An anatomic consideration of C2 vertebrae artery groove variation for individual screw implantation in axis

C2 pedicle screw can be used as a successful anchor for correction and fusion of a variety of atlantoaxial and occipitocervical problems [5‐7]. Biomechanical studies have shown the stability of C2 pedicle screws in a highly mobile region [8, 9]. Pedicle screws could offer adequate fixation of the axis and comparable fusion rates compared with those obtained with transarticular screws. However, placement of these screws is technically demanding and places the vertebral arteries and nerve roots at risk of damage in circumstances of cortical breach [10].

Stulik et al. [11] reported 5.4 % of screws malpositioned placed in C2. Ondra et al. [12] reported the result of their 150 C2 pedicle screws placed in 79 patients, and there were 8 VA foraminal breaches on postoperative CT scans. Yeom et al. [13] analyzed the incidence of cortical breaches for VAG in C2 pedicle screws using postoperative fine-cut CT scans and CT angiography with multi planar and three-dimensional reconstructions. They reported a higher VAG violations rates (20 %). They speculated that the frequency of VAG violation may be underestimated by many authors due to inaccurate evaluation methods. As the intraoperative lateral C arm fluoroscopic monitoring and postoperative radiographs may not be enough for assistant, avoid this kind of risk in C2 transpedicle screw placement. Therefore, it is necessary to investigate the anatomic character of C2 VAG furtherly and set up an individual surgery strategy may be helpful for decrease the risk of VAG violation.

It has been reported that anatomic variations of C2 VAG found on preoperative imaging have impact on surgical planning [14]. However, there is no well-accepted guidelines on which radiographic parameters can predict risks for cortical breach with C2 pedicle screw placement. Preoperative CT has been shown to improve surgeon ability to detect important differences in vertebral anatomy [15]. Hassan et al. [16] reported that the C2 pedicle screws placement risk could be judged on the size of pedicle shown on presurgical thin-slice CT scan, as the diameter of C2 pedicle less than 6 mm was associated with a nearly twofold higher risk of cortical breach than the group more than 6 mm (37 vs 21 %); therefore, measurement of the pedicle diameter on CT scan could act as an useful parameter for evaluation of the risks of screw placement. However, measurement of the diameter of C2 pedicle varies among different authors, usually with different methods and on different slices of the CT scan.

Computer-assisted three-dimensional reconstruction is a good way for the evaluation of C2 VGA; however, it is time-consuming and complicated. We are seeking a simple and useful way for C2 VGA evaluation. By using consecutive thin-slice CT scan (1 mm thickness), a kind of CT scan spectrum of C2 VAG could be easily obtained. Compared with the single-slice image of C2 pedicle, the consecutive CT scan spectrum could provide us a kind of holography of the C2 VAG with more integrated and rich information. Through this C2 VAG CT spectrum, we could reconstruct a kind of model figure of C2 VAG easily just like the three-dimensional reconstruction CT image based on parameter a and parameter e, and a “safe zone” for pedicle screw placement could be easily found on the model figure of C2 too.

When inserting the screw through C2 pedicle, the room surrounded by the “a and e” in the coronal section of pedicle will provide a “safe zone” for screws placement. The larger the a/e value is, the safer for pedicle screw implantation. When the a/e lesser than 4.5/4.5 mm (type II subgroup), it is difficult and dangerous to place a 3.5 mm screw, as violation could happen easily.

For the subgroup of types I, III, IV, the “safe zone” of “a and e” is bigger than 4.5 × 4.5 mm, which could provide a relative safe room for pedicle placement. However, in the subgroup type II, the “safe zone” of “a and e” is lesser than 4.5 × 4.5 mm, which should be regard as contradiction for pedicle screw placement. Therefore, an alternative method of C2 translaminar screw should be recommended. The major advantage of C2 translaminar screw is the elimination of the potential risk of arterial injury by placing screws only within the posterior column [17]. In our 45 consecutive patients, there were 74 transpedicle screws and 16 translaminar screws used according the above strategy rules. The postoperative CT scan shows that there were two pedicle screws violated into the artery groove (2.7 %), and no translaminar screw breached into the vertebrae canal, which show a smaller breach rate than the lecture reported by Yeom and other authors [11‐13]. Yeom et al. claimed that the risk of pedicle violation cannot be completely avoided, even with careful preoperative planning and intraoperative C arm fluoroscopic imaging. We think that choosing an individual screw placement method (transpedicle or translaminar) based on presurgical thin-slice CT analysis of C2 VAG variations could provide an useful personalized strategy for C2 screw fixation, diminishing complications and risks, and lower the violation rates.