According to previous reports , the incidence of ASD after lumbar fusion surgery and non-fusion surgery was 5–77% (mean 26.6%) and 10% respectively. However, the  . ASD after spinal fusion is considered to be multifactorial. Many literatures have reported that the degree of intervertebral disc degeneration was closely related to age (over 60 years old), genetic factors, high body mass index (BMI ≥ 25), pre-existing stenosis or degeneration of adjacent segments, lumbar insufficiency, multi-segmental lumbar fixation and fusion [10–12]. Therefore, this study aimed to eliminating interference of these factors, and there is no statistical difference of these factors between two groups before operation. In addition, the operation itself is also one of the important reasons resulting in ASD. Ekman et al found that lumbar fusion accelerated ASD. after long-term follow-up . Some scholars reported that the incidence of cephalic ASD examined by X-ray 2–3 years after lumbar fixation and fusion was 38.5% . This study also focuses on the cephalic adjacent intervertebral disc. Radcliff et al pointed out that the rate of ASD after fusion was significantly higher than that in patients without decompression , and concluded that excessive distracting by the fusion cage to the intervertebral space was an important risk factor for ASD . In a retrospective study, Biden et al suggested that floating fusion, in which the lower end of the fusion vertebra located at L5, is a risk factor for ASDis . In addition, floating fusion was more likely to develop ASD in patients with posterolateral lumbar fixation .
Although various reasons were attributed to ASD from the different views of many studies, the author speculates that surgery-related biomechanical changes of the spine is one of the most reasonable mechanisms.
In 1983, Kirkaldy-willis put forward the theory of three-joint complex (composed of intervertebral disc and two posterior facet joints), and believed that this structure plays an important role in maintaining the stability of the spine . Liu et al, after six-year follow-up of patients accepting L4-5 fusion, found that the incidence of ASD was the highest in patients undergoing laminectomy . Imagama et al followed up 52 patients after L4-5 laminectomy or L4-5 fenestration fusion for five years, revealing that patients with fenestration were less likely to develop ASD . The results showed that the preservation of the structure of the posterior column of the lumbar spine is an important factor to avoid ASD. Lumbar fusion requires extensive peeling off of paraspinal muscles, removal of part of ligaments and bony structure, destruction of the stability of the three-joint complex, resulting in abnormal load distribution of the whole spine, prone to vertebral spondylolisthesis or fracture and other diseases . Therefore, it changes the original equilibrium relationship between the diseased vertebral body and the adjacent vertebral body, and aggravates the postoperative adjacent segmental degeneration (ASD) [21–23]. Ma et al. [found in the human cadaver model that the increase in stress on the facet joints after fusion may affect the degeneration of adjacent segments . Through the analysis of three-dimensional finite element model, the biomechanical load of the adjacent vertebral facet joint above the fusion segment is obviously abnormal [25, 26].
Makino at al reported that the incidence of ASD in 41 L4-5 PLIF patients with minimum intervertebral space distraction (12.2%) was significantly lower than that of previous ASD with PLIF distraction (31.8%) . It is considered that the use of a smaller fusion cage to minimize the opening of the intervertebral space may prevent ASD. In a biomechanical study of a finite element model fused at the L4/5 level, stress on the L3/4 endplate and intervertebral disc increased during flexion/extension movement . In addition, in the cadaveric L3/4 fixation model, Cunningham et al observed an increase of pressure in the L2/3 intervertebral disc by 45% during flexion/straightening . It can be seen that the cadaveric experiment showed that the pressure in the proximal intervertebral disc of the adjacent intervertebral disc increased to a fixed level .
Therefore, we think that the occurrence of ASD after fusion may be related to mechanical factors, the destruction and disorder of local structure, the range of motion of its upper adjacent segments and the compensatory load of facet joints.
Because the nucleus pulposus tissue is a colloidal semi-liquid substance with flow characteristics, the volume of the intervertebral disc will be further degraded and absorbed over time after nucleus pulposus resection . Therefore, the removal of the nucleus pulposus of PTED leads to the decrease of the bearing capacity of intervertebral disc, which in turn leads to the decrease of the upper vertebral body. At the last follow-up, the height of the intervertebral space in the upper adjacent segment was lower than that before operation, and there was statistical significance (P < 0.05). It may be related to the natural process of aging. However, compared with TLIF, PTED can not only retain more spinal range of motion, but also retain as much intervertebral disc tissue as possible on the basis of ensuring the curative effect, which provides a pathological basis for self-repair and secondary stability in the later stage, and may reduce the incidence of ASD or delay the occurrence of ASD.
Many studies have shown that the decrease, disappearance or kyphosis of lumbar physiological curvature is closely related to the degeneration of intervertebral disc. Studies have shown that lumbar physiological curvature changes in patients with lumbar disc herniation may be the result of lumbar mechanical structural imbalance caused by lumbar degeneration . Umehara believes that the reduction of lumbar kyphosis will increase the posterior column load and cause changes in the mechanical load of the adjacent segments . Djurasovic et al found lumbar kyphosis deformity or loss of lumbar lordosis after lumbar fusion . In this study, the changes of adjacent segments and Cobb angle in group F were significantly lower than those in group X, indicating that PTED can maintain physiological curvature and mechanical balance of spinal structure to some extent, and reduce the incidence of lumbar degenerative diseases.
Intervertebral disc degeneration can directly and indirectly affect the area of intervertebral foramen. Cinotti et al found that intervertebral disc height loss can lead to intervertebral foramen stenosis by measuring 160intervertebral foramen in dry cadaver specimens and 50 intervertebral foramina in fresh cadaveric spine . In this study, the cross-sectional area of intervertebral foramen decreased before operation and at the last follow-up, and the change in group F was lower than that in group X. The stenosis of intervertebral space caused by intervertebral disc degeneration can significantly reduce the height of intervertebral foramen, especially the minimum sagittal diameter of intervertebral foramen. It may be due to the natural degeneration of the intervertebral disc or the change of posture during the examination of the patient.
In the past, many scholars have shown that the foraminal endoscope had a definite effect for LDH in early stage, and could significantly improve the pain symptoms . In 588 patients with LDH treated by intervertebral foramen endoscopy and followed up for more than 2 years, the excellent and good recovery rate was 95.3%, and the recurrence rate was 3.6% . Since imaging adjacent segmental degeneration is not necessarily related to symptoms after spinal fusion surgery, there was no significant difference in JOA scores between the two groups at the last follow-up, which is consistent with previous studies that about 1/4 of 1/3 ASD of patients can progress ASDis[38–40]. At the same time, compared with open surgery, foraminal endoscopic surgery was performed under local anesthesia, the operator can observe patient’s feedback well, and there is no need to expose the herniated intervertebral disc by pulling the nerve root and dural sac during the operation, which reduces the risk of nerve injury . Therefore, this study also found that minimally invasive surgery has a low complication rate and a low recurrence rate.
Considering that the treatment of osteoporosis is another technique for preventing ASD. Therefore, the two groups of patients with osteoporosis received conservative drug treatment. Considering that the contraindication of the hole mirror is the adaptability of XLIF, which affects the accuracy of this study. Besides,this study is a retrospective study, with a small sample size in a single center and a short follow-up time. In addition, we strive to reduce systematic errors by establishing and implementing strict inclusion and exclusion criteria in order to maintain the homogeneity of the sample. In order to design standardized surgical procedures, the same surgical team is responsible for the same surgical instruments; the last follow-up symptoms are evaluated by the same spinal surgeon. All imaging parameters were independently measured by three neurosurgeons who turned a blind eye to the study. Therefore, the data of this study is real and the conclusion is reliable.