of adjacent segments has always existed, and the clinical effect of PELD in the treatment of symptomatic ASD after lumbar fusion is only temporary. It reported that the failure rate of 9 cases of percutaneous transforaminal endoscopic treatment of symptomatic ASD after lumbar fusion was 33% after 2 years operation, which may be related to the small number of patients included by the author and the presence of adjacent segmental instability in the included patients. The cohort study of the Ba Z. team reported the clinical efficacy of transforaminal lumbar endoscopic decompression surgery and lumbar fusion surgery for the treatment of ASD after single-segment lumbar fusion. The results showed the clinical efficacy of the two surgical methods have no significant difference, and endoscopic surgery has the advantages of less surgical trauma and shorter hospital stay . In this study, ASD patients with stable adjacent segments underwent PELD with good results in terms of recovery of spinal cord function and improvement of low back and leg pain. Evaluation according to the modified MacNab standard, the excellent and good rate reached 90.63%. Therefore, it is necessary to strictly controll the surgical indications in clinical diagnosis and treatment, and take into account the patient's clinical and imaging manifestations. For patients with radicular radiating pain and intermittent claudication, the first choice is nerve decompression, while the patients with low back pain caused by segmental instability of the lumbar spine tend to undergo fixation and fusion surgery to reconstruct spinal stability.
There are some limitations and shortcomings in the study. The first was that this study was a retrospective study and the sample size was small, so the results of prospective randomized controlled studies with large sample sizes in the future will be more convincing. The second is that the follow-up time of this study is relatively short, long-term observation should be carried out to clarify the effectiveness and safety of the PELD.