Different from congenital pathological factors of developmental lumbar spinal stenosis [3, 25-26]. Degenerative CLSS is caused by hypertrophic facets, hypertrophy and ossification of the ligamentum flavum, and disc herniation [1-4]. The dorsal compression of the dural sac is usually not severe, and even for severe CLSS, adipose tissue can still be seen on the dorsal side of the dural sac in axial MRI images (Fig.6). Except for the pathogenic factors on the dorsal side of the dural sac, such as ossification of the ligamentum flavum, the main cause of degenerative CLSS is the narrowing of the transverse diameter of central spinal canal. Due to the cystic structure of the dural sac, the lateral pressure of the spinal canal reduces the area of the dural sac.
The main purpose of CLSS surgical treatment is to completely decompress and minimize surgical trauma and continuous sequelae [2, 3, 9]. With the development of minimally invasive spinal surgery techniques, surgical trauma is significantly reduced, and the incidence of iatrogenic lumbar fusion surgery is reduced. MED-assisted unilateral laminectomy and bilateral decompression have obvious advantages over traditional laminectomy [11,13], but still have inevitable muscle anatomy and difficulty in completely decompressing the contralateral side .Endoscopic bilateral decompression through a unilateral interlaminar approach has the advantages of small trauma, less bleeding, and less damage to ligaments and joint structures. Many scholars have used percutaneous endoscopic interlaminar decompression(PEID) to treat CLSS and achieved excellent results in the literature[15-16].PEID has become an excellent minimally invasive surgical decompression technique for CLSS. However, this technology also has some limitations. In the first place, PEID requires general anesthesia, but elderly patients usually have multiple comorbidities and cannot tolerate general anesthesia. Secondary, PEID is difficult to completely decompress the contralateral side, especially the ventral side of the contralateral nerve root. Additional, for patients with CLSS, especially in elderly patients, lateral recess stenosis and intervertebral foramen stenosis often coexist, usually accompanied with bilateral symptoms. But PEID is not effective for intervertebral foramen stenosis. Finally, in some elderly patients, the interlaminar space is often narrowed by hypertrophic osteophytes and ossified ligamentum flavum. Decompression through interlaminar approach requires a large amount of time to remove the dorsal bone and the ligamentum flavum, especially in the upper lumbar segment with a small interlaminar window.
Some authors have also reported good clinical results of endoscopic lumbar unilateral decompression for lateral recess stenosis through transforaminal approach [17, 21, 28-29]. However, transforaminal unilateral decompression is often insufficient for CLSS. In 2019, Bao  reported that PETD treats central spinal stenosis, which is different from our research. Their study ruled out the pathological factor of bony central spinal stenosis and calcified disc herniation. And they only performed unilateral decompression with a unilateral approach. While our study included pathological factor of ossification compression and performed bilateral decompression. The main pathogenic factors of degenerative CLSS are the hypertrophic facets, hypertrophy and ossification of the ligamentum flavum, and disc herniation, which is usually accompanied by bilateral lateral recess stenosis. Therefore, bilateral decompression is necessary, especially for patients with bilateral buttock and lower extremity pain. The usage of deep-restricted trephine can be safer and more effective in removing bone structure, which can significantly reduce the operation time. In our study, the operating time of bilateral decompression was still lower than their unilateral approach (116.4min vs. 161 min).
Transforaminal bilateral approach can effectively relieve the lateral and ventral compression of the dural sac. In this study, the area of postoperative dural sac was significantly improved compared with preoperative (74.28±13.08 mm2vs.104.45±12.51 mm2), and the patient's symptoms improved significantly, indicating that transforaminal endoscopic surgery can also effectively decompress CLSS. In this study, we were enrolled in patients with radicular symptoms and intermittent claudication, which were caused by the compression of the dural sac and nerve roots due to degenerative changes of the lumbar spine. For CLSS, the working target area of the operation is the entire spinal canal, which requires removal of the epiphysis, intervertebral disc and ligamentum flavum which compress the dural sac and nerve roots. To achieve sufficient decompression space, the second foraminoplasty or even multiple foraminoplasties are always required. The specially designed deep-restricted trephine combined with high-speed drill can make foraminoplasty more efficient and safe. For patients with CLSS, the decompression needs to reach the dorsal side of the dural sac (Fig.3). The signs of adequate decompression are the following: the dural sac is well filled, and normal pulsation is restored, the dural sac has clear and adequate peripheral clearance.
