Evidence & Methods
Whether transarticular or pedicle screws (at C1–C2) pose greater risk to the vertebral arteries is unclear.
In this computer simulation using actual patient CT scans, the authors found no difference in risk between the two techniques unless a high-riding vertebral artery was noted, in which case pedicle screws appear safer.
The study provides valuable information that might help improve safety for patients.
—The Editors
Posterior C1–C2 transarticular screw fixation can result in vertebral artery injury, with potentially catastrophic results such as vertebrobasilar insufficiency, brain stem and posterior fossa infarct, and even death [1], [2], [3]. The rate of vertebral artery injury has been reported to be as high as 8.2% [1], [2], [3]. Vertebral artery injuries commonly occur if a drill, tap, or screw perforates or occludes the vertebral artery in the vertebral artery groove of C2, which is located on the lower surface of C2 and lateral to the pedicle (Fig. 1) [1], [2], [3], [4], [5], [6], [7], [8]. It has been suggested that C2 pedicle screw combined with C1 lateral mass screw placement is inherently safer than C1–C2 transarticular screw fixation in view of the risk of vertebral artery injury at C2, as the medially directed trajectory of the C2 pedicle screw carries the screw away from the vertebral artery, which normally is located lateral to the screw [9], [10], [11]. Although this is a commonly held belief, no large series have been performed to assess the validity of this assumption. Knowing whether the risk of vertebral artery injury is higher with C1–C2 transarticular screw fixation versus C2 pedicle screw placement is important, as the knowledge would provide surgeons with information on which to choose the appropriate fixation method, particularly in cases where the anatomy may be abnormal (eg, in the cases of a high-riding vertebral artery or narrow pedicle).
To date, two reports comparing the risk of vertebral artery injuries by transarticular screw versus pedicle screw have been published [12], [13]. Both studies stated that the anatomic risk of vertebral artery injury is not significantly different between transarticular and pedicle screws, contrary to the previous suggestion favoring pedicle screw [9], [10], [11]. Although these studies carefully analyzed the risk of each fixation method, like all studies, they have limitations. One study [13] used mainly 2D images with three-dimensional (3D) images used only in 10 patients. As stated by Miyata et al. [14], a 3D evaluation is essential for establishing the anatomic relationship between the course of the vertebral artery and the intended screw trajectory. Furthermore, both studies analyzed a small number of patients without a power analysis, raising the possibility that they were underpowered to detect a difference between the two fixation methods (beta error).
In addition to analyzing the safety of each technique in the general patient population, it may be even more important to evaluate the risk of vertebral artery injury in those with a relatively small space to accommodate these screws. These are the individuals in whom the risk of vertebral artery is increased, and the choice of appropriate screw fixation is even more important. Two anatomic variations have been suggested to be associated with vertebral artery injury. First, it is well known that the risk of vertebral artery injury during C1–C2 transarticular screw placement increases with a high-riding vertebral artery [2], [5], [6], [15], [16]. However, we are unaware of any reports comparing the risk of vertebral artery injury by C1–C2 transarticular screw versus C2 pedicle screw in those with a high-riding vertebral artery, and no references on the topic could be found in a computerized search using MEDLINE. Second, narrow pedicles of C2 theoretically can increase the risk of vertebral artery injury by C2 pedicle screw. Yoshida et al. [12] stated in their 3D simulation study that pedicle screw placement was limited by the width of the C2 pedicle. However, again, as far as we know, no studies have been performed comparing the risk of vertebral artery injury with transarticular screw versus pedicle screw in those with a narrow pedicle.
Given the limitations of the existing studies, the primary purpose of our study was to compare the anatomic risk of vertebral artery groove violation by transarticular screw versus pedicle screw using a large number of computed tomography (CT) scan images and 3D screw trajectory simulation software. The comparison was performed in three groups of patients: the overall patient population and two subsets, those with a high-riding vertebral artery and those with narrow pedicles. Of note, neither C1–C2 transarticular screws nor C2 pedicle screws are approved by the US Food and Drug Administration.