The slipped vertebra reduction remains a controversial issue in the surgical treatment of lumbar spondylolisthesis. In 2011, Audat et al. [5] reported that spondylolisthesis reduction is unnecessary as the clinical outcomes remain the same with or without reduction. However, other studies confirmed marked decreases in SD. Sears et al. [6] reported a good clinical outcome in 83% of cases after slip reduction of spondylolisthesis. In a prospective study involving 40 patients with degenerative spondylolisthesis, Wegmann et al. [7] demonstrated that reduction strongly correlates with improvements in quality-of-life scores (QLS). Spondylolisthesis reduction can, therefore, restore spinal canal volume, correct tapering of the nerve root hole, protect nerve roots from being pulled, and regain the overall physical arrangement of the vertebral body. In addition, decrease in SD may have a positive effect on LL correction. Kawakami et al. [8] reported that a decrease in SD was highly associated with enhanced LL correction. If the SD reduction of the slipped vertebra in lumbar spondylolisthesis is successful, compressive techniques with posterior instrumentation can be readily performed. There is no technical difficulty in treating lumbar spondylolisthesis with an additional screw in the proximal vertebral body. In fact, the upper screws can serve as a leverage point and the upper and lower displaced vertebral bodies can act as anchors to facilitate a middle bridge-shaped pulling reduction, under direct vision. Adding a screw to the proximal vertebral body produces a stronger lifting force, a more uniform stress distribution, and an enhanced secure reduction effect. The risk of pedicle screw extraction can also be reduced, particularly in patients with osteoporosis. In this study, SD was significantly decreased in the 6S group, compared to the 4S group at all points after surgery. This indicated that SD was significantly decreased by introducing the additional upper pedicle screws in PLIF surgery. Hence, patients in the 6S group, relative to the 4S group, received sufficient distraction reduction.
Lumbar spondylolisthesis almost always occurs in the lumbosacral segment, which is similar to the thoracolumbar segment, owing to its unique anatomical and biomechanical features. Prior studies reported [9, 10] that about 70% of the global LL are located in the last two lumbar levels. During lumbar spinal fusion surgery, LL recovery is a priority. However, it is difficult to mediate, especially, in terms of the acquisition of total LL during surgery. Takahashi et al. [11] reported that an increased SL in turn increases LL. Therefore, SL acquisition leads to LL. In this study, the postoperative LL and SL of both groups were significantly improved, compared to before surgery. After PLIF surgery, no significant differences were observed in LL and SL values between the two groups, but the LL and SL values in the 6S group were better maintained at the last follow-up. We speculate that this is due to the varying fixation methods used during surgery. The 6 screws in 3 vertebrae were obviously more superior to the 4 screws in 2 vertebrae, particularly, in terms of fixation strength and stress distribution, in the fixation mode of lumbar spondylolisthesis.
The sagittal sacropelvic morphology and orientation intercede lumbar spinal geometry. In case of abnormal sacropelvic morphology and orientation, a disturbed global sagittal spinal balance is achieved. PI is a fixed parameter that is independent of pelvis orientation. PI is equal to the arithmetic sum of SS and PT [12] . The standard PI value is approximately 53° ± 9° [13]. In this study, PI was significantly higher, compared to the standard value, which is coincident with other publications [14]. There was also a close relationship between PI and LL (the ideal relationship is: LL = PI ±9° [15]. Patients with a high PI value require more extended LL after surgery in order to maintain sagittal balance. PT and SS, on the other hand, are positional and are related to pelvic orientation. The standard PT value is approximately 13 ± 6° [16]. Kim et al. [17] reported that PT has a strong correlation with good clinical outcomes. Additionally, PT improvement after PLIF surgery may be one of the reasons for low back pain relief in patients with lumbar spondylolisthesis. In this study, we showed no statistically significant differences in PI and SS values between the two groups. Although there was significant difference between the two groups in PT values at the last follow-up, the difference was small. Therefore, it appears that short and long segment pedicle screw fixations do not massively influence the spinopelvic sagittal plane after surgery. However, the postoperative patients in this study lacked in performing daily activities, mostly due to old age. Meanwhile, the follow-up time was short, and could not accurately reflect the effect of screw numbers on spinal-pelvic sagittal balance.
Compared to the preoperative status in this study, VAS and ODI values decreased significantly after surgery in both groups. Although most patients reported satisfied clinical functional improvement after surgery, three patients in both groups suffered from chronic low back pain. Rajnics et al. [18] reported that failed restoration of LL may cause chronic low back pain post surgery. This is likely due to the painful compensation with hyperextension at the upper adjacent levels, due to the increase in traction load on the posterior spinal arrangement. LL restoration did not change, and even worsened, in these three patients with chronic low back pain in our study.
Lee et al. [19] reported that the incidence of adjacent segment disease (ASD) is markedly higher after PLIF surgery. This is because the spinal fixation instruments and fusion cage can significantly increase segment stiffness and stress transmission to the adjacent segment, thus accelerating the postoperative degeneration process of the adjacent segment. Similarly, some scholars speculate that long segment fixation will promote normal segment degeneration, since the longer lever arm from the multi-segment fusion will generate more stress on the remaining free segment [20]. Gene et al. [21] advocated the new-onset substantial mechanical back pain as a possible symptom of ASD. However, Kumar et al. [22] reported that longer level fusions or a PLIF addition does not increase ASD risk. Till date, studies have not definitively revealed whether the radiological alterations and clinical deterioration of ASD are the result of spinal fusion or iatrogenic production of a rigid motion segment. The exact mechanism of ASD is uncertain, but some risk factors, such as, pre-existing degeneration of the adjacent segment, exert a negative impact on ASD. Moreover, secondary degeneration of the proximal segment of spondylolisthesis may occur due to alterations in the normal spinal sequence after lumbar spondylolisthesis. Lee et al. [23] reported that pre-existing disc degeneration increases biomechanical changes in adjacent segment after surgery. Some surgeons might perform a complete PLIF at the adjacent level. However, Zhang et al. [24] demonstrated that a distraction of the intervertebral space and facet fusions in the adjacent segment can effectively prevent ASD. During PLIF surgery, bone graft fusion is generally performed to correct spondylolisthesis. However, in this study, the normal segment was only fixed without fusion in the 6S group, which achieved augmented LL and SL, compared to the 4S group. In terms of longer segmental fixation, such as, L1-S1 fixation, the L4-S1 fixation is known to avoid the complete loss of lumbar activity, induce less surgical trauma, and reduce the cost of internal fixation. Therefore, in this study, two segment fixation was recommended for patients with isthmic L5–S1 spondylolisthesis, with a long course of disease, degeneration of adjacent segments, and low bone density.
There were certain limitations in our study. Firstly, this was a retrospective study. Hence, our nonrandomized design may have unintentionally introduced selection bias. Secondly, our patient population was relatively small. Finally, the mean follow-up time was relatively short (less than 2 years). In future investigations, we recommend conducting prospective multi-center studies, involving large patient population, and long-term follow up.