In this study, pain in the buttocks and lower extremities improved significantly after surgery and continued to improve during the follow-up. The area of postoperative dural sac was significantly improved compared with preoperative. At the same time, the patient's ODI and MacNab criteria were significantly improved compared with preoperative. These results are comparable to other endoscopic decompression techniques (Table 2). In this study, the average age of patients was 72 years, and 80.9% (38/47) of patients combined with various medical diseases. The average age and the incidence of preoperative medical diseases were higher than other reports, but ultimately those clinical parameters such as VAS and ODI showed significant improvements [16, 27, 31-33].These results indicate that the advantages of endoscopic decompression through bilateral transforaminal approach not only have a good cosmetic appearance and anesthesia tolerance, but also can quickly and effectively relieve pain. The above advantages are mainly attributed to the minimally invasive endoscopic procedure. In addition, this technique of percutaneous endoscopic bilateral decompression can provide a better treatment option for elderly or medically compromised patients who are difficult to perform general anaesthesia.
It has been reported that incomplete decompression is one of the major drawbacks of minimally invasive bilateral decompression through a unilateral interlaminar approach, especially in the contralateral nerve root ventral decompression . This is due to the limited visibility of the operating device and the limited physical space. Percutaneous endoscopic transforaminal bilateral decompression can avoid this problem perfectly. The advantages of the transforaminal approach can better deal with lateral and ventral compression of the dural sac. Although the operation time in this study is slightly higher than other decompression methods [16, 27, 31-33], the whole spinal canal including bilateral intervertebral foramina, lateral recesses and central spinal canal can be effectively treated through bilateral transforaminal approach. And bilateral nerve roots and dural sac can be fully decompressed which ensure the surgical decompression effect. Some researchers have already identified that limited visualization of neural structures in endoscopic surgery may result in a higher rate of dural incision or nerve damage . However, compared with previous studies of endoscopic decompression techniques, the incidence of surgical-related complications in this study was not high (10.6%). (Table 2). There were 2 dural tears (4.2%) in the current study, which is comparable to previous studies. Both dural tears were small cracks without special treatment. At present, it is still controversial whether the dural sac tear needs to be repaired in the spinal endoscopic decompression surgery[34-36].We are more inclined to conservative views. The dural tear under endoscopy is small crack. If there is no cauda equina hernia, the dural tears do not need repair. Because the surgical trauma is small and the muscle is not damaged, only the incision needs to be sutured tightly, and a closed cavity will be created locally. In our cases, there were no cases of cerebrospinal fluid leakage, infection, or negative consequences such as revision surgery. Three patients of transient postoperative dysthesia were present in this study, similar to other literature [16, 27,31-33].Postoperative dysthesia is considered to be related to the burning of the radiofrequency electrode around the nerve root, and high-intensity radiofrequency bipolar should be avoided around the nerve structure. The continuous intraoperative saline flushing provides a safer space for the differentiation of the nerve structure from the surrounding structure during endoscopic decompression. Careful operation and necessary pre-hemostasis to ensure a clear surgical area can effectively reduce complications during the operation. The use of deep-restricted trephine combined endoscopic drill can not only reduce radiation exposure, but also remove bone structure more effectively,while reducing the risk of dural tear and nerve damage.
Biomechanical studies have suggested that a 50% retention of both facet joints is necessary to preserve atability , Osman SG studied the lumbar flexibility and pathological anatomy changes after posterior and transforaminal decompression . The anterior medial third of the superior facet, the anterior portion of the inferior facet, and the joint part between them were resected after transforaminal decompression. Spinal flexibility changes were not observed after transforaminal decompression while the flexibility of extension and axial rotation was obviously increased after posterior decompression. The transforaminal decompression technique used in our study was similar to the above study. Even after bilateral transforaminal decompression, no iatrogenic segmental instability was recorded during the follow-up. Because the posterior structures, including spinous processes, ligaments and lamina were intact, transforaminal decompression had less damage to facet joints, which had less impact on the integrity of the spinal anatomy. The risk of iatrogenic instability caused by this surgical technique was minimized.
Compared with other MIS procedures, bilateral transforaminal decompression has the following advantages. In the first place, it can achieve adequate decompression of the central spinal canal, bilateral lateral recesses and intervertebral foramina, providing a new treatment strategy for CLSS. Moreover, decompression through the transforaminal approach minimizes facet joints damage and has no posterior ligament structure and muscle damage, which reduces iatrogenic segmental instability and avoid spinal fusion surgery. Last but not least, this procedure can be performed under local anaesthesia with high patient tolerance, and it has the advantages of MIS surgery such as less trauma, less bleeding, and faster recovery, which can provide a better treatment option for elderly or medically compromised patients who are difficult to perform general anaesthesia.