Pedicle screw fixation in osteoporotic spine is gradually becoming common because of the aging population. Therefore, we will have to face the increasing incidences of pedicle screw loosening and instrumentation-associated complications . According to the literature, 17% of revision surgeries are associated with pedicle screw failure . Thus, CAPSI has been widely used for patients with poor bone quality due to its reliable biomechanical stability [15, 24-26]. But the treatment of multilevel LDD with cement-augmented pedicle screws is more complicated and challenging compared with 1 or 2-level LDD because of the increase of CL and mechanical overload . Many studies either assessed only the results of short segment augmentation or assessed the results of long and short segment augmentation together, and most data are not related to osteoporotic LDD. So far there is currently little data about the effectiveness and potential risks of CAPSI treating the multilevel LDD.
Although the application of augmented pedicle screws for multilevel LDD can achieve better stability and less screw loosening, it also carries a high risk of CL, PCE, and wound infections. The CL rate of the present study was 34.52%. Several studies indicated that CAPSI has a high risk of asymptomatic cement leakage and serious complications were also reported in the literature. Methods to reduce CL in LDD include: (1) Using high viscosity cement or toothpaste viscosity cement . (2) Reducing the volume of cement and the number of augmented screws [29, 30]. Experimental studies have indicated that screw stability does not significantly improve when the volume of PMMA exceeded 2.8 ml each screw . Therefore, the volumes of bone cement should be optimized according to the severity of osteoporosis. (3) Planning the trajectory and the size of pedicle screws by preoperative CT to achieve the accurate placement of pedicle screws. Our previous study has found that a smaller distance between the tip of screw and the midline of vertebra is closely related to the epidural CL. (4) We suggest injecting cement with small doses and slow speed (the cement was injected by every 0.1 ml increment in this study) or creating a small cavity in the vertebral body prior to the cement injection . (5) The injection of cement should be confirmed consecutively by fluoroscopy.
In the present study, PCE was detected in two cases (4.35%) that one patient was symptomatic. Clinical research has shown the incidence rate of pulmonary embolism ranged from 3.5% to 23% after percutaneous vertebroplasty and kyphoplasty[17, 31]. Most of the pulmonary embolism cases are asymptomatic while the incidence of symptomatic is about 1.4%~4.1% . Severe PCE is rare but fatal [17, 32]. The optimal treatment strategy for PCE remains controversial. Several researches have suggested that for asymptomatic PCE, it is feasible to adopt clinical observation and regular follow-up without anticoagulants. And for symptomatic PCE, long-term anticoagulation therapies, including initial heparin during hospitalization and oral coumarin therapy, can prevent cardiovascular complications. Surgery for removing cement embolus is also an alternative strategy for patients with severe embolism.
Wound infection may lead to nonunion and instrumentation removal. The infection rate of CAPSI group was very high (6.52%, 3/46). We believe it was related to advanced age, long operation time, large intraoperative bleeding, and other comorbidities, such as diabetes. The average age of these patients was 78 years old, average operation time 326.67 min, average blood loss 1433.33ml, and two of them have diabetes. Few clinical studies reported augmented screw loosening, but it can’t mask the fact that it is one of the non-ignorable complications in CAPSI. Two patients (three augmented screws) were found loosening in our study, one of the screws is caused by tumble and the others poor distribution of bone cement around them. Due to the worse balance control, lower extremity pain, and musculoskeletal degeneration, LDD patients become more prone to fall, even after spinal surgery . When bone cement was poorly-dispersed around the pedicle screw, the biomechanical stability were dramatically lower than those well distributed one[14, 23]. In addition, according to our experience, when a solid screw with PMMA augmentation is implanted late and cement became solid, the sclerotic wall of the cement does not allow this screw to interdigitate into the PMMA, and thus the screw may deviate from augmented trajectory.
We attempted to inject PMMA and then placed screws immediately for multilevel augmentation from the beginning, but it brought a risk that screws cannot interdigitate into sclerotic cement (Fig.3). Later we found using fenestrated screws leads to better control of PMMA injection (especially when more than two vertebra need to be augmented). Thus, we propose using fenestrated screws for multilevel augmentation, especially for inexperienced surgeons. But we tend to insert solid screws in S1 to reduce the leakage caused by fenestrated screws penetrating the presacral bone cortex . Otherwise, we usually use augmented screws on two vertebral bodies at a time rather than three bodies, which minimizes the difficulty of operation and facilitates C-arm fluoroscopy.
Screw loosening of multilevel fixation often occurs in the cranial and caudal segment and far more frequently in the caudal (S1 in particular). Consistent with previous studies [9, 27], twenty-four S1 conventional screws (44.44%) were found loosening in the CAPSI group, and thirty-three screws (8.46%) in CPS group, including seven screws at cranial level and twenty-six at caudal level (eight in L5, eighteen in S1). Wu et al analyzed 658 screws of 126 patients and found that 25 patients experienced screw loosening, 18 patients (72%) had loosened screws in caudal vertebra while 7 (28%) in the cranial vertebra. Li et al also found that the failure of internal fixation usually develops at the caudal level in cases of multilevel pedicle screw fixation. Therefore, we recommend increasing properly the bone cement dose in cranial and caudal screw to reduce the need for sacroiliac screw fixation and other pelvic fixation .s
Since the application of pedicle screws with cement on all segments will increase the operating time, CL, PCE, and incision infection, only cementing the selected strategic vertebrae, such as cranial or caudal pedicle screw alone, could be a worthy strategy. Based on the clinical and radiological data of 31 patients treated with cement injections only for the selected strategic vertebrae and augmentation for all segments, Erdem  found the rate of PCE and operative time were markedly higher in all segments cement injections group, but there is no significant difference in screw loosening and clinical efficacy between two groups. Therefore, selective cement augmentation for both cranial and caudal pedicle screws could be a potential optimal procedure to decrease the side complications of CAPSI.
There are several limitations of our study. First, since we have not analyzed other risk factors of the screw loosening and fusion, such as the strength of paravertebral muscles, single or double cage, the length/diameter of screw and the repeated placement of screws during operation, as well as the specific time of screw loosening and asymptomatic adjacent segment degeneration. Second, as a retrospective study, it is difficult for us to control the uniformity of the patients and there is a degree of selective bias in this study, such as the inclusion of cases, the grouping of patients, and the selection of surgical methods. In addition, the actual rate of loosening may be lower than those reported in this paper, because the patients with screw loosening are more likely to receive follow-up, while patients with good curative effect may be reluctant to return to the hospital.