Study Design. This study examines the C2 vertebrae using both direct anatomic and computed tomographic measurements.
Objective. To define the relation of the C2 vertebrae bony elements to the vertebral artery and the spinal canal, to determine individuals at risk for vertebral artery injury during C1–C2 transarticular screw placement.
Summary of Background Data. Recent literature assessing the safety of upper cervical spine transarticular screws has concentrated on technique, including the optimal point of entry and path projection of the screw. The actual dimensions of the C2 isthmus of the pars interarticularis has not been examined in a large number of specimens.
Methods. C2 isthmus width and height measurements were made on 205 human cadaveric C2 vertebrae. Vertebrae predicted to be at risk for vertebral arterial injury were imaged by computed tomography.
Results. There were 102 female and 103 male specimens with mean isthmus widths of 8.2 ± 1.5 mm and 7.2 ± 1.3 mm, respectively. Five specimens (2.4%) had an isthmus width less than 5 mm. The mean isthmus heights were 8.6 ± 2.0 mm and 6.9 ± 1.5 mm for male and female specimens, respectively. In twenty-four specimens (11.7%), one or both isthmi had a height of less than 5 mm. Six of these specimens were male and 18 were female. The right C2 isthmus was generally smaller than the left. Computed tomographic measurements closely approximated those of the actual dimensions of the isthmi.
Conclusions. Placing a 3.5 mm screw in a patient with narrow C2 isthmus dimensions (smaller than 5 mm in either the height or width) is technically difficult. Because of narrow C2 isthmus width and/or height, approximately 10% of patients may be at risk for a vertebral artery injury with placement of C1–C2 transarticular screws.
C1–C2 transarticular screw fixation is frequently performed to correct the instability caused by numerous traumatic and nontraumatic conditions of upper cervical spine. Recent literature examining the safety of C1–C2 transarticular screws has concentrated on surgical techniques. 6,16 These researchers described the optimal point of screw entry and path for correct screw placement to minimize neural and vascular injury.
There is much confusion in the nomenclature in describing anatomy of C2 because of its unique morphology. 1,11 The true anatomic C2 pedicle lies anterior to the articular process connecting the dorsal elements to the vertebral body. The narrow bridge of bone most often described as the pedicle of C2 is actually the isthmus of the pars interarticularis. This nomenclature is used in this study (Figure 1). Properly placed, the path of the C1–C2 transarticular screw passes through the lamina and the narrow isthmus of the C2 pars interarticularis and then through the posterior portion of the C1–C2 facet joint.
Precise placement of the screw is required because of the proximity of the vertebral artery and neural elements to the screw’s path. The ideally placed screw is confined totally within the bony elements for both stability and safety. Screws placed either in the anterior portion of the C1–C2 facet joint or lateral to the C2 isthmus may result in vertebral artery damage, and a screw placed medial to the isthmus projects into the spinal canal itself.
In this study, the morphology of the C2 vertebrae was examined, to define the relation of bony elements to the vertebral artery and the spinal canal and determine individuals at risk for improper screw placement, despite the use of proper surgical techniques. Two anatomic measurements of the second cervical vertebrae were obtained and indicated the presence of an at-risk population for vertebral artery disruption during placement of C1–C2 transarticular screws. The potential use of computed tomography (CT) for preoperative evaluation to identify at-risk patients was also studied.