Posterior lumbar fusion surgery (PLIF) is a very mature technique that has been applied to clinical treatment by many spine surgeons (7). It is suitable for patients with minimally invasive and difficult operation and complicated conditions, and full visual field exposure can ensure the safety and operability of surgery. Today, PLIF and related fusion methods are the gold standard of spinal fusion, a technique first proposed by Briggs H and Milligan P (8), the earliest use of excised laminae as intervertebral space plant fusion, with the development of technology and the rise of various materials, The use of different materials and different types of fusion apparatus and internal fixation in clinical treatment, such as surgical titanium and polyether ether ketone (PEEK), has reduced the failure of internal fixation. In our study, all patients underwent PLIF surgery, pedicle nail rod system fixation and interbody implant fusion device, which was made of polyether ether ketone (PEEK). Titanium cages can improve the rate of bone fusion, but also increase the settlement caused by the hardness of titanium relative to bone. PEEK materials have more skeletal elasticity and exhibit less settling than titanium cages (9). Cage made of different PEEK materials can be selected according to the patient's condition to stretch the vertebral space recovery height (10). Indirect decompression is achieved by stretching the ligaments of the surrounding tissue through the intervertebral height recovered by the fusion apparatus. At the same time, the interbody fusion device was used as a bone graft carrier to achieve interbody fusion and restore stability to the unstable segment (11). Post-operative fusion subsidence may affect the effect of indirect decompression and spinal stability, and may also cause local inflammation resulting in postoperative low back pain (12). Therefore, the fusion device subsidence has important clinical significance. This study further analyzes the high-risk factors causing the fusion device subsidence and clarifies its correlation by analyzing various factors.
At present, the clinical research on the fusion device settlement mostly starts from the stress balance and biomechanics between the fusion device and the end plate. Hou et al. (13) found through biomechanical tests that bone mineral density was closely related to the destructive load of the end plate of the vertebral body, and the reduction of bone mineral density could lead to the reduction of the destructive load of the end plate, thus increasing the risk of interbody fusion fusion. Liu Lei et al. (14) also found that bone density was closely related to the height of fusion sink, and the lower the bone density, the greater the possibility and height of fusion sink. In this study, it was also found that bone density was closely related to fusion sink, and the better the bone density, the lower the possibility of fusion sink. Chen et al. (15) pointed out in their study that women and body mass index were also risk factors for fusion sink, but no significant correlation was found in this study. In addition to the patient's own factors, surgical skills and methods are also the key factors of fusion device subsidence. Yang et al. (16) pointed out in their study that excessive spacing of the vertebral space would increase the stress in the fusion stage and accelerate the onset of settlement. Intraoperative vertebral space correction height should be selected according to the patient's preoperative degeneration and the height of the vertebral space. In this study, it was found that when the correction height of the vertebral space was too large, the probability of fusion fusion would be increased. In addition, intraoperative end-plate damage also increases the probability of fusion device subsidence, and intraoperative end-plate damage reduces the end-plate failure load, resulting in fusion device subsidence (17). WEWEL et al. (18) pointed out in their study the importance of intraoperative end-plate protection to prevent fusion device subsidence, and this study also found that end-plate damage was a risk factor for fusion device subsidence. Clinical studies have also pointed out that segmental lordosis can be an independent risk factor for fusion device subsidence (14). When the corrected segmental lordosis Angle is too large, the stress behind the fusion device is too large, and the failure load of the end plate will cause fusion device subsidence. However, the Angle of segmental lordosis correction has not been clearly studied in clinic. In this study, it is found that when the Angle of segmental lordosis correction is too large, the possibility of fusion settling will increase significantly. In addition, studies have found that PI-LL is also a risk factor for fusion device subsidence. Schwab et al. (19) proposed that PI-LL < 10° is spinal pelvic matching, which has important significance for spinal stability. Clinically, PI-LL was divided into 3 degrees: <10°, 10°-20° and > 20°. The higher the PI-LL value, the greater the likelihood of postoperative complications of lumbar spine, and the incidence of the latter 2 degrees was 1.1 times and 5.3 times of PI-LL < 10°, respectively (20). The surgeon should reasonably restore the height of the intervertebral space and correct the segmental lordosis Angle according to the patient's degeneration and sagittal position parameters, so as to reduce the probability of fusion device subsidence.
Clinical studies on the risk factors of fusion fusion are detailed, but the correlation between paravertebral muscle and intervertebral bone fusion time and fusion fusion fusion has not been reported. In this study, the CSA and FI of the paravertebral and psoas major muscles were measured, which are key parameters for evaluating paravertebral atrophy (21). The paravertebral muscle is an important muscular system that helps stabilize the spine during the maintenance of normal lumbar physiological lordosis and dorsal extension (22). In severe paraspinal atrophy, spinal stability is reduced and the stress between the fusion apparatus and the endplate is constantly changing, resulting in fatigue damage to the endplate and resulting fusion apparatus subsidence (23). Singhatanadgige et al. (2) In the follow-up of patients who underwent MIS-TLIF surgery, it was found that paravental atrophy was a risk factor for fusion fusion. In this study, it was found that paravental muscle rFCSA was an independent risk factor for fusion fusion. When the paravertebral muscle atrophy, the stress between the fusion-endplate increases, which also causes the fusion-plate subsidence. Wang Sinian et al. (24) also pointed out that paravertebral muscle degeneration is closely related to sagittal position parameters, and the paravertebral muscle affects the sagittal position force line to maintain spinal sagittal balance through a compensatory mechanism. The larger the paravertebral muscle rFCSA was, the less likely the fusion sink was to occur. However, the biomechanical relationship between the paravertebral muscle and the fusion apparatus needs further study. In addition, the time of interbody fusion is also closely related to fusion fusion sink. Lee et al. (25) found in the follow-up of 79 patients undergoing spinal fusion surgery that when interbody fusion occurred and the new bone was able to withstand the load at the interbody fusion fusion-vertebral endplate interface, the settlement would no longer progress. Prolonged intervertebral fusion can lead to osteolysis and absorption of the implanted bone in the intervertebral space (26), and the stress between the fusion organ and the end plate increases, and the fusion organ subsidence occurs when the load exceeds the end plate failure. When the fusion apparatus sinks, the instability of the intervertebral space will further affect the intervertebral fusion time. The two can influence each other and cause and effect each other. In this study, it was concluded that the longer the intervertebral fusion time, the greater the likelihood of fusion chamber subsidence. Therefore, osteogenic drugs should be actively used after surgery to reduce the time of intervertebral bone fusion and reduce the possibility of fusion sink.