C1 pedicle screw placement risks in patients with ponticulus posticus: a CTA retrospective simulation study.

Abstract

Background: Ponticulus posticus (POPO) may distort C1 surface landmarks for C1 pedicle screw (CPS) placement, raising concern for vertebral artery compromise (VAC). We aimed to establish a CTA-based workflow to identify POPO anatomies potentially suitable for CPS placement.

Methods: This single-center retrospective CTA simulation study included 141 subjects: 46 bilateral POPO, 65 unilateral POPO, and 30 pure non-POPO patients. CPS trajectories were primarily simulated using PedicleScrewSimulator. D1 and D2 were analyzed as supra-arch and infra-arch localization variables. Ten morphometric parameters were measured; key safety metrics included D3 and D5. Simulated CPS feasibility and VAC were evaluated for 2.5-, 3.0-, 3.5-, and 4.0-mm screws. A side was considered geometrically feasible when D5 ≥ screw radius (r), D6/2 ≥ r, and D7/2 ≥ r. Simulated VAC was defined as violation of D5 or D7/2 clearance. Outcomes were analyzed using patient-clustered generalized estimating equations.

Results: D1 and the D1-D2 shift differed markedly among groups (p < 0.001). D5 was smaller in bilateral POPO (2.93 ± 0.99 mm vs. 3.44 ± 1.14 mm vs. 3.81 ± 1.13 mm; p < 0.001). D3 did not differ significantly (p = 0.211). At 3.5 mm, overall simulated feasibility was 96.0%. Actual POPO sides showed 93.5% simulated feasibility and 6.5% simulated VAC, versus 99.2% and 0.8% in regular sides. In actual POPO sides, increasing diameter from 2.5 to 4.0 mm reduced simulated feasibility from 98.7% to 85.6% and increased VAC from 1.3% to 14.4%. The exploratory D3/D5 rule (D3 ≥ 3.4 mm and D5 ≥ 2.0 mm) achieved 88.4% sensitivity, 100% specificity, and 100% positive predictive value for avoiding simulated 3.5-mm VAC. Reliability for the four remeasured variables ranged from 0.702 to 0.799.

Conclusions: CTA-based D3 and D5 thresholds can identify POPO anatomies potentially suitable for CPS placement, but simulation results reflect planning feasibility rather than validated operative safety. The D3 ≥ 3.4 mm and D5 ≥ 2.0 mm rule may serve as a conservative rule-in screening tool, pending clinical validation. POPO should not automatically preclude consideration of CPS but should trigger side-specific CTA analysis and conservative screw-diameter selection.