Axis vertebral dimensions for safe screw placement: A CT normative data analysis

Morphometric evaluation of the pedicle and isthmus of second cervical vertebra (C2) (Axis) is extremely vital before contemplating any surgical stabilization involving the Craniovertebral region, in view of its proximity to the vertebral artery and the cervical nerve root. The dimensions of pedicles and isthmuses in C2 vary between individuals and there is paucity of data in the Indian population. This study strives to measure the average pedicle and isthmus dimensions in a sample of population, which would enable selection of screws with safest diameters to be used in C2; thereby avoiding injury to adjacent neurovascular structures.

One Hundred patients in the age group between 18 and 70 years who underwent CT scan of head and neck region were included in the study. The aim of this study was to assess the anatomic suitability of transarticular and pedicle screw placement in Axis vertebrae of Indian population and determine the maximum safe diameter for screw placement. The following parameters were measured in millimeters: Pedicle width, Pedicle angle, Internal height and Isthmic height.

The Mean maximum diameter of potential pedicle screw was 4.99 ± 1.1 mm for the right side with the left side being slightly wider at 5.20 ± 1.16 mm. Twenty eight (28%; 56 out of 200 pedicles) had a measurement < 4.5 mm. The internal height in sagittal images representing the pedicle height was found to be 4.79 ± 0.96 mm on the right side and 4.75 ± 1.04 mm on the left side. Sixty five (65) out of 200 pedicles (32.5%) had measurements < 4.5 mm in sagittal plane. The Mean maximum diameter of potential Transarticular screw (outer diameter of isthmus) was 5.05 ± 0.78 mm for the right side and 5.18 ± 0.84 mm on the left side.

Isthmic height < 4.5 mm could potentially violate the vertebral foramen when a 3.5 mm screw is used. In our study 22.5% isthmuses were narrow (<4.5 mm). The mean maximum safe diameter for a potential transarticular screw in the present study was 5.11 mm. Though our patients had smaller isthmus dimensions compared with literature, 77.5% of C2 could take a 4 mm transarticular screw quite comfortably considering the 0.5 mm margin on either side. In the present study, 28% of pedicles were found to be inappropriately sized (<4.5 mm) to accommodate the standard 3.5 mm screw. The mean maximum diameter of a potential pedicle screw in our study was 5.09 mm; and in 72% of patients a 4 mm screw could be placed with confidence. Though our patients on an average can accommodate a 4 mm screw comfortably, we suggest a protocol of obtaining CT measurements of C2 prior to operative intervention for identifying those individuals at risk of neurovascular injury; 22.5% for transarticular screw and 28% for pedicle screw.