Objective: To explore the feasibility and effectiveness of C1 lateral mass fixation via the posterior arch using pedicle screws of 3.5-mm diameter in patients whose atlas in the vertebral artery groove (C1 pedicle) was ≤3.5 mm in height.

Methods: A total of 14 patients who underwent transposterior arch lateral mass screw fixation between 2014 and 2017 due to atlantoaxial instability were retrospectively studied. The height of the atlas pedicle was ≤3.5 mm on one or both sides. The position of the screw and damage of the screw trajectory were assessed using the postoperative 3-dimensional computed tomography. The patients were regularly followed up to observe the fracture union and bone graft fusion.

Results: In total, 27 atlas pedicle heights were ≤3.5 mm and 22 screws were successfully inserted without any neurovascular complications. However, the pedicle wall was found to have a certain degree of damage with damaged superior/inferior wall in 4 of 6 pedicles respectively. Clinical symptoms were relieved to varying degrees, and patients demonstrated bony fusion during the postoperative follow-up.

Conclusions: Pedicle screws of 3.5-mm diameter could be fixed with a high success rate using the viscoelasticity and expansivity of the bone tissue even if the atlas pedicles were ≤3.5 mm in height. Although the screws had a certain probability of breaking the superior/inferior wall, they provided sufficient stability and enabled fracture or bone graft healing. Extraverted atlas pedicle screws should be considered in some cases.