Traditional trajectory (TT) technique was the most common form of posterior fixation for lumbar spine fusion surgery. In the current rapidly growing aging population and the number of patients suffering from spinal diseases, posterior fixation of the lumbar spine in patients with osteoporosis still faces challenges [20–21], and complications were the common problems [22]. In order to provide the superior fixation for patients, various attempts have been made by domestic and foreign scholars from the design of screw shape to screw track consolidating. However, expandable screws and hydroxyapatite-coated screws were expensive and cannot be widely used [23–24], while bone cement was expensive and had certain complications and safety risks. Since cortical bone does not deform and degenerate significantly with the aging [25], the basis for the proposal and clinical application of CBT has been laid.
Biomechanical Properties Of The Mcbt
Previously, several studies were conducted by our team. MCBT effectively avoided the facet joint violation, stress tolerance, and fixation stability was superior than the CBT and traditional technique [10]. Hybrid fixation techniques of MCBT and traditional trajectory at the L4-L5 segment provided superior stability compared to the single fixation system using the same trajectory craniocaudally [12]. The ideal diameter for the L4 vertebral body using MCBT is 5.0 mm, in addition, the pullout strength and stability of the screw were improved significantly [11]. Compared with CBT, the volume of stress area and stress concentration point of MCBT were significantly reduced [13].
Comparison Of Screw Track Bone Density
Some scholars use the Hounsfield Units (HU) of CT images for bone mineral density (BMD) and bone strength positively correlated with dual-energy X-ray (DEXA), and HU assessment has the advantage of being reliable, convenient, and affordable, and also predicts postoperative screw loosening and pedicle screw pull-out strength in the lumbar spine [26–29]. The most important risk factor for postoperative screw loosening is osteoporosis [22]. The main fixation carrier of the pedicle screw posterior fixation technique was cancellous bone. When osteoporosis occurs, for every 10 g/cm3 decrease in bone density, the maximum screw withdrawal resistance decreases by 60 N [30].
Related studies have confirmed that the HU of CBT screw track was greater than that of TT. Jin et al. [17] found that CBT is 1.7–2.3 times higher than TT, zhang et al. [18] found that CBT is 1 times higher than TT, and Kojima et al. [19] found that CBT is 4 times higher than TT, mainly because of the high cortical bone content in the travel area of CBT. The above findings in the literature are consistent with our findings, and the comparative CT values of the screw tracks in this study resulted in MCBT > CBT > TT, and the comparisons between the groups were statistically significant.
Reviewing the anatomy of the lumbar spine, the thickness of the bony cortex around the pedicle varies as follows: inferior wall > superior wall > medial wall > lateral wall [31], with the cortical bone of the endplate being thinner in the central zone and thicker in the marginal zone at the junction with the cortical bone of the vertebral body [32]. In traditional CBT, the screw entry point was closer to the lateral wall of the arch, which was still distant from the medial wall of the arch, where the bone cortex was relatively thicker [15–16]. The MCBT technique moves the screw entry point 2.0–3.0 mm toward the midline based on CBT, which not only increases the bone thickness around the screw placement point and the holding force with the medial wall of the arch, but also reduces the incision exposure during screw placement. In addition to the changes in the screw entry point, the MCBT technique further increases the contact between the screw and the cortical bone around the screw path, especially the bone cortex of the medial wall of the pedicle and the lateral edge of the upper endplate of the vertebral body, by increasing the abduction angle and decreasing the cephalic tilt angle, based on the 10° of screw abduction angle of the traditional CBT technique. It further improves the biomechanical properties [7–8, 33].
The results of this study further confirms that the bone density of the MCBT is significantly greater than that of CBT.
The CT value of the screw track was positively correlated with the pull-out strength of pedicle screw [29]. Ueno et al. [34] found that the pull-out strength of CBT was greater than that of TT, regardless of the type of screw. CBT increases the uniaxial yield pullout load by 30% and 1.7 times higher torque than TT screw [4–5]. Our finding was that MCBT screw also had better pullout resistance than TT screw [11]. By comparing the anatomical characteristics of the CBT and MCBT, and by combining the mean CT values of the MCBT screw track with those of the CBT and statistically significant (P < 0.05). We concluded that the pullout resistance of MCBT screw is mainly attributed to the superior qualities of the screw track.
Length Of The Screw Path
The fixation strength of posterior fixation was not only related to the strength of the bone, but also to the length of the screw [35]. Varghese et al. [36] confirmed through biomechanical study that the torque of the screw during insertion into the bone is the best predictor of the final bone-screw interface failure. However, it is worth noting that the most important factor affecting the insertion torque of CBT was the screw length within the lamina, independent of the length within the vertebra or the total screw length (Fig. 3) [16]. We believe that this conclusion was reached precisely because of the inherent defects of the traditional CBT and the failure to effectively utilize the cortical bone throughout the screw trajectory, especially the medial and inferior wall of the pedicle and the bony cortex at the lateral edge of the upper endplate of the vertebral body [8–9]. Fujiwara et al. [33] found that the insertion torque of the CBT screw was positively correlated with screw length, negatively correlated with the distance between the screw and the medial wall of the pedicle and the distance from the screw to the superior endplate. Concept of MCBT placement with longer screw tracks and closer to the medial wall of the pedicle was also in line with this findings.
Chen et al. [37] demonstrated the anatomical parameters of CBT screw placement proper to the Chinese adults. The common length specification of screws in Chinese adult lumbar traditional CBT technique was 35–40 mm, the upper limit of screw diameter safety at each level was L1: 5.5 mm, L2: 5.5-6.0 mm, L3: 6.5-7.0 mm, L4: 7.5 mm, L5: 8.0 mm, the average abduction angle of screw placement at each level of lumbar vertebra was 9.20 ° ± 2.11 °, and the average head inclination angle was 26.41 ° ± 4.22 °. The abduction angle of the MCBT screw placement technique was 10 ° in the L1 and L2 vertebral bodies and 15 ° in the L3 vertebral body, while the L4 and L5 vertebral bodies were 15 ° − 20 °, with a smaller head inclination angle than CBT, and the distance this trajectory traveled in the lumbar spine [9]. As showing in the Fig. 4, AD > BC, a1 < a2, β1 > β2. This corresponds to the results of our study and other paper [33], in the comparison of the length of the screw track, the length of the MCBT in each group was greater than that of CBT, and it was statistically significant.
However, It is difficult to insert the MCBT screw with freehand in high accuracy because it has a subtle difference from CBT including the difference of several millimeters between the entry points of the two trajectories and few degrees between the mediolateral and craniocaudal angles. Penner et al. [38] obtained the comparable with the higher accuracy of CBT reported for the robot-assisted pedicle screw placement using preoperative 3D CT planning. 3D printed navigation template was investigated by our team previously and demonstrated that it improves the accuracy of the MCBT screw placement [39]. Robot-assisted screw placement not only improved its accuracy but also reduced complications [40]. In addition, emerging artificial intelligence (AI) technology plays an important role in solving many problems faced in spine surgery [41]. We strongly believe that the MCBT technique will be perfectly combined with robotics and AI in the near future.
In addition, McLachlin et al. [42] found the "teeter-totter" phenomenon by CT reconstruction of the loosened TT, early loosened screw makes use of the cortical bone as the fixation point, with the anterior head swinging in the cancelous bone. MCBT due to the extension of the screw path, the head of the screw reaches the lateral edge of the upper endplate of the vertebral body, while the entry point of the MCBT close to the thicker medial wall of the pedicle, so that the screw head and tail fixed which reduces the incidence of the "teeter-totter" phenomenon, screw loosening, and postoperative revision rate. According to the study above, combined with the anatomical basis of the MCBT and the results of our study, the MCBT screw with longer tract has better extraction resistance than CBT to a certain extent.
Limitations
Certain limitations were inherent in the present study: First, the sample size was not enough. Second, size of the screws selected was fixed, screw with different diameters and lengths were not analyzed. Third, only the patients with normal bone density were analyzed. Anatomical characteristics of vertebral body and changes with age will be discussed in the future.