Cervical fixation using pedicle screws is becoming increasingly popular for various cervical diseases due to their advantages, such as three-column fixation, sagittal correction, and excellent biomechanical stability when compared with spinous process wire, lateral mass screws or facet screws(19, 20). However, the CPS has not become the routine for posterior cervical fixation due to the high risk and high skills required.
In addition to the CPS, lateral mass screw (LMS) is another commonly used method for posterior subaxial cervical fixation. Previous researches and clinical practices showed that these two methods have their advantages and disadvantages(20). The placement of LMS is comparatively simple, and the incidence of neurovascular complications during operation is lower(21). However, LMS showed a small amount of bony purchase and thereby lesser pullout strength especially in the presence of osteoporosis in biomechanical experiments when compared with CPS. By contrast, the biomechanical result showed the pullout strength was significantly higher for the CPS, and thus it was relatively difficult to loosen(20). The technique for CPS placement is more difficult than LMS although much successful clinical use of CPSs has been described. Reports of the CPS insertion failure have shown the possibility of serious iatrogenic injury (17, 22).
The most challenging process of placing CPSs safely into the cervical vertebrae is making the exact cancellous pathway between the vertebral artery and cervical spinal cord for the pedicle screw. Lateral pedicle perforation when placing CPSs may injury the vertebral artery, while medial pedicle perforations can injury the cervical spinal cord.
The cortex of the medial margin of the cervical pedicle is much thicker and stronger than that of the lateral margin, so it is predisposed to lateral perforation during pathway preparation, tapping, and insertion of the screws. Previous literature has confirmed this point of view. A multicenter study from Japan on the complications of CPS placing during the conventional free-hand technique showed that 75% (57/76) of all misplaced screws were lateral pedicle perforation, while only 25% (19/76) were medial pedicle perforation(16). Therefore, attention needs to be paid to how to reduce lateral pedicle perforation while designing a new CPS placing technology.
The most representative conventional free-hand technique for subaxial CPS placing is the method proposed by Abumi et al. (5)in 1994. The contents include: the entry point at the posterior cortex of the articular mass was determined slightly lateral to the center of the articular mass and close to the posterior margin of the superior articular surface. The intended angle of the screws based on measurements of preoperative CT images was 30-40° medial to the midline in the transverse plane, and parallel to the upper end-plate in the sagittal plane. The insertion of the screw was greater than two-thirds of the AP vertebral body depth. Subsequently, Jeanneret(6), Miller(7), and Liu(8) proposed various improved free-hand methods for CPS placement. There were also studies reported the CPS placement using the medial funnel technique(23) and medial cortical pedicle screw technique(9). Burcev et al.(10) introduced a standardized and fast method for subaxial CPS: screw insertion based on the simple angles to the bony landmarks. However, the accuracy of conventional CPS placement methods needs to be improved though there are many methods to choose from.
In recent years, digital navigation technology, 3D printing technology, and robotic technology are also arising in CPS placement. For example, Ishikawa et al.(24)placed 108 cervical CPSs using an intraoperative, full-rotation, 3D image (O-arm)-based navigation system. The results showed that 96 of them (88.9%) were grade 0, 9 were grade 1 and 3 were grade 2. There were no complications such as vascular and nervous complications, indicating that a combination of intraoperative 3D image-based navigation with other techniques may result in more accurate CPS placement. But not all the results about using the navigation in CPSs placement were optimistic. Uehara et al. (15)inserted the CPSs by using a CT-based navigation system during operations. The results showed that the combined rate of grades 2 and 3 perforations was 20.0 % (116/579). Therefore, the authors concluded careful insertion of pedicle screws is necessary, especially at C3 to C5, even when using a CT-based navigation system. There were a few studies even showing more perforations with navigated screws than with the free-hand technique(25, 26). Although most of the studies support that navigation technology can improve the accuracy of pedicle screw placement(11), none of the high-tech can completely avoid the occurrence of screw perforations. The navigation system and other computer-aided technology always associated with complex operation procedures and expensive equipment costs(12). Moreover, due to the high mobility of the cervical spine, cervical spine alignment can easily change during operation(27), which may lead to inaccurate synchronization to preoperative images. All the shortcomings limit the application of digital navigation technology, especially in developing countries.
Therefore, it is of great clinical significance to develop a cost-effective and safe method for free-hand CPS placement. Raphael Vialle et al.(13)developed the "sliding technique" for safe pedicle screw placement in the thoracic spine in 2004. The key point of this novel technique was to use the cortex of the anterior aspect of the transverse process and the lateral margin of the pedicle as a ‘‘slide’’ to permit correct probe positioning during pedicle probe insertion. Inspired by it, we devised a novel method for free-hand CPSs insertion also called “slide technique”, to increase the accuracy of CPS placement via a direct slide on the cortex of the medial margin of the pedicle.
Preliminary clinical results of this novel technique showed a higher rate of correct screw position, comparable to CPS placement with navigation system. Meanwhile, no neurovascular complications occurred stemming from malpositioning of pedicle screws. In the process of this free-hand technique, screw perforations inevitably occurred, with lateral pedicle perforation accounted for the majority, which was consistent with previous studies.
Several points should be paid attention to in the clinical application of the "sliding technique". First, it is necessary to carefully read and analyze the imaging data before an operation, especially to accurately measure the diameter of the pedicle and pay attention to the variation of anatomical structure (such as pedicle sclerosis, pedicle slenderness, vertebral foramen malformation, local bone destruction, etc.). Second, the pedicle probe should be rotated while gently being advanced, and be stopped in case of resistance. No violence should be used during the whole operation. To ensure the accuracy of the pathway, intraoperative fluoroscopy is necessary. Third, when CPS placement is difficult, lateral mass screw or other fixation may be selected.
Please note that the patients included in the current study were all with cervical trauma, whose pedicle variation rate was small, so the accuracy of screw placement was relatively easy to achieve. The accuracy of the screw may decrease when applied to difficult situations such as cervical deformity.