Pedicle screw fixation provides effective stabilization with simultaneous decompression to the cervical spine, and is typically used to treat those patients with local kyphosis, segmental instability, or anterior conditions which had previously been operated on [5, 6, 19]. As DCS has been reported to occur at a high rate in patients with cervical spondylotic myelopathy or cervical spinal cord injuries [1, 2, 20, 21], the application of cervical pedicle screws may be of value in these DCS patients. The risk of neurovascular injury as a result of poor screw positioning, however, limits the application of CPS insertion [8, 9]. DCS is often caused by dysplasia of the rear attachment structures of the cervical spine in children [1, 22]. Most studies have found that DCS patients often exhibit short pedicle, flat lamina, and the overlap of the lamina and the rear edge of the lateral mass line in lateral radiographs [23, 24]. Some studies have further produced morphological comparisons of the cervical posterior structures between DCS and NDCS patients [16, 25, 26]. Wang Z et al. [25] found through CT-based measurements that the majority of evaluated parameters differed between DCS and NDCS patients in northeastern China. However, the safety of subaxial CPS insertion in DCS patients has not previously been clarified. Namely, if the short pedicle with narrow canal is relatively small, posterior pedicle screws insertion maybe more dangerous for these patients. If not, cervical pedicle screws maybe more safe for a short and wide pedicle.
We therefore performed morphological measurements on CT reconstruction images, comparing these measurements between DCS and NDCS patients grouped according to the Pavlov ratio, which is commonly used in clinical settings [18]. These measurements not only provide objective information regarding CPS insertion in DCS patients, but also provide insight into the question of whether CPS insertion in DCS patients is more difficult. Morphological features relevant to CPS insertion include linear parameters, such as pedicle width, pedicle height, and pedicle length as well as angular parameters, such as the pedicle transverse angle and the pedicle sagittal angle. Previous studies have shown that pedicle width was the primary determinant of the feasibility and safety of CPS insertion, while the pedicle transverse angle was more important than sagittal angle with respect to the potential for neurovascular complications [11, 15, 27]. In the present study, we therefore measured the pedicle width and transverse angle carefully. We additionally assessed the incidence of small pedicles with a POW < 4 mm, and we calculated the range of safe angle. As previous comparisons of these parameters between male and female patients have yielded consistent results [11, 15], gender differences were not focused in this study.
The mean overall cervical pedicle width values and trends in our study were similar to those in previous reports [11,12,13,14,15,16]. Onibokun et al. [11] found in a multiplanar CT anatomical assessment of 122 subjects that the overall pedicle outer width ranged from 4.7 to 6.5 mm, with a trend towards increasing caudally. Miyazaki et al. [16] found in a CT myelography analysis of 52 subjects comparing DCS and NDCS patients that the POW ranged from 4.8 to 5.9 mm in DCS patients and from 4.7 to 6.4 mm in NDCS patients. The mean POW in the DCS group was significantly less than that in the NDCS group at C6 and C7, with no significant differences at C3, C4, or C5. Unlike this latter study, we observed that the POW in DCS patients was wider than that in NDCS patients at every level. This may be due to the different classification methods used for establishing study groups. This previous studies classified study groups according to the spinal canal longitudinal diameter (SCLD), whereas we relied upon the Pavlov ratio. Moreover, racial variations may have contributed to these different study outcomes. Indeed, our results indicated that subaxial CPS insertion in DCS patients may be easier than that in NDCS patients.
The smallest CPS diameter available is 3.5 mm in clinical practice. As such, pedicles with a diameter of 4.0 mm were assumed to be the smallest into which a screw could feasibly be inserted, as documented in previous studies [15, 28]. In our study we identified instances of TCP in both patient groups from C3 to C6 (8.3 to 11.6% in the DCS group; 15.0 to 35.0% in the NDCS group), with no instances at C7. A comparative analysis revealed there were fewer TCPs in DCS patients from C3 to C5. Thus, preoperative CT scanning and trajectory planning is essential for safe and effective CPS insertion. Our study also indicated that greater consideration should be taken for pedicle insertion in NDCS patients owing to the greater number of cervical pedicles in these individuals.
Proper angulation is an important factor for CPS insertion [27, 29]. Injury to the medial breach may result in cerebrospinal fluid leakage or spinal cord injury, whereas injury to the lateral breach may cause vertebral artery injury. In our study, observed pedicle transverse angles closely paralleled those reported in previous studies [16, 30,31,32]. Tan et al. [32] reported an average PTA range of 30° to 46° from C3 to C7 in a study of Chinese Singaporeans, with the smallest average PTA at C7. Miyazaki et al. [16] found the transverse angles from C3 to C7 ranged from 30.6° to 41.9° in DCS patients, and from 30.8° to 42.4° in NDCS patients, with no significant difference between these groups. Our study supported the finding of these previous studies, revealing a similar PTA range of 35° to 47° in the DCS and NDCS groups. In addition, we further measured the maximum transverse angle, the minimum transverse angle, and the range of safe angle in the two groups included in our study. Our results revealed that the RSA in the DCS patients was greater than that in NDCS patients at all levels except for C3. However, this range of safe angle at every level in the two groups was very small (about 10°), even neglecting the diameter of CPS. That no significant differences were detectable at C3 may be due to the small POW at this level, resulting in limited change between the maximum and minimum transverse angles. In addition, we observed large individual variations in this study as evidenced by the relatively wide PTA range and the large standard deviation for each level. Given this wide degree of variability, a preoperative CT evaluation of the PTA is vital in all patients to ensure safe CPS insertion.