Morphologic Considerations of C2 Isthmus Dimensions for the Placement of Transarticular Screws

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.