Lumbar spondylolisthesis with osteoporosis has great requirements for the biomechanical stability of pedicle screws fixation system (11, 24). Studies have shown that CPSs augmented by PMMA to reconstruct the displaced vertebral body and perform bony fusion is still the main surgical method for these patients (19, 20, 24–28). Theoretically, the more CPSs used the greater holding force supplied by the internal fixation. However, overuse of CPSs may increase the risk of complications related to PMMA leakage including allergic reaction, venous or pulmonary embolism, and difficulty in 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 literatures and fund that some studies using CPSs in bilateral sides (25–28), while other studies using in unilateral side partially (17, 19, 20). Therefore, it has great clinically value to explore whether unilateral PMMA-augmented CPSs can provide adequate stability compared with bilateral PMMA augmentation, as well as whether any difference exist in the effectiveness and complications between the two methods.
In this study, a clinical finding was broadly shown. The CPSs augmented by PMMA, unilaterally or bilaterally, could achieve satisfactory improvement of reduction in the postoperative slip degree. This finding could be concluded based on the significant differences in the intervertebral disc height and Taillard index compared preoperative with postoperative in both groups. In the period of follow-up, intervertebral disc height is a key indicator for treatment success. Previous studies have confirmed that reduction can restore physiologic alignment and balance, especially for high-grade spondylolisthesis (29, 30). Furtherly, Chalee-valayer et al (31) and Roussouly et al (32) reported loss of intervertebral disc height is positively correlated with lower back pain. In group UC and BC, the intervertebral disc height was lost in mean value at last follow-up in the present study which was consistent with the literature (27). However, this change is not statistically significant compared to immediately after surgery, and the clinical symptoms of the patients were not aggravated by this loss. The reason for this phenomenon can be explained by physiological progress. Interbody fusion cages are possible to sank after surgery because of osteoporosis. It should be noted that unilateral and bilateral fixation were equally effective in maintaining disc height by comparing the loss of intervertebral space height between group UC and group BC.
Taillard index is another key indicator to evaluate the maintenance of spinal reduction. Floman et al (33) and Goyal et al (34) suggested that displaced vertebral body should be anatomically restored as much as possible for lumbar spondylolisthesis, so as to increase the area of intervertebral fusion. Kim et al (35) and Wang et al (36) reported CPSs had better ability to restored displaced vertebral body than traditional screws. Similarly, our results showed that PMMA-augmented CPSs could avoid vertebral body slipping again, and unilateral and bilateral fixations both showed a long-term maintenance of spinal stability after surgery.
Previous studies revealed that screw loosening rate increased in patients with osteoporosis, which might lead to serious consequences such as screw fracture, non-fusion, and pseudarthrosis (37–40). In this study, no screw loosening was observed. It confirmed by screw displacement less than 1 mm at last follow-up in all cases. However, the incidence of complications related to PMMA will increase with the amount of PMMA used in a single vertebral body. It implies that bilateral PMMA-augmented CPSs has a greater risk of PMMA leakage. In fact, the PMMA-leakage rate of CPSs in various studies had great difference, Angel et al (25) and Wang et al (27) reported 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 in the previous studies. The reason might be related to the different designs of CPSs in different studies. However, the leakage rate for unilateral augmentation was 7% in group UC, which was significantly lower than that in bilateral cases. Unilateral CPSs may reduce the risk of PMMA leakage by reducing the amount of PMMA used.
The biomechanical properties changes of vertebral body after surgery have 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. No significant fractures were observed during follow-up in the present study, and this could be related to the small number of patients enrolled or the relatively short follow-up period. Some effects of alterations to biomechanical properties are difficult to observe in the short term, although they may occur in the long term.
Singh V et al (43) did a systematic analysis for PMMA-augmented cannulated pedicle screw. Their results summarized from published studies 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. The average improvement ODI for assessment of functional recovery was 42.1. In this study, we had similar results, the VAS and ODI score significantly improved after surgery immediately and at the last follow-up (P < 0.05) compared with those before operation in both groups. Additionally, there were significant differences of VAS and ODI scores between immediately after surgery and at the final follow-up more than 6 months (P < 0.05). The result indicates 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). These results reveal that unilateral PMMA-augmented CPSs is less invasive and can be performed less time than BC. This is 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.
This study has the following limitations that should be considered. First of all, the measurement is not accurate for showing the changes at the screw tip. The analysis also can be incorrectly measured and subjected highly to individual variants, which is not tested by different radiologists due to the projection or obliquity of the x-rays view. A CT scan would be much superior in analyzing the evidence of screws loosening to obtain a stronger conclusion. Finally, the study was a retrospective study with defects in study design and the sample size of this study was relatively small, which reduced the credibility of the study.