CBT screw was first used for posterior spinal fixation in patients with osteoporosis reported by Santoni et al., which has the characteristics of maximizing contact with cortical bone [8]. Because it has a medial-to-lateral direction and a caudocephalad trajectory, the CBT screw can reduce soft tissue separation, minimize trauma and blood loss, shorten the operation time [9]. Sakaura et al. found that CBT can reduce the incidence of radiological adjacent segment disease changes (R-ASD) and symptomatic adjacent segment disease (S-ASD) by protecting cephalic facet joints [10]. Biomechanical studies have shown that CBT screws have 1.7 times the pull-out resistance of traditional pedicle screws [11], and the clinical efficacy in posterior lumbar fusion is equivalent to that of traditional pedicle screws [12], as an alternative technique of internal fixation of the spine, the CBT screw has been highly concerned by orthopedic surgeons.
In recent years, some scholars have adopted the method of simultaneously inserting pedicle screws and CBT screws in the same pedicle to solve some complicated spinal diseases. Ueno used the double-trajectory technique in a patient with severe osteoporosis and achieved satisfaction fixed effects [13]. Analiz Rodriguez et al. used a navigation system to insert CBT screws in the same pedicle with pedicle screw to procedure the revision surgery of ASD, as well as Chen et al. used free-hand technique, they both achieved satisfactory results [3, 4]. Obviously, the CBT screw used for ASD revision surgery does not need to remove the original fixation, which has many advantages such as small surgical incision, more minor trauma, short operation time, and less blood loss, etc. The longer the original surgical segment, the more apparent these advantages.
However, the difficulty of this procedure was that the pedicle screw and CBT screw were placed in the same pedicle at the same time. Although the pedicle screw and CBT screw have different start points and trajectories, in many cases, these two screw placement channels still will be a certain degree of overlap, and the double screws will collide in the pedicle (Figure 6). After radiological measurements and research, Mullin found that the success rate of inserting the pedicle screw and CBT screw in the same pedicle is about 50% [14], so it is essential to analyze CT carefully before surgery and plan a feasible screw placement trajectory. Intraoperative navigation system had been used to improve the accuracy of screw placement [3]. However, the navigation system is expensive and complicated to operate, it generally requires special operating room layout and technical personnel to assist operations, and the use of navigation system is reported to have higher radiation exposure [15]. More importantly, even if the preoperative CT scan is confirmed to have a feasible screw trajectory, it is still challenging to find the trajectory again during the operation. Due to the influence of factors such as the surgical position and the positioning of the navigation system, the trajectory may be different from the preoperative planning, and the optimal trajectory may deviate. When the optimal trajectory has been selected with preoperative CT, the production of navigation templates is a very mature technology with low cost.
The use of individual templates in orthopedic was first reported by Radermacher in 1998 [16]. With the improvement of 3D printing technology, it has been used more widely nowadays, especially in spine surgery. The technology of making screw navigation templates is mature and relatively economical. Many researchers use 3D printing technology to make navigation templates to assist the placement of cervical pedicle screws, thoracic pedicle screws, CBT screws, or assist screw placement in special cases such as severe spinal deformity and complex revision surgery, which have been proven to have higher accuracy and safety and help reduce the radiation exposure of both the patients and operating room staff [17-20]. In this study, we used 3D printed navigation templates to assist the placement of CBT screws in the vertebral body which has been inserted pedicle screw in 18 cases, and the rest of the surgical procedures were the same as conventional techniques. Compared with the navigation technology, our operation time is shorter (154±58min), and the blood loss during the operation is less (144±176ml) [3]. The cost of making the navigation template is relatively low. We estimate that the cost of making the navigation template is 400 dollars per segment, and the cost of using CT navigation equipment may be as high as 1500-2000 dollars. The work of making the navigation template is completed before the operation. Although the workload is slightly increased, there are almost no extra steps during the operation. In addition, this technique doesn't have any extra learning curves, and 3D-printing devices are easy to get (or print the navigation templates by a cooperation company). Considering factors such as difficulty and cost. We believe that screw placement assisted by the 3D-printed navigation template has certain advantages compared with the intraoperative CT navigation system, and it is easier to be promoted and applied, especially in primary hospitals.
In this study, the newly inserted CBT screws run along the superior or inferior of the original pedicle screw from the inner to the outer side, mainly based on preoperative CT planning, the space that allows the new CBT screw to be inserted (Figure 6). In the preoperative planning, some of the CBT screws could not reach the cortical bone of the superior endplate because of the original pedicle screws blocking. We increased the abduction angle of the screw so that the screw just reached or penetrated the pedicle and the outer edge of the vertebral cortex, considering that there are no essential nerves, blood vessels and other anatomical structures, the screw fixation strength can be increased to prevent internal fixation failure. This increases the number of screws which is classified into Grade A-B when we evaluate the accuracy of the screw after surgery, but because most of our screws slightly break through the outer wall of the pedicle at the end, except for one Grade B screw, which had nerve root stimulation after breaking through the inferior wall of the pedicle, none of the other screws have caused postoperative complications related to internal fixation. Ueno reported that the starting point of the newly inserted CBT screw based on the original pedicle screw should be located above of the conventional CBT screws, so the CBT screws will run along the superior of the original pedicle screw [13], but in this way the CBT screw cannot interact with the inner and inferior wall of the pedicle, which may weaken the fixation strength of the screw. It is also reported in the literature that the CBT screw should be run obliquely from the inner and inferior of the pedicle screw to the outer and superior so that the screw has a stronger fixation strength [21]. However, this may be limited by the size of the pedicle and the position of the original pedicle screw, and because the nerve root is often close to the inner and inferior wall of the pedicle, this method of screw placement is more likely to cause nerve injury. One of our patients had right pedicle pain after surgery. Considering that the CBT screw broke through the inner and inferior wall of the pedicle. In our study, we evaluated the stability of fixation by measured the intervertebral height pre-and postoperative, the result showed that the postoperative and the last follow-up intervertebral height were significantly greater than that before the operation. Meanwhile, the postoperative data and the last follow-up data had no statistical difference. The CT scan also showed good fusion at the last follow-up, so we considered that the fixation using CBT screws had satisfactory stability. But this method should be testified by biomechanics test in the future.
The accurate attachment of the navigation template is the key to the accuracy of the screw position. In an RCT study of 3D-printed navigation template assisted screw placement in patients with spinal deformity, Riccardo et al. reported that 9.8% of the screws in the 3D printed navigation template group were graded as Grade B or C [17], Evan D. Sheha believes that this situation is mainly due to the poor fit between the navigation template and the bone surface [22]. According to this study, we summarize the experience in the process of making the navigation template: Because the facet joints of the revision surgery are mostly damaged during the first operation, and the surrounding soft tissues and scars are severely proliferated, it is difficult to clean the surface of the facet joint during the revision process, so it is not suitable as a navigation template attachment point, in contrast, the original internal fixation screws fixed in position and the surface soft tissue is easy to clean and remove. In Mimics modeling, due to the high Hounsfield Unit(HU) of original screws on CT, the density of the surrounding soft tissue and bone tissue differs, the boundary is clear and the modeling accuracy is high. Therefore, we used the inner edge of the original fixation screw and the spinous process bone surface as the main contact surface of the screw navigation template. Meanwhile, a connect bridge was necessary because it could also increase the stability of the navigation template.
Limitation
There have been some limitations of this study. Firstly, this study is a retrospective study, and there is no control group, and the total number of cases is only 18. Although 18 patients achieved satisfactory fusion at the last follow-up, future studies with larger sample sizes are needed to validate the fusion rate due to the small number of cases. Secondly, the CBT screws cannot always be used for the revision surgery of adjacent segments because of its anatomical limitations. Despite that, we still recommend this technique for clinical application because multiple studies including us have confirmed the good clinical results [3,4]. Once the screws were inserted by the above-mentioned technique successfully, the patient will benefit greatly. Thirdly, due to the existence of the original pedicle screw, the trajectory of the CBT needs to be compromised, which may cause some CBT screws to fail to achieve the maximum cortical contact as it was said that the "4-point cortical contact" [23], systematic biomechanical stability research of this fixation method is required in the future.