In recent years, with the improvement of medical condition and living standard, there are more and more elderly people, and the number of patients with LSS is also increasing gradually. LSS refers to various forms of spinal canal, neural canal and intervertebral foramen stenosis, which would compress dural sac, spinal cord or nerve root, and result in the corresponding nerve dysfunction. Intermittent lameness is a typical symptom. According to the anatomical classification, LSS can be divided into the following three types: central type, foraminal type and lateral recess type (16). Meanwhile, the vertebral canal can be divided into four walls: the anterior wall is the vertebral body, intervertebral disc and posterior longitudinal ligament, the posterior wall is the vertebral lamina, articular process and most of LF, and the bilateral walls are pedicle, intervertebral foramen and part of LF. And the focus of this study is the patients with LSS secondary to hypertrophy of LF.
The LF is composed of a large number of elastic fibers, which are nearly vertical, starting at the lower edge of the upper vertebral arch and ending at the superior edge of the next vertebral arch. LF consists of both superficial and deep components. The superficial component of the LF inserts onto the superior edge and the posterosuperior surface of the caudal lamina, and the deep LF inserts for a variable distance onto the anterosuperior surface of the caudal lamina (17, 18). When the spine is tilted or bent, the LF extends and the tension increases; on the contrary, when the spine is in a neutral position, the LF contracts and normally does not form wrinkles. However, it can cause degeneration, hyperplasia and hypertrophy of LF on the basis of repeated indirect injury and chronic strain, then the LF would squeeze the nerves and cause local circulatory disturbance. Some studies had suggested that the degree of hypertrophy of LF is positively correlated with chronic low back pain, and it is difficult to relieve (19). In addition, LSS secondary to LF hypertrophy is common in the elderly, and most of them are located in L4/5 level (20–22), which is also consistent with the lesion segments of the patients in this study.
Conservative treatment is often the first choice for such patients, including bed rest, physiotherapy, traction, medication and acupuncture (23–25). The above treatments are symptomatic, but for elderly patients with LSS, the lesions are mostly caused by degeneration of the tissue around the nerve root, and will not be cured by these treatments. At present, there is no enough evidence to recommend any particular type of non-operative treatment for LSS, and most conservative treatments are based on expert advice and treatment experience. Moreover, many studies also revealed that a variety of conservative treatments have limited effects and are prone to relapse (26–31). Therefore, most patients who have failed to respond to conservative management will still choose surgery.
In the past few decades, lumbar interbody fusion has been considered as the standard procedure for the treatment of LSS, but the surgical trauma was great, accomplished by the destruction of bone, increasing the risk of postoperative intervertebral instability (32). Compared with minimally invasive surgery, patients with traditional open surgery showed longer time of lying in bed, and the incidence of complications was significantly higher, such as pulmonary infection, symptomatic deep venous thrombosis, urinary tract infection and so on (33, 34). More importantly, most of the patients were elderly patients, the above complications would have a greater negative impact. Besides, the normal biomechanics of the spine was changed after fusion, which always limited the movement of the surgical segment and increased the movement and load of the adjacent segment, thus this type of surgery often caused adjacent segment degeneration, including disc degeneration, spondylolisthesis, adjacent vertebrae fracture and so on (35, 36). In the long run, such surgery often lead to the limitation of lumbar movement and even long-term low back pain due to the loss of the active segment of the patients (37).
Nowadays, the expectation of elderly patients for surgical treatment is not only for relieving pain, but also for quick postoperative recovery so that they can be able to return quickly to normal life. As a result, minimally invasive spine surgery technique emerged as the times require (38, 39). The initial indications of minimally invasive spine surgery were limited to lumbar disc herniation, and various types of LSS were contraindications at that time. However, thereafter, as surgical techniques and instruments improve, the operative indications has gradually extended, and LSS has become one of that. Ahn pointed out that different approaches should be used for different types of LSS: interlaminar approach is suitable for central canal stenosisi in order to be decompressed thoroughly, lateral recess stenosis can be operated by interlaminar or transforaminal approach and transforaminal approach can be used for foraminal stenosis (40). In addition, several reports have shown that transforaminal approach is often used in LSS, while interlaminar approach is used in this study for the following reasons: 1) In this study, these patients with LSS were mainly caused by the hypertrophy of LF, and most of the LF was located in the posterior wall of vertebral canal, so the interlaminar approach was more suitable than the transforaminal approach to decompress the spinal canal; 2) Most of the hypertrophy of ligamentum flavum is at the L4/5 level, and the diseased segments of the patients in this study are at L4/5 or L5/S1, the operation of the transforaminal approach is more difficult because of the influence of the iliac crest; 3) The anatomy of interlaminar approach is closer to that of open surgery, which is more convenient for operation, and reduce the risk of inadvertent iatrogenic injury.
Compared with traditional surgery, the advantages of spinal endoscopy are as follows:
In the process of resecting the ligamentum flavum, only 1 ~ 2 mm of the upper and lower lamina was removed to maintain spinal stability (41, 42). What’s more, in this study, unilateral approach with bilateral decompression was adopted to better maintain the overall stability and provide advantages to the patient in postoperative rehabilitation. It is not difficult to see that patients in the endoscopy group have significantly better scores after surgery, including VAS, ODI and JOA score; b) We had accurate positioning during the surgery so that we could have a definite object in view and make decompression more effective; c) The operative vision was presented on the screen and magnified to several times, therefore, the nerve would be exposed more clearly for more accurate decompression. d)The minimally invasive surgery could effectively reduce local scar tissue, thereby reducing the probability of iatrogenic LSS occurrence after operation (43). e) The continuous perfusion of normal saline during the operation, on the one hand, the water pressure could play the role of local hemostasis and reduce the bleeding volume during the operation effectively, on the other hand, it also played an important role in ensuring a clear field of vision in the process of operation, thus improving the efficiency of operation and reducing the time of operation. It is clear from this study that the endoscopy group was lower than the control group whether the operation time or the quantity of bleeding.
Although there were no obvious complications in both groups of patients in this study, it did not mean that percutaneous endoscopic decompression would not lead to the occurrence of various complications, such as dural rupture, subdural hematoma, nerve root injury, infection and other complications all might happen (44, 45). According to this research, our experience of achieving better surgical outcomes and effectively guarding against related complications could be concluded as follows. Because the operating space of minimally invasive surgery was relatively small, the operation plan should be made carefully before operation according to the patient's medical history, symptoms, physical examination and various imaging data in order to ensure the same therapeutic effect as open surgery. Besides that, the incision location was identified by the relative position of the hypertrophic LF and the adjacent vertebral lamina and articular process so as to avoid unnecessary bone and soft tissue injury. Except for the adequate preoperative preparation, the limited operative space also suggested that we need operate meticulously, stop bleeding timely and keep clear operative field, especially in the process of stripping the nerve root or dural sac from the surrounding tissue. Moreover, the process of entering the working cannula should be done in a step-by-step manner. More specifically, before the contralateral decompression, we should ensure that the ipsilateral decompression is completed and the operative field is clear, then tilt the cannula as far as possible, properly remove the basal part of the spinous process so that the cannula can enter the contralateral spinal canal and continue contralateral decompression, and attention should also be paid to the protection of ipsilateral nerve and dural sac during contralateral decompression. Another phase that requires caution is adequate decompression, it requires us to actively deal with other pathological changes that cause LSS such as herniated or free disc and the osteophyte formation of the articulated facet when we remove the hypertrophic LF during the operation. Lastly, the original intention of minimally invasive surgery is safe, effective, less bed rest time and quick recovery, which is in line with the concept of enhanced recovery after surgery (ERAS). Therefore, we gave all patients NSAIDs for analgesia and encouraged patients to get out of bed and walk at the bedside on the first postoperative day, which was not only conducive to postoperative recovery but also helped to reduce the occurrence of postoperative complications.
Despite these promising results, questions remain. First, this study was a retrospective study and the number of patients in this study was small. Secondly, the follow-up time of this study is not long enough and the long-term effect remains to be observed. Therefore, larger, prospective, randomized controlled studies with longer treatment periods are our next-step research direction to confirm the results.