Although the clinical efficacy of CPS has been widely recognized by various researchers, a series of complications caused by CL has always been a concern. Previous studies [15–17] report that the intraoperative CL rate varies from 5–80% based on X-ray. Nevertheless, few researchers have investigated the reasons behind the wide variation in CL and its risk factors. The causes and specific risk factors that render the variation in CL need to be further clarified. This study is the first to provide evidence of the risk factors for CL through a large sample of spinal degenerative diseases. Our results indicate that BMD, the number of CPSs implanted, the thoracic vertebra augmentation, and certain types of diseases are risk factors for CL. We should focus on risk factors during surgery to avoid complications associated with CL.
BMD values is a risk factor that affects CL rate. Hu et al. [18] suggested a significant correlation between CL rate and BMD when a certain volume of bone cement was applied to the thoracolumbar spine for augmentation (2 mL per screw for thoracic vertebra and 3 mL for lumbar vertebra, mostly by bilateral augmentation). It was also noted that lower BMD indicates higher CL risk. Similar with previous studies, the present study likewise showed a significant correlation between BMD and CL rate (Tables 4). We further divided the study objects into three groups according to the degree of osteoporosis. The results indicated that BMD of less than − 3.0 SD would be a risk factor for CL compared with that between − 2.5 and − 3.0 SD (Table 5). Therefore, our findings suggest that in the management of patients with severe osteoporosis, the risk of leakage should be considered when using CPS to augment the biomechanical stability.
Consistent with the findings of Janssen et al. [19], the present study showed a correlation between the CL risk and the number of CPSs implanted (Tables 4 and 5). Consequently, it is necessary to reduce the application of non-essential CPSs during surgery to reduce the CL rate. However, the current strategy for the use of CPSs has never been clearly defined [20, 21]. Based on our clinical experience, we recommend the strategy of "the application of CPSs must be effective and as few as possible". Technically, the following are recommended: 1) for patients with lumbar disc herniation and lumbar spinal stenosis, unilateral augmentation is feasible; 2) for patients with osteoporotic vertebral compressive fracture, unilateral augmentation is feasible and, bilateral augmentation is used if necessary; and 3) unilateral augmentation is feasible for lumbar spondylolisthesis of degree I and II, while bilateral augmentation is feasible for lumbar spondylolisthesis of degree III and Ⅳ. Furthermore, CPSs are used at the distal and proximal ends of the fixed vertebral body in ankylosing spondylitis with kyphosis. In degenerative scoliosis, CPSs are used at the proximal end and the remaining vertebral body depends on the intraoperative situation.
Guo et al. [8] reported that the augmentation of the right side of vertebral body is a risk factor for CL. It was speculated that the right inferior vena cava is shorter than the left inferior vena cava, and the blood returns at a faster rate. Therefore, when unilateral augmentation is performed, it suggests that right-side augmentation need to be avoided in order to prevent CL, and left-sided augmentation can be used as an recommended method.
In contrast with previous studies, the present study indicated that augmented position affected the CL rate. CPSs used in the thoracic vertebra is a risk factor for CL. Hsieh et al. [15] indicated that it is necessary to be vigilant during the cement-augmentation of thoracic vertebrae in clinical practice, as Type S leakage is more prone to pulmonary artery embolism. Moreover, Zhu et al. [10] pointed out that a higher degree of positive end-expiratory pressure could decrease the rate of CL. we speculate that it may be due to the small volume of the thoracic vertebra and its proximity to the thoracic cavity, which has low intravenous pressure. Therefore, CPSs used in thoracic vertebra is more prone to Type S leakage. On the other hand, the thoracic vertebra is closer to the thoracic cavity and more susceptible to intrathoracic pressure; as a result, increasing the positive end-expiratory pressure ventilation during surgery may potentially decrease Type S leakage in the thoracic vertebral. However, it is still unknown whether it would potentially increase Type B and Type C leakage rate.
Compared with the augmentation of the thoracic vertebra, that of the lumbar vertebra is not a risk factor for Type S and Type C leakages but a risk factor for Type B leakage. We hypothesize that the anterior lumbar vertebral vein may be distant from the thoracic cavity. The intravenous pressure is relatively high due to the negative intrathoracic pressure and blood reflux during respiration, which suggests that Type B leakage is more likely to occur. In Type B leakage, the bone cement easily enters the spinal canal via segmental veins. Although the lumbar spinal canal is wider than the thoracic spinal canal, it is still necessary to be cautious of the symptoms of neurological compression that CL may cause.
Our results also present that disease types affect the CL rate. Osteoporotic vertebral compressive fracture, revision surgery, ankylosing spondylitis with kyphosis, and degenerative scoliosis were risk factors for CL. A lower BMD combined with the fracture disrupting the integrity of the cortical bone of the vertebral body leads to an increased leakage rate. For patients with ankylosing spondylitis, the hyperplastic sclerosis of the vertebral cortical bone combined with the administration of hormones alters the micromorphological structure of the bone tissue, resulting in more CL. With respect to the high leakage rate in degenerative scoliosis, we believe it is due to the long-fixed segments and the increased use of CPSs, which affect the overall leakage rate in the patient.
Meanwhile, osteoporotic vertebral compressive fracture and ankylosing spondylitis with kyphosis are independent risk factors for Type S leakage. Presumably, most of these two lesions involve the thoracolumbar segment. And the augmentation of the thoracic vertebral body is an independent risk factor for Type S leakage. It’s reported Type S leakage is more prone to pulmonary embolism [9], the patient's vital signs should be closely monitored in the event of Type S leakage during surgery. Although lumbar disc herniation and lumbar spondylolisthesis are not risk factors for CL, they are risk factors for Type B leakage. We believe it is related to the higher venous pressure on the anterior vertebral veins in the lumbar spine, especially, in patients with lumbar spondylolisthesis.
Guo et al. [8] reported that the closer the screw tip to the midline of the vertebral body, the higher the probability of Type B leakage. Bokov et al. [22] also indicated that the proximity of the screw tip to the midline of the vertebral body is also a significant risk factor for epidural CL. Conversely, Hu et al. [18] showed in their study that the closer the screw tip to the midline of the vertebral body, the better the dispersion effect and screw holding force. Therefore, surgeons could further investigate whether it is necessary to adjust the angle of the screw placement so that the tip is away from the midline of the vertebral body, thereby reducing the probability of Type B leakage.
Although our study reached some substantial conclusions, it still has its limitations. First, this study has a single-center retrospective analysis design, which may decrease its evidence level and credibility. Second, some groups in this study enrolled a small number of cases; thus, further in-depth research with an increased sample size is necessary.