Traumatic atlantoaxial (upper cervical spine) leads to instability in weightbearing movement and neurological deficit. Presently, C1 (axial) lateral mass or pedicle screws for fixation are the most popular because of excellent mechanical performance for internal fixation. C1 pedicle screw fixation can reduce intraoperative blood loss and postoperative occipital neuralgia more than C1 lateral mass screws. However, screws cannot be inserted completely through the pedicle in some patients due to C1 size. We aimed to determine the ideal pedicle screw entry point, angle of screw projection, and pedicle height in the Thai population. Patient data were collected and measured using the INFINITT program at Mukdahan Hospital from September 2020 to June 2021. The C1 measurements, i.e., distance from the midline to the medial edge of the posterior arch (DPA) and medial edge transverse foramen (DTF), angle of screw projection, and length and height of the pedicle were recorded. Descriptive statistics and -test were used to analyze the data. The mean Thai pedicle dimensions were DPA = 14.17 mm (range: 11.19-19.70 mm), DTF = 22.09 mm (range: 18.13-26.44 mm), ideal screw entry point = 18.13 mm (range: 15.19-22.00 mm), ideal angle of screw projection medial angulation = 2.67 degrees (range: 0-7 degrees), and height of posterior arch (pedicle) = 4.77 mm (range: 2.68-7.22 mm). Forty of 167 patients (24.0%) had a pedicle height less than 4.0 mm (bilateral 11 patients and unilateral 29 patients). The ideal C1 pedicle screw entry point is approximately 18.13 mm from the midline. In the Thai samples with C1 pedicle height less than 4.0 mm, the screws cannot be inserted completely through the pedicle. Therefore, screw insertion should be partially through the pedicle (notching technique).
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