Overview of MCBT
Zdeblick et al. [15] pointed out that the torque when the screw was inserted into the vertebral body was the best predictor of the bone-screw interface failure. However, it is worth noting that the most important factor affecting the insertional torque of CBT was the length of the screw in the lamina [16], independent of screw length in the vertebral body and overall length. We believed that the reason lay in the defects of the screw path of the CBT, which can not make effective use of the cortical bone throughout the screw path, especially the medial wall of the pedicle and the cortical bone of the lateral edge of the superior endplate. To compensate for these shortcomings of CBT, improvement measures were taken and related research articles have been published previously [8, 9, 17]. The screw insertion point of the MCBT technique moved 2.0–3.0 mm inward based on CBT, increasing the bony cortex around the screw insertion point and medial & inferior wall of the pedicle and the anterior lateral edge of the upper endplate of the vertebral body by increasing the mediolateral angle from 10° to 20°and decreasing the craniocaudal angle from 30° to 25°compared to the CBT technique [8, 9]. 10° was recommend for L1 and L2, 15° for L3, and 15°-20° for L4 and L5. MCBT achieved the tri-cortical fixation effect, basically from the head to the tail [8]. Fujiwara et al. [18] demonstrated that the closer the screw head is to the vertebral endplate of the body, the greater the insertional torque is. As previously demonstrated by our team that it was better to use an MCBT screw with a diameter of 5.0 mm for the L4 vertebral body compared to a traditional pedicle screw. In addition, the pull-out strength and stability of the MCBT screw were superior to the pedicle screw [14].
Advantages of finite element analysis
Santoni et al. [3, 19] found that the CBT screw had superior pull-out strength and torque, which was inconsistent with other scholars. For example, in the study of Baluch et al. [20], there was no significant difference between the pull-out strength of traditional pedicle screws and CBT screws. Moreover, in the study by Perez-Orribo et al. [21], the performance of the CBT screw is equivalent to the traditional pedicle screw in rotation, flexion, and extension. The reason for the differences in the experimental results of these scholars may be that there was a certain deviation in the selection of the screw insertion point and the angle, resulting in the bias of the experimental results. In addition, when the first screw setting was not ideal and the screw path was changed, it may affect the strength of the second screw setting, resulting in poor repeatability of the experiment. However, the FE analysis can be carried out under the premise of copying the material properties of the specimen and three-dimensional reconstruction of the complete structure of the lumbar spine, and the screw placement can be done under the guidance of a completely accurate and ideal screw insertion point and angle, which ensures the accuracy and repeatability of the experiment and the interference of confounding factors to the experimental results can be avoided with high performance and low cost [22, 23]. In this study, the bone properties and distribution of the vertebral body are assigned according to the actual situation, and the screw insertion point and screw path direction are strictly by the two CBT screw placement technical standards. With the establishment of the FE model, the calculation and analysis of the data will be carried out by the general standard procedures and methods. In addition, The results of the CBT obtained in this study were consistent with Matsukawa et al. [11], and Shao et al. [12]. Perez-Orribo et al. [21], confirmed the reliability of the FE analysis of the paper (Table 2).
Pull-out strength of MCBT
The pull-out strength of the screw is related to the stability of the screw-bone interface [24]. Matsukawa et al. [25] found that under the premise of consistent screw performance, the proportion of cortical bone in the total length of the screw path and its firmness will play a vital role in the pull-out strength, especially on osteoporotic vertebrae. As seen from the data in Table 2, the pull-out strength of the MCBT was higher than CBT. Weinstein et al. [26] showed that 60% of the overall strength of the screw was in the pedicle of the spine, and the cortical bone of the medial wall of the pedicle was thicker than the lateral wall. The MCBT makes full use of this crucial position to make the screw thread closely fit with the medial wall of the pedicle. When the screw continues to reach the cancellous bone of the vertebral body, it will increase by 75%-80% [25]. If the screw continues to penetrate the anterior cortex of the vertebral body, the fixation force can reach 80%-85% [24]. Wu et al. [27] reported that due to the higher bone mineral density near the endplate when the head of the screw reaches the upper endplate, it will provide better stiffness for the screw. Compared with the CBT, MCBT has a larger mediolateral angle, so that the head of the screw fully contacts the cortical bone of the lateral edge of the upper endplate of the vertebral body, achieving a fixation effect similar to the tri-cortical fixation (lamina-medial wall of the pedicle-lateral edge of the upper endplate). The bi-cortical fixation technique of traditional pedicle screw needs to penetrate the anterior wall of the vertebral body, which may lead to the injury of blood vessels and other important structures in front of the vertebral body, resulting in adverse complications [27], while head of the MCBT is far away from the blood vessels.
Screw stability of MCBT
Zindrick et al. [28] showed that increasing the screw diameter and length can enhance the stability of the fixation. The diameter of the screw is one of the important factors affecting its stability of the screw. Pedicle screws with different diameters have significantly different biomechanical characteristics. Generally speaking, the larger the screw diameter, the greater the holding strength, and the better its stability. In this study, the CBT screw is in a diameter of 6.0mm and a length of 35 mm, while the MCBT screw is relatively thin with a diameter of 4.5 mm and a length of 40 mm. The diameter of the former was 25% higher than that of the latter. Interestingly, in the FE analyses of screw stability, the CBT was not as good as MCBT, which indicated that the surrounding bone quality of the MCBT was much better than CBT. The insertion point is moved inward, the mediolateral angle is larger, and the craniocaudal angle is smaller (Fig. 2) so that the screw in the lamina and the vertebral body is lengthened and can reach the lateral edge of the upper endplate of the vertebral body, length of the screw is at least increased by 5mm compared with the CBT. After the screw was inserted into the vertebral body, in addition to the pull-out force, it was also subjected to stresses in other directions, which affected its stability [29]. For patients with osteoporosis, the above-mentioned stress may exert a more significant impact on the pedicle screw, resulting in a loosened screw [30]. In this study, the MCBT showed superiority under various stress, which is crucial for the overall stability of the screw. This property can avoid the domino linkage effect due to the poor mechanical properties of one of the stresses, which will eventually affect the overall stability between the screw and bone.
Vertebral body stability of MCBT
Biomechanical properties of the MCBT are superior in flexion and extension, compared to the CBT, but not obvious in lateral bending and axial rotation. In the lateral bending, the screw insertion point of the MCBT is more inward with a larger mediolateral angle, so that the head end of the screw is located at the lateral edge of the upper endplate of the vertebral body (Fig.. 5 E), which reaches the level of the middle column or even the anterior column of the vertebral body. As is shown in Fig. 2 and Fig. 5B, E, the sectorial areas of MCBT (1758mm²) were much larger than that of CBT (1476mm²). The MCBT further increases the resistance of the screw to rotation and lateral bending, so that the rotary force acting on the screw can be uniformly transmitted and distributed along the three-column structure of the vertebral body as far as possible. To facilitate a more vivid understanding, we use "human hand holding the cup" to explain (Fig. 5C, F). In the MCBT, the screw penetrates the three columns of the vertebral body and has a longer moment arm, which can effectively prevent the vertebral body from lateral bending and excessive axial rotation. However, the effective length of the CBT screw is smaller than that of the MCBT screw (Fig. 5A, D), which is only fixed in the posterior column and a small part of the middle column, with a small moment arm and poor resistance to vertebral body rotation (Fig. 5B, E).
Limitations of this study
Several limitations were in this study. First, only four cadaveric lumbar vertebrae were selected, and the sample size was small. Only the L4 vertebral body was included. Lack of ligaments, muscles, and other tissues, may affect the experimental results, and the overall stress of the fixation system cannot be analyzed. As a novel technique, the MCBT has not been verified by similar cadaver specimens or FE analysis by other scholars.