The biomechanical stability of pedicle screw fixation systems is particularly important in patients with lumbar spondylolisthesis and osteoporosis (11, 24). Studies have shown that the use of PMMA-augmented CPSs to reconstruct the displaced vertebral body and perform bony fusion is still the main surgical method for these patients (19, 20, 24-28). Theoretically, more CPSs require a greater holding force by internal fixation; however, overuse of CPSs may increase the risk of complications related to PMMA leakage, including allergic reactions, venous or pulmonary embolism, and difficulty with revision. There are very few reports on the accurate and reasonable application guidelines of CPSs to improve the rationality in clinical practice. We reviewed the published literature and found that some studies used CPSs on bilateral sides (25-28), while others used only the unilateral side (17, 19, 20). Therefore, it is clinically important to explore whether unilateral PMMA-augmented CPSs can provide stability that is equal to, or better than that of bilateral PMMA augmentation. In addition it is important to determine any differences in the effectiveness and complications between the two methods.
In the current study, the CPSs augmented by PMMA, either unilaterally or bilaterally, could improve the reduction in the postoperative slip degree. This finding was based on the significant differences in the intervertebral disc height and Taillard index preoperatively and postoperatively in both groups. During follow-up, intervertebral disc height is a key indicator for treatment success, and previous studies have confirmed that reduction can restore physiological alignment and balance, especially for high-grade spondylolisthesis (29, 30). Furthermore, Chalee-Valayer et al. (31) and Roussouly et al. (32) reported that loss of intervertebral disc height was positively correlated with lower back pain. In the UC and BC groups in the current study, the mean intervertebral disc height was lost at the last follow-up, which is consistent with the literature (27). However, this change was not statistically significant compared to immediately after surgery, and the clinical symptoms of the patients were not aggravated by this loss; this phenomenon can be explained by physiological progress. Furthermore, interbody fusion cages are possible to sink after surgery because of osteoporosis. In the current study, unilateral and bilateral fixation were equally effective at maintaining disc height, as demonstrated by comparing the loss of intervertebral space height between the UC and BC groups.
The Taillard index is another key indicator for evaluating the maintenance of spinal reduction. Floman et al. (33) and Goyal et al. (34) suggested that the displaced vertebral body should be anatomically restored as much as possible for patients with lumbar spondylolisthesis, so as to increase the area of intervertebral fusion. Kim et al. (35) and Wang et al. (36) reported that CPSs were better able to restore displaced vertebral bodies than traditional screws. Similarly, our results showed that PMMA-augmented CPSs could avoid vertebral body slipping, and unilateral and bilateral fixations both showed long-term maintenance of spinal stability after surgery.
Previous studies revealed that the screw loosening rate was increased in patients with osteoporosis, which might lead to serious consequences, such as screw fracture, non-fusion, and pseudarthrosis (37-40). However, no screw loosening was observed in the current study, as confirmed by screw displacement less than 1 mm at the last follow-up in all cases. However, the incidence of complications related to PMMA is known to increase with the amount of PMMA used in a single vertebral body; this implies that bilateral PMMA-augmented CPSs have a greater risk of PMMA leakage. In fact, the PMMA-leakage rate of CPSs differed greatly among previous studies, and Angel et al. (25) and Wang et al. (27) reported that the rate was in the range of 29.3%–36.1% for bilateral augmentation. In the present study, the rate was 11.9%, which was lower than that reported in previous studies; this may be related to different designs of CPSs used in different studies. However, the leakage rate for unilateral augmentation was 7% in the UC group, which was significantly lower than that in the BC group. Unilateral CPSs may reduce the risk of PMMA leakage by reducing the amount of PMMA used.
The biomechanical properties of the vertebral body after surgery have also attracted the attention of researchers. Baroud et al. (41) and Uppin et al. (42) demonstrated that PMMA augmentation increased the fracture risk for the vertebral body or the adjacent ones. In the present study, no significant fractures were observed during follow-up, which could be related to the small number of patients enrolled or the relatively short follow-up period. Indeed, the effects of alterations to biomechanical properties are sometimes difficult to observe in the short term, although they may be obvious in the long term.
Singh et al. (43) performed a systematic analysis of a PMMA-augmented CPS. Their findings indicated that the average VAS score before operation was 8.4 (range, 8–9.2) compared to 2.3 (range, 1.42–4.8) at the last follow-up. Moreover, for assessment of functional recovery, the average improvement in the ODI was 42.1. These results were in line with those of the current study, in which the VAS and ODI scores significantly improved immediately after surgery and at the last follow-up (P < 0.05) compared to those before surgery in both groups. Additionally, there were significant differences in VAS and ODI scores immediately after surgery and at the final follow-up (> 6 months after surgery) (P < 0.05). These results indicate that satisfactory mid-term clinical outcomes can be achieved in both groups.
The operation time, blood loss, and cerebrospinal fluid leakage in the UC group were significantly lower than those in the BC group (P <0.05), demonstrating that unilateral PMMA-augmented CPSs are less invasive and can be performed less time than bilateral CPSs; these factors are especially important for elderly patients with comorbidities. Because lumbar spondylolisthesis usually occurs in adults older than 50 years, the patients in this study were older and may have had many comorbidities and severe osteoporosis; thus, complex surgical methods could not be tolerated by these patients.
This study has several limitations that should be considered. First, the measurement method cannot accurately demonstrate the changes at the screw tip. Second, the analysis can also be subjected highly to individual variants, which is not tested by different radiologists due to the projection or obliquity of the X-ray view. In this context, a computed tomography (CT) scan would be superior to analyze the evidence of screw loosening, and provide a more robust conclusion. Finally, the study was a retrospective study with defects in the study design, and the sample size of this study was relatively small, which reduced the credibility of the study.