Pedicle screw fixation in osteoporotic patients is gradually becoming common because of the aging of the population. Therefore, an increasing incidence of pedicle screw loosening and instrumentation-associated complications will inevitably be faced . According to the literature, 17% of revision surgeries are associated with pedicle screw failure . Thus, CAPSI has received growing attention due to its reliable fixation and simple surgical technique, which has been shown to increase pedicle screw pull-out force from 147% to 278% in both vitro and in vivo clinical studies [15, 24-26]. But the treatment of multilevel LDD with cement-augmented pedicle screws is more complicated and challenging compared with 1-level or 2-level LDD for 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 do not relate to osteoporotic LDD. There is currently little data on the effectiveness and potential risks of CAPSI when treating LDD affecting 3 or more levels.
Although the application of cemented pedicle screw on multilevel LDD can achieve better stability with less screw loosening, it’s also bears a high risk of CL, PCE, and wound infections. The CL rate occurring in the present study was 34.52% and 93.97% CL was leakage into segmental veins and basivertebral veins. Several studies indicated that CAPSI bears a very high risk of asymptomatic cement leakage, of which a considerable fraction becomes clinically symptomatic. Methods to reduce CL in LDD include: (1) Using high viscosity cement or toothpaste viscosity of cement (the waiting period after mixing powder and liquid was 390 seconds in this study). (2) Reducing the volume of cement and the number of augmented screws. The large volume of bone cement and augmented screw resulting in increasing possibility of secondary venous wall rupture and cement pressed in [29, 30]. Experimental studies also have indicated that the screw stability doesn’t significantly improve with the volumes of PMMA above 2.8 ml each screw . Therefore, the volumes of injected PMMA should be optimized according to the different osteoporotic spine and should 1.5-2.5 ml per pedicle screw. (3) Planning the angle of screw placement and the selection of screw length by preoperative CT careful, so that surgeons can reduce the incidence of screw penetrating the bone cortex and the tip of screw approaching to the midline of the vertebral body. Our previous study has found that a smaller distance between the tip of the screw and the midline was a significant risk factor for epidural CL. (4) We propose injecting small doses slowly (the cement was injected by every 0.1 ml increment in this study) or creating a small cavity in the vertebral body prior to cement injection . (5) The injection of cement should be confirmed consecutively by fluoroscopy.
In this study, PCE was detected in two cases (4.35%), including one patient was symptomatic. Researchers have shown the risk of pulmonary embolism after percutaneous vertebroplasty and kyphoplasty ranging from 3.5% to 23%. This variation is relate to the radiographic technique used to evaluate embolism: CT images are more sensitive in detect small embolism [17, 31]. Most of the pulmonary embolism is asymptomatic and the incidence of symptomatic pulmonary embolism is about 1.4%~4.1% . The happening of deaths is scarce and related to fulminant or severe PCE [17, 32]. The optimal treatment strategy for PCE remains controversial. Many authors have suggested that in case of asymptomatic PCE, clinical observation and regular follow-up without anticoagulants is feasible. For patients with symptomatic PCE, long-term anticoagulation therapy can improve the safety of patients, included initial heparin during hospitalization and oral coumarin therapy at the 6-month follow-up. In severe symptomatic cases, surgical intervention to remove cement embolus is also reported in some patients.
A wound infection could be a risk factor for a concomitant vertebral body infection and instrumentation removal. The rate of infection in CAPSI group was very high (6.52%), we analyzed our data to explain the high infection rate, and we found that the infect patients with average age of 78 years old, average operation time of 326.67 min, average blood loss of 1433.33ml, and two of them have diabetes. We believe that the high infection rate was related to the elderly age, long operation time, a large amount of bleeding and other comorbidities, such as diabetes. We are very aggressive in the management of possible deep infection. Whenever it is suspected, rapid and aggressive surgical debridement is performed. Martín-Fernández  also think it is necessary to control infection early and rapidly, in his study, all surgical incision was performed to rapid and aggressive surgical debridement and to antibiotic treatment. Interbody and posterior segmental instrumentation can usually be left in place early on, as several authors have reported high success rates using this hardware preservation strategy in the management of early surgical site.
Few clinical studies reported the loosening of the augmented screw, but which can’t mask the fact that it is one of the complications in CAPSI. Two patients and three augmented screws were found loosening in our study, one of which is caused by tumble and the others are caused by poor distribution of bone cement around the screws. Individuals with LDD always have sensory, proprioception and musculoskeletal system alterations, which may result in balance impairment. Although these people’s balance control, pain, and functional activities were observed improved after lumbar surgery, they were still less stable than those age-matched healthy adults after surgery. Therefore, these LDD patients are more likely to fall, especially those who underwent multilevel fixation. When bone cement was poorly-dispersed around the pedicle screw, the augmented bone intensity, pull-out strength, periodic loading times and compression-fatigue performance were markedly lower than those well distributed one[14, 23]. In addition, according to our experience, when solid screw with PMMA augmentation was implanted late and cement become solidified, the sclerotic wall of the cement did not allow the screw to interdigitate into the PMMA, and thus the screw may deviate from augmented trajectory while the cement-screw interface was poorly-dispersed. This is another factor of augmented screw loosening.
We attempted to inject PMMA and then place the screws immediately on multilevel augmentation from the beginning, but it was with the risk that screw cannot interdigitate into sclerotic cement (Fig.3). Later we found using fenestrated screws allows for better control of PMMA injection (especially more than two levels need to be instrumented), and augment of the levels based on both the BMD and primary attachment of the screw. Thus, we propose using fenestrated screws on multilevel augmentation, especially for inexperienced surgeons. But we tend to insert the solid screws instead of fenestrated 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, minimizing the difficulty of operation and facilitating C-arm fluoroscopy.
Screw loosening of multi-segment fixation often occurs in the cranial and caudal segment, much more frequently in the caudal (S1 in particular). Coincided 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 in cranial and twenty-six caudal (eight in L5, eighteen in S1). Wu et al analyzed 658 screws in 126 patients and found that 25 patients experienced screw loosening, 18 patients (72%) had loosened screws in the inferior portion of the spinal construct, while 7 (28%) in the superior portion. Li et al also found that instrument failure (broken screw or rod) usually develops at the lowest segment in cases of multilevel pedicle screw fixation. The cause of this trend may be the mechanical overload on the cranial and caudal, which results in stress concentration on the cranial or caudal pedicle screw. Moreover, S1 pedicles have a larger diameter and shorter length than the lumbar ones, and the bone-screw interface located in the weaker trabecular bone of the sacrum may also contribute to the loosing of S1 screws. Therefore, we recommend that it is necessary for properly increasing bone cement dose in the cranial and caudal screw and the augment of S1 screw because these methods can reduce the need for sacroiliac screw fixation and other pelvic fixation .
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 the cranial and caudal pedicle screw alone, could be a worthy strategy of choice. Based on the results of clinical and radiological data compared 31 patients, with degenerative spinal pathologies, who experienced treatment of pedicle screws and cement injections on all segments to those who treated with cement injections only on the strategic vertebrae selected, Erdem argued that the cement embolism, symptomatic chest discomfort, and duration of surgery were significantly higher in all segments cement injections. Therefore, cementing the strategic vertebrae alone can enhance the fixation strength and endurance as well as decrease the side complications of cement application.
There are limitations of our study, since we have not analyzed the potential impacts 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 for the specific time of screw loosening and asymptomatic adjacent segment degeneration. 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 postoperative recovery may be reluctant to return to the hospital